A comparison of genetic polymorphism in populations of Onchocerca volvulus from untreated- and ivermectin-treated patients

Virtual screening, MMGBSA, and molecular dynamics approaches for identification of natural products from South African biodiversity as potential Onchocerca volvulus pi-class glutathione S-transferase inhibitors

We investigated 1012 molecules from natural products previously isolated from the South African biodiversity (SANCDB, https://sancdb.rubi.ru.ac.za/), for putative inhibition of Onchocerca volvulus pi-class glutathione S-transferase (Ov-GST2) by virtual screening, MMGBSA, and molecular dynamics approaches. ADMET, docking, and MMGBSA shortlisted 12 selected homoisoflavanones-type hit molecules, among which two namely SANC00569, and SANC00689 displayed high binding affinities of -46.09 and -46.26 kcal mol-1, respectively towards π-class Ov-GST2, respectively. The molecular dynamics results of SANC00569 showed the presence of intermolecular H-bonding, hydrophobic interactions between the ligand and key amino acids of Ov-GST2, throughout the simulation period. This hit molecule had a stable binding pose and occupied the binding pockets throughout the 200 ns simulation. To the best of our knowledge, there is no report of any alleged anti-onchocerciasis activity referring to homoisoflavanones or flavonoids. Nevertheless, homoisoflavanones, which are a subclass of flavonoids, exhibit a plethora of biological activities. All these results led to the conclusion that SANC00569 is the most hypothetical Ov-GST2, which could lead the development of new drugs against Onchocerca volvulus pi-class glutathione S-transferase. Further validation of these findings through in vitro and in vivo studies is required.

Ivermectin as a novel malaria control tool: Getting ahead of the resistance curse

Reduction in malaria clinical cases is strongly dependent on the ability to prevent Anopheles infectious bites. Vector control strategies using long-lasting insecticidal nets and indoor residual spraying with insecticides have contributed to significantly reduce the incidence of malaria in many endemic countries, especially in the Sub-Saharan region. However, global progress in reducing malaria cases has plateaued since 2015 mostly due to the increased insecticide resistance and behavioral changes in Anopheles vectors. Additional control strategies are thus required to further reduce the burden of malaria and contain the spread of resistant and invasive Anopheles vectors. The use of endectocides such as ivermectin as an additional malaria control tool is now receiving increased attention, driven by its different mode of action compared to insecticides used so far and its excellent safety record for humans. In this opinion article, we discuss the advantages and disadvantages of using ivermectin for malaria control with a focus on the risk of selecting ivermectin resistance in malaria vectors. We also highlight the importance of understanding how ivermectin resistance could develop in mosquitoes and what its underlying mechanisms and associated molecular markers are, and propose a research agenda to manage this phenomenon.

Single-Nucleotide Polymorphism Associates' β-Tubulin Isotype-1 Gene in Onchocerca volvulus Populations in Ivermectin-Treated Communities in Taraba State, Nigeria

PURPOSE

The occurrence of Single-Nucleotide Polymorphisms (SNPs) associated with repeated ivermectin treatment and sub-optimal responses reported by previous findings is of great concern in Onchocerciasis endemic areas. This study investigated SNPs' occurrence after 15 years of ivermectin intervention in Onchocerciasis endemic communities in two Local Government Areas of Taraba State, Nigeria.

METHODS

Microfilariae samples were collected by skin snip from individuals treated with ivermectin for 10-15 years of annual distribution and preserved in RNAlater^® in a 1.5 ml micro-centrifuge tube. Genomic DNA was extracted from microfilariae and residual skin, amplification in two regions within the β-tubulin gene, sequenced and analyzed for SNPs using Bioinformatics tools.

RESULTS

Three distinct SNP positions: 1183 (T/G), 1188 (T/C) and 1308 (C/T) on the β-tubulin gene on the targeted 1083-1568 bp fragment, associate's with the ivermectin-treated population. Furthermore, SNPs positions detected in this study are 1730 (A/G) and 1794 (T/G) in the β-tub gene in the 1557-1857 (bp) region. The 1794 (T/G) SNP position (Phe243Val) in the exon within the β-tubulin gene region were observed in this study.

CONCLUSION

The present study indicates that SNPs are observed in Onchocerca volvulus, thus strengthening the warning that genetic changes could occur in some parasite populations in some ivermectin-treated areas.

Development of a recombinant vaccine against human onchocerciasis

INTRODUCTION

Human onchocerciasis caused by the filarial nematode parasite Onchocerca volvulus remains a major cause of debilitating disease infecting millions primarily in Sub-Saharan Africa. The development of a prophylactic vaccine, along with mass drug administration, would facilitate meeting the goal of onchocerciasis elimination by 2030.

AREAS COVERED

Models used to study immunity to Onchocerca include natural infection of cattle with Onchocerca ochengi and O. volvulus infective third-stage larvae implanted within diffusion chambers in mice. A vaccine, comprised of two adjuvanted recombinant antigens, induced protective immunity in genetically diverse mice suggesting that it will function similarly in diverse human populations. These antigens were recognized by immune humans and also induced protective immunity against Brugia malayi. We describe the development of a fusion protein composed of the two vaccine antigens with the plan to test the vaccine in cows and non-human primates as a prelude to the initiation of phase 1 clinical trials.

EXPERT OPINION

The adjuvanted O. volvulus vaccine composed of two antigens Ov-103 and Ov-RAL-2 was shown to be consistently effective at inducing protective immunity using multiple immune mechanisms. The vaccine is ready for further evaluation in other animal models before moving to clinical trials in humans.

Genomic Epidemiology in Filarial Nematodes: Transforming the Basis for Elimination Program Decisions

Onchocerciasis and lymphatic filariasis are targeted for elimination, primarily using mass drug administration at the country and community levels. Elimination of transmission is the onchocerciasis target and global elimination as a public health problem is the end point for lymphatic filariasis. Where program duration, treatment coverage, and compliance are sufficiently high, elimination is achievable for both parasites within defined geographic areas. However, transmission has re-emerged after apparent elimination in some areas, and in others has continued despite years of mass drug treatment. A critical question is whether this re-emergence and/or persistence of transmission is due to persistence of local parasites-i.e., the result of insufficient duration or drug coverage, poor parasite response to the drugs, or inadequate methods of assessment and/or criteria for determining when to stop treatment-or due to re-introduction of parasites via human or vector movement from another endemic area. We review recent genetics-based research exploring these questions in Onchocerca volvulus, the filarial nematode that causes onchocerciasis, and Wuchereria bancrofti, the major pathogen for lymphatic filariasis. We focus in particular on the combination of genomic epidemiology and genome-wide associations to delineate transmission zones and distinguish between local and introduced parasites as the source of resurgence or continuing transmission, and to identify genetic markers associated with parasite response to chemotherapy. Our ultimate goal is to assist elimination efforts by developing easy-to-use tools that incorporate genetic information about transmission and drug response for more effective mass drug distribution, surveillance strategies, and decisions on when to stop interventions to improve sustainability of elimination.

Reaching the last mile: main challenges relating to and recommendations to accelerate onchocerciasis elimination in Africa

BACKGROUND

Onchocerciasis (river blindness), caused by the filarial worm species Onchocerca volvulus, is a serious vector-borne neglected tropical disease (NTD) of public health and socioeconomic concern. It is transmitted through the bite of black flies of the genus Simulium, and manifested in dermal and ocular lesions. Ninety-nine percent of the total global risk and burden of onchocerciasis is in Africa. This scoping review examines the key challenges related to the elimination of onchocerciasis by 2020-2025 in Africa, and proposes recommendations to overcome the challenges and accelerate disease elimination. To find relevant articles published in peer-reviewed journals, a search of PubMed and Google Scholar databases was carried out.

MAIN TEXT

Rigorous regional interventions carried out to control and eliminate onchocerciasis in the past four decades in Africa have been effective in bringing the disease burden under control; it is currently not a public health problem in most endemic areas. Notably, transmission of the parasite is interrupted in some hyperendemic localities. Recently, there has been a policy shift from control to complete disease elimination by 2020 in selected countries and by 2025 in the majority of endemic African countries. The WHO has published guidelines for stopping mass drug administration (MDA) and verifying the interruption of transmission and elimination of human onchocerciasis. Therefore, countries have revised their plans, established a goal of disease elimination in line with an evidence based decision to stop MDA and verify elimination, and incorporated it into their NTDs national master plans. Nevertheless, challenges remain pertaining to the elimination of onchocerciasis in Africa. The challenge we review in this paper are: incomplete elimination mapping of all transmission zones, co-endemicity of onchocerciasis and loiasis, possible emergence of ivermectin resistance, uncoordinated cross-border elimination efforts, conflict and civil unrest, suboptimal program implementation, and technical and financial challenges. This paper also proposes recommendations to overcome the challenges and accelerate disease elimination. These are: a need for complete disease elimination mapping, a need for collaborative elimination activities between national programs, a need for a different drug distribution approach in conflict-affected areas, a need for routine monitoring and evaluation of MDA programs, a need for implementing alternative treatment strategies (ATSs) in areas with elimination anticipated beyond 2025, and a need for strong partnerships and continued funding.

CONCLUSIONS

National programs need to regularly monitor and evaluate the performance and progress of their interventions, while envisaging the complete elimination of onchocerciasis from their territory. Factors hindering the targeted goal of interruption of parasite transmission need to be identified and remedial actions should be taken. If possible and appropriate, ATSs need to be implemented to accelerate disease elimination by 2025.

Identification and characterization of the Onchocerca volvulus Excretory Secretory Product Ov28CRP, a putative GM2 activator protein

Onchocerca volvulus is the nematode pathogen responsible for human onchocerciasis also known as "River blindness", a neglected tropical disease that affects up to 18 million people worldwide. Helminths Excretory Secretory Products (ESPs) constitute a rich repertoire of molecules that can be exploited for host-parasite relationship, diagnosis and vaccine studies. Here, we report, using a range of molecular techniques including PCR, western blot, recombinant DNA technology, ELISA, high performance thin-layer chromatography and mass spectrometry that the 28 KDa cysteine-rich protein (Ov28CRP) is a reliable component of the O. volvulus ESPs to address the biology of this parasite. We showed that (1) Ov28CRP is a putative ganglioside GM2 Activator Protein (GM2AP) conserved in nematode; (2) OvGM2AP gene is transcriptionally activated in all investigated stages of the parasitic life cycle, including larval and adult stages; (3) The full-length OvGM2AP was detected in in-vitro O. volvulus ESPs of adult and larval stages; (4) the mass expressed and purified recombinant OvGM2AP purified from insect cell culture medium was found to be glycosylated at asparagine 173 and lacked N-terminal signal peptide sequence; (5) the recombinant OvGM2AP discriminated serum samples of infected and uninfected individuals; (6) OvGM2AP competitively inhibits MUG degradation by recombinant β-hexosaminidase A but not MUGS, and could not hydrolyze the GM2 to GM3; (7) humoral immune responses to the recombinant OvGM2AP revealed a negative correlation with ivermectin treatment. Altogether, our findings suggest for the first time that OvGM2AP is an antigenic molecule whose biochemical and immunological features are important to gain more insight into our understanding of host-parasite relationship, as well as its function in parasite development at large.

Insights Into Onchocerca volvulus Population Biology Through Multilocus Immunophenotyping

We have developed a serologically based immunophenotyping approach to study Onchocerca volvulus (Ov) population diversity. Using genomic sequence data and polymerase chain reaction-based genotyping, we identified nonsynonymous single-nucleotide polymorphisms (SNPs) in the genes of 16 major immunogenic Ov proteins: Ov-CHI-1/Ov-CHI-2, Ov16, Ov-FAR-1, Ov-CPI-1, Ov-B20, Ov-ASP-1, Ov-TMY-1, OvSOD1, OvGST1, Ov-CAL-1, M3/M4, Ov-RAL-1, Ov-RAL-2, Ov-ALT-1, Ov-FBA-1, and Ov-B8. We assessed the immunoreactivity of onchocerciasis patient sera (n = 152) from the Americas, West Africa, Central Africa, and East Africa against peptides derived from 10 of these proteins containing SNPs. Statistically significant variation in immunoreactivity among the regions was seen in SNP-containing peptides derived from 8 of 10 proteins tested: OVOC1192(1-15), OVOC9988(28-42), OVOC9225(320-334), OVOC7453(22-36), OVOC11517(14-28), OVOC3177(283-297), OVOC7911(594-608), and OVOC12628(174-188). Our data show that differences in immunoreactivity to variant antigenic peptides may be used to characterize Ov populations, thereby elucidating features of Ov population biology previously inaccessible because of the limited availability of parasite material.

Polymorphism in ABC transporter genes of Dirofilaria immitis

Dirofilaria immitis, a filarial nematode, causes dirofilariasis in dogs, cats and occasionally in humans. Prevention of the disease has been mainly by monthly use of the macrocyclic lactone (ML) endectocides during the mosquito transmission season. Recently, ML resistance has been confirmed in D. immitis and therefore, there is a need to find new classes of anthelmintics. One of the mechanisms associated with ML resistance in nematodes has been the possible role of ATP binding cassette (ABC) transporters in reducing drug concentrations at receptor sites. ABC transporters, mainly from sub-families B, C and G, may contribute to multidrug resistance (MDR) by active efflux of drugs out of the cell. Gene products of ABC transporters may thus serve as the targets for agents that may modulate susceptibility to drugs, by inhibiting drug transport. ABC transporters are believed to be involved in a variety of physiological functions critical to the parasite, such as sterol transport, and therefore may also serve as the target for drugs that can act as anthelmintics on their own. Knowledge of polymorphism in these ABC transporter genes in nematode parasites could provide useful information for the process of drug design. We have identified 15 ABC transporter genes from sub-families A, B, C and G, in D. immitis, by comparative genomic approaches and analyzed them for polymorphism. Whole genome sequencing data from four ML susceptible (SUS) and four loss of efficacy (LOE) pooled populations were used for single nucleotide polymorphism (SNP) genotyping. Out of 231 SNPs identified in those 15 ABC transporter genes, 89 and 75 of them were specific to the SUS or LOE populations, respectively. A few of the SNPs identified may affect gene expression, protein function, substrate specificity or resistance development and may be useful for transporter inhibitor/anthelmintic drug design, or in order to anticipate resistance development.

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In the D. immitis genome, all ABC-A, -B, -C and -G transporter genes were identified.

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Within 15 ABC transporter genes identified in D. immitis, 231 SNP loci were found.

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Four exonic SNPs caused changes in predicted secondary structure of ABC proteins.

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D. immitis populations have low genetic variability among ABC transporter genes.

Quantification of resistant alleles in the β-tubulin gene of field strains of gastrointestinal nematodes and their relation with the faecal egg count reduction test

BACKGROUND

Benzimidazole (BZ) resistance in gastrointestinal nematodes is associated with a single nucleotide polymorphism (SNP) at codons 167, 198 and 200 in the isotype 1 of beta-tubulin gene although in some species these SNPs have also been associated with resistance to macrocyclic lactones. In the present study we compared the levels of resistance in Teladorsagia circumcincta and Trichostrongylus colubriformis by means of the faecal egg reduction test (FECRT) and the percentage of resistant alleles obtained after pyrosequencing. The study was conducted in 10 naturally infected sheep flocks. Each flock was divided into three groups: i) group treated with albendazole (ABZ); ii) group treated with ivermectin (IVM); iii) untreated group. The number of eggs excreted per gram of faeces was estimated at day 0 and 14 post-treatment.

RESULTS

Resistance to ABZ was observed in 12.5% (1/8) of the flocks and to IVM in 44.4% (4/9) of them. One flock was resistant to both drugs according to FECRT. Coprocultures were performed at the same dates to collect L3 for DNA extraction from pooled larvae and to determine the resistant allele frequencies by pyrosequencing analysis. In T. circumcincta, SNPs were not found at any of the three codons before treatment; after the administration of ABZ, SNPs were present only in two different flocks, one of them with a frequency of 23.8% at SNP 167, and the other 13.2% % at SNP 198. In relation to T. colubriformis, we found the SNP200 before treatment in 33.3% (3/9) of the flocks with values between 48.5 and 87.8%. After treatment with ABZ and IVM, the prevalence of this SNP increased to 75 and 100% of the flocks, with a mean frequency of 95.1% and 82.6%, respectively.

CONCLUSION

The frequencies observed for SNP200 in T. colubriformis indicate that the presence of resistance is more common than revealed by the FECRT.

The Immunomodulatory Role of Adjuvants in Vaccines Formulated with the Recombinant Antigens Ov-103 and Ov-RAL-2 against Onchocerca volvulus in Mice

BACKGROUND

In some regions in Africa, elimination of onchocerciasis may be possible with mass drug administration, although there is concern based on several factors that onchocerciasis cannot be eliminated solely through this approach. A vaccine against Onchocerca volvulus would provide a critical tool for the ultimate elimination of this infection. Previous studies have demonstrated that immunization of mice with Ov-103 and Ov-RAL-2, when formulated with alum, induced protective immunity. It was hypothesized that the levels of protective immunity induced with the two recombinant antigens formulated with alum would be improved by formulation with other adjuvants known to enhance different types of antigen-specific immune responses.

METHODOLOGY/ PRINCIPAL FINDINGS

Immunizing mice with Ov-103 and Ov-RAL-2 in conjunction with alum, Advax 2 and MF59 induced significant levels of larval killing and host protection. The immune response was biased towards Th2 with all three of the adjuvants, with IgG1 the dominant antibody. Improved larval killing and host protection was observed in mice immunized with co-administered Ov-103 and Ov-RAL-2 in conjunction with each of the three adjuvants as compared to single immunizations. Antigen-specific antibody titers were significantly increased in mice immunized concurrently with the two antigens. Based on chemokine levels, it appears that neutrophils and eosinophils participate in the protective immune response induced by Ov-103, and macrophages and neutrophils participate in immunity induced by Ov-RAL-2.

CONCLUSIONS/SIGNIFICANCE

The mechanism of protective immunity induced by Ov-103 and Ov-RAL-2, with the adjuvants alum, Advax 2 and MF59, appears to be multifactorial with roles for cytokines, chemokines, antibody and specific effector cells. The vaccines developed in this study have the potential of reducing the morbidity associated with onchocerciasis in humans.

Vaccination of Gerbils with Bm-103 and Bm-RAL-2 Concurrently or as a Fusion Protein Confers Consistent and Improved Protection against Brugia malayi Infection

Background

The Brugia malayi Bm-103 and Bm-RAL-2 proteins are orthologous to Onchocerca volvulus Ov-103 and Ov-RAL-2, and which were selected as the best candidates for the development of an O. volvulus vaccine. The B. malayi gerbil model was used to confirm the efficacy of these Ov vaccine candidates on adult worms and to determine whether their combination is more efficacious.

Methodology and Principle Findings

Vaccine efficacy of recombinant Bm-103 and Bm-RAL-2 administered individually, concurrently or as a fusion protein were tested in gerbils using alum as adjuvant. Vaccination with Bm-103 resulted in worm reductions of 39%, 34% and 22% on 42, 120 and 150 days post infection (dpi), respectively, and vaccination with Bm-RAL-2 resulted in worm reductions of 42%, 22% and 46% on 42, 120 and 150 dpi, respectively. Vaccination with a fusion protein comprised of Bm-103 and Bm-RAL-2 resulted in improved efficacy with significant reduction of worm burden of 51% and 49% at 90 dpi, as did the concurrent vaccination with Bm-103 and Bm-RAL-2, with worm reduction of 61% and 56% at 90 dpi. Vaccination with Bm-103 and Bm-RAL-2 as a fusion protein or concurrently not only induced a significant worm reduction of 61% and 42%, respectively, at 150 dpi, but also significantly reduced the fecundity of female worms as determined by embryograms. Elevated levels of antigen-specific IgG were observed in all vaccinated gerbils. Serum from gerbils vaccinated with Bm-103 and Bm-RAL-2 individually, concurrently or as a fusion protein killed third stage larvae in vitro when combined with peritoneal exudate cells.

Conclusion

Although vaccination with Bm-103 and Bm-RAL-2 individually conferred protection against B. malayi infection in gerbils, a more consistent and enhanced protection was induced by vaccination with Bm-103 and Bm-RAL-2 fusion protein and when they were used concurrently. Further characterization and optimization of these filarial vaccines are warranted.

Onchocerciasis and Lymphatic filariasis (LF) are debilitating neglected tropical diseases (NTDs). Practical challenges in implementation of mass drug administration (MDA) such as prolonged treatment regime requirements and the possible emergence of drug resistance will likely impede the elimination of these NTDs. Hence, the availability of an efficacious prophylactic vaccine would be an invaluable tool. The objective of the present studies was to use the B. malayi-gerbil model of filariasis as a surrogate system to test the efficacy of filarial molecules as vaccine targets for an onchocerciasis vaccine. The vaccine efficacy of Onchocerca volvulus recombinant proteins Ov-RAL-2 and Ov-103 was recently demonstrated using a mouse diffusion chamber model. In this communication, we provide encouraging results on the vaccine efficacy of Bm-RAL-2 and Bm-103, individually or in combination. Our data demonstrate that vaccination with Bm-RAL-2 and Bm-103 concurrently and as a fusion protein confers not only a consistent and significant protection against B. malayi infection in gerbils, but also reduces the fecundity of female worms as demonstrated in embryogram analyses. Our results support the contention that Ov-RAL-2 and Ov-103 are excellent onchocerciasis vaccine candidates and that further investigations leading to their development as a vaccine are warranted.

Does Increasing Treatment Frequency Address Suboptimal Responses to Ivermectin for the Control and Elimination of River Blindness?

The first 3 years of biannual ivermectin distribution in Ghana have substantially reduced Onchocerca volvulus infection levels in 10 sentinel communities, but longitudinal analysis indicates that some communities are still consistently responding suboptimally to treatment, with implications for onchocerciasis elimination.

Background. Several African countries have adopted a biannual ivermectin distribution strategy in some foci to control and eliminate onchocerciasis. In 2010, the Ghana Health Service started biannual distribution to combat transmission hotspots and suboptimal responses to treatment. We assessed the epidemiological impact of the first 3 years of this strategy and quantified responses to ivermectin over 2 consecutive rounds of treatment in 10 sentinel communities.

Methods. We evaluated Onchocerca volvulus community microfilarial intensity and prevalence in persons aged ≥20 years before the first, second, and fifth (or sixth) biannual treatment rounds using skin snip data from 956 participants. We used longitudinal regression modeling to estimate rates of microfilarial repopulation of the skin in a cohort of 217 participants who were followed up over the first 2 rounds of biannual treatment.

Results. Biannual treatment has had a positive impact, with substantial reductions in infection intensity after 4 or 5 rounds in most communities. We identified 3 communities—all having been previously recognized as responding suboptimally to ivermectin—with statistically significantly high microfilarial repopulation rates. We did not find any clear association between microfilarial repopulation rate and the number of years of prior intervention, coverage, or the community level of infection.

Conclusions. The strategy of biannual ivermectin treatment in Ghana has reduced O. volvulus microfilarial intensity and prevalence, but suboptimal responses to treatment remain evident in a number of previously and consistently implicated communities. Whether increasing the frequency of treatment will be sufficient to meet the World Health Organization's 2020 elimination goals remains uncertain.

Macrocyclic lactones and their relationship to the SNPs related to benzimidazole resistance

Haemonchus contortus is an abomasal nematode of ruminants that is widely present across the world. Its ability to cause death of infected animals and rapidly develop anthelmintic resistance makes it a dangerous pathogen. Ivermectin (IVM) and moxidectin (MOX) are macrocyclic lactones (MLs). They have been successfully used to treat parasitic nematodes over the last three decades. A genetic association between IVM selection and single nucleotide polymorphisms (SNPs) on the β-tubulin isotype 1 gene was reported in H. contortus. These SNPs result in replacing phenylalanine (F, TTC) with tyrosine (Y, TAC) at position 167 or 200 on the β-tubulin protein. Recently we reported a direct interaction of IVM with α- and β-tubulin. It had been hypothesized that the SNPs (F167Y and F200Y) may change tubulin dynamics and directly affect IVM binding. The goal of the current study was to observe the effects of SNPs (F167Y and F200Y) on tubulin polymerization and IVM binding. It was also of interest to evaluate the differences between IVM and MOX on tubulin polymerization. We conclude that the SNPs cause no difference in the polymerization of wild and mutant tubulins. Furthermore, neither of the SNPs reduced IVM binding. Varying results were obtained in the degree of polymerization of parasitic and mammalian tubulin for IVM and MOX, i.e., the extent of polymerization was greater for IVM compared with MOX, for H. contortus tubulin, and vice versa for mammalian tubulin. Molecular modeling showed that IVM and MOX docked into the taxane binding pocket of both mammalian and parasitic wild type and mutant tubulins. However the binding was stronger for mammalian tubulin as compared to parasitic tubulin.

Host ABC transporter proteins may influence the efficacy of ivermectin and possibly have broader implications for the development of resistance in parasitic nematodes

ABC transporter proteins function to extrude compounds from the cell. These proteins present an obstacle for treatment and for overcoming drug resistance as they are expressed by both host and parasite, and function similarly. The contribution of host ABC proteins to drug efficacy was examined using ivermectin and a Brugia malayi model system. Parallel in vitro and in vivo experiments were conducted using equal concentrations of ivermectin. The motilities and fecundity of B. malayi exposed to ivermectin in vitro were significantly lower than those treated in vivo. The higher motilities were correlated with low concentrations of ivermectin in worms extracted from treated hosts. The expression of ABC proteins was significantly higher in worms treated in vitro compared to those treated in vivo as well as in gerbils treated with ivermectin than in non-treated controls. The results suggest that host ABC transporters may influence the efficacy of ivermectin.

Doxycycline Leads to Sterility and Enhanced Killing of Female Onchocerca volvulus Worms in an Area With Persistent Microfilaridermia After Repeated Ivermectin Treatment: A Randomized, Placebo-Controlled, Double-Blind Trial

Sub-optimal responses to ivermectin (IVM) have emerged. Targeting the Onchocerca volvulus Wolbachia endosymbionts with doxycycline is effective in clearing microfilariae in onchocerciasis patients with persistent microfilaridermia and in enhanced killing of adult worms after repeated standard IVM treatment.

Background. Ivermectin (IVM) has been the drug of choice for the treatment of onchocerciasis. However, there have been reports of persistent microfilaridermia in individuals from an endemic area in Ghana after many rounds of IVM, raising concerns of suboptimal response or even the emergence of drug resistance. Because it is considered risky to continue relying only on IVM to combat this phenomenon, we assessed the effect of targeting the Onchocerca volvulus Wolbachia endosymbionts with doxycycline for these individuals with suboptimal response.

Methods. One hundred sixty-seven patients, most of them with multiple rounds of IVM, were recruited in areas with IVM suboptimal response and treated with 100 mg/day doxycycline for 6 weeks. Three and 12 months after doxycycline treatment, patients took part in standard IVM treatment.

Results. At 20 months after treatment, 80% of living female worms from the placebo group were Wolbachia positive, whereas only 5.1% in the doxycycline-treated group contained bacteria. Consistent with interruption of embryogenesis, none of the nodules removed from doxycycline-treated patients contained microfilariae, and 97% of those patients were without microfilaridermia, in contrast to placebo patients who remained at pretreatment levels (P < .001). Moreover, a significantly enhanced number of dead worms were observed after doxycycline.

Conclusions. Targeting the Wolbachia in O. volvulus is effective in clearing microfilariae in the skin of onchocerciasis patients with persistent microfilaridermia and in enhanced killing of adult worms after repeated standard IVM treatment. Strategies can now be developed that include doxycycline to control onchocerciasis in areas where infections persist despite the frequent use of IVM.

Clinical Trials Registration. ISRCTN 66649839.

Vaccines to combat river blindness: expression, selection and formulation of vaccines against infection with Onchocerca volvulus in a mouse model

Human onchocerciasis is a neglected tropical disease caused by Onchocerca volvulus and an important cause of blindness and chronic disability in the developing world. Although mass drug administration of ivermectin has had a profound effect on control of the disease, additional tools are critically needed including the need for a vaccine against onchocerciasis. The objectives of the present study were to: (i) select antigens with known vaccine pedigrees as components of a vaccine; (ii) produce the selected vaccine antigens under controlled conditions, using two expression systems and in one laboratory and (iii) evaluate their vaccine efficacy using a single immunisation protocol in mice. In addition, we tested the hypothesis that joining protective antigens as a fusion protein or in combination, into a multivalent vaccine, would improve the ability of the vaccine to induce protective immunity. Out of eight vaccine candidates tested in this study, Ov-103, Ov-RAL-2 and Ov-CPI-2M were shown to reproducibly induce protective immunity when administered individually, as fusion proteins or in combination. Although there was no increase in the level of protective immunity induced by combining the antigens into one vaccine, these antigens remain strong candidates for inclusion in a vaccine to control onchocerciasis in humans.

The contribution of mass drug administration to global health: past, present and future

Mass drug administration (MDA) is a means of delivering safe and inexpensive essential medicines based on the principles of preventive chemotherapy, where populations or sub-populations are offered treatment without individual diagnosis. High-coverage MDA in endemic areas aims to prevent and alleviate symptoms and morbidity on the one hand and can reduce transmission on the other, together improving global health. MDA is the recommended strategy of the World Health Organisation to control or eliminate several neglected tropical diseases (NTDs). More than 700 million people now receive these essential NTD medicines annually. The combined cost of integrated NTD MDA has been calculated to be in the order of $0.50 per person per year. Activities have recently been expanded due, in part, to the proposed attempt to eliminate certain NTDs in the coming two decades. More than 1.9 billion people need to receive MDA annually across several years if these targets are to be met. Such extensive coverage will require additional avenues of financial support, expanded monitoring and evaluation focusing on impact and drug efficacy, as well as new diagnostic tools and social science strategies to encourage adherence. MDA is a means to help reduce the burden of disease, and hence poverty, among the poorest sector of populations. It has already made significant improvements to global health and productivity and has the potential for further successes, particularly where incorporated into sanitation and education programmes. However logistical, financial and biological challenges remain.

Reproductive status of Onchocerca volvulus after ivermectin treatment in an ivermectin-naïve and a frequently treated population from Cameroon

BACKGROUND

For two decades, onchocerciasis control has been based on mass treatment with ivermectin (IVM), repeated annually or six-monthly. This drug kills Onchocerca volvulus microfilariae (mf) present in the skin and the eyes (microfilaricidal effect) and prevents for 3-4 months the release of new mf by adult female worms (embryostatic effect). In some Ghanaian communities, the long-term use of IVM was associated with a more rapid than expected skin repopulation by mf after treatment. Here, we assessed whether the embryostatic effect of IVM on O. volvulus has been altered following frequent treatment in Cameroonian patients.

METHODOLOGY

Onchocercal nodules were surgically removed just before (D0) and 80 days (D80) after a standard dose of IVM in two cohorts with different treatment histories: a group who had received repeated doses of IVM over 13 years, and a control group with no history of large-scale treatments. Excised nodules were digested with collagenase to isolate adult worms. Embryograms were prepared with females for the evaluation of their reproductive capacities.

PRINCIPAL FINDINGS

Oocyte production was not affected by IVM. The mean number of intermediate embryos (morulae and coiled mf) decreased similarly in the two groups between D0 and D80. In contrast, an accumulation of stretched mf, either viable or degenerating, was observed at D80. However, it was observed that the increase in number of degenerating mf between D0 and D80 was much lower in the frequently treated group than in the control one (Incidence Rate Ratio: 0.25; 95% CI: 0.10-0.63; p = 0.003), which may indicate a reduced sequestration of mf in the worms from the frequently treated group.

CONCLUSION/SIGNIFICANCE

IVM still had an embryostatic effect on O. volvulus, but the effect was reduced in the frequently treated cohort compared with the control population.

Haemonchus contortus as a paradigm and model to study anthelmintic drug resistance

Anthelmintic resistance is a major problem for the control livestock parasites and a potential threat to the sustainability of community-wide treatment programmes being used to control human parasites in the developing world. Anthelmintic resistance is essentially a complex quantitative trait in which multiple mutations contribute to the resistance phenotype in an additive manner. Consequently, a combination of forward genetic and genomic approaches are needed to identify the causal mutations and quantify their contribution to the resistance phenotype. Therefore, there is a need to develop genetic and genomic approaches for key parasite species identified as relevant models. Haemonchus contortus, a gastro-intestinal parasite of sheep, has shown a remarkable propensity to develop resistance to all the drugs used in its control. Partly because of this, and partly because of its experimental amenability, research on this parasite has contributed more than any other to our understanding of anthelmintic resistance. H. contortus offers a variety of advantages as an experimental system including the ability to undertake genetic crosses; a prerequisite for genetic mapping. This review will discuss the current progress on developing H. contortus as a model system in which to study anthelmintic resistance.

New strategies to combat filariasis

Two of the major filarial infections, lymphatic filariasis (LF) and onchocerciasis, affect 150 million people, while 1 billion living in endemic areas are at risk of infection. Public health programs to control these infections have successfully existed for years and have evolved from activities driven by the WHO into global programs with public-private partnerships. Currently, these programs use yearly mass application of drugs that mainly kill the larval stages (the microfilariae), with the aim of preventing uptake by the transmitting insect vectors and thus, to block transmission and reduce the infections to such levels that in 15-30 years from now, they will no longer pose a public health problem. While the programs have been very successful in general, there are drawbacks such as coverage being too low within the population, reappearance of infection by migration of infected people into controlled areas, targeting of a stage (the microfilaria) that does not induce pathology in LF and thus lowers compliance, and the potential development of drug resistance, first indications of which have been clearly observed in onchocerciasis. In addition, even without drawbacks, program scopes are not the eradication of filarial infections, which is, however, an ultimate goal of control activities. There is therefore an unequivocal call for the development of higher efficient, complementary chemotherapeutical approaches that lead to a long-lasting reduction of the pathology-inducing worm stages; that is, microfilariae in onchocerciasis and adult worms in LF, or to a macrofilaricidal effect. The recent discovery that depletion of Wolbachia endosymbionts by tetracycline antibiotics leads to long-lasting sterility of adult female worms in onchocerciasis and a macrofilaricidal effect in LF fulfils these requirements. Successful regimens have already been published and agreed upon for use by expert panels. While these regimens are still too long for mass application, the antiwolbachial chemotherapy can currently be applied in the form of a suitable doxycycline regimen for 6 weeks for the treatment of individuals, and exploited in the future for the development of new drugs suitable for mass application. In addition, first data suggest that Wolbachia may also be major mediators of lymphangiogenesis and that their depletion is associated with reduction of lymph vessel-specific vascular endothelial growth factors and reduced lymph vessel size.

Extracts of Euphorbia hirta Linn. (Euphorbiaceae) and Rauvolfia vomitoria Afzel (Apocynaceae) demonstrate activities against Onchocerca volvulus microfilariae in vitro

Background

Onchocerciasis transmitted by Onchocerca volvulus is the second major cause of blindness in the world and it impacts negatively on the socio-economic development of the communities affected. Currently, ivermectin, a microfilaricidal drug is the only drug recommended for treating this disease. There have been speculations, of late, concerning O. volvulus resistance to ivermectin. Owing to this, it has become imperative to search for new drugs. World-wide, ethnomedicines including extracts of Euphorbia hirta and Rauvolfia vomitoria are used for treating various diseases, both infectious and non-infectious.

Method

In this study extracts of the two plants were evaluated in vitro in order to determine their effect against O. volvulus microfilariae. The toxicity of the E. hirta extracts on monkey kidney cell (LLCMK2) lines was also determined.

Results

The investigations showed that extracts of both plants immobilised microfilariae at different levels in vitro and, therefore, possess antifilarial properties. It was found that all the E. hirta extracts with the exception of the hexane extracts were more effective than those of R. vomitoria. Among the extracts of E. hirta the ethyl acetate fraction was most effective, and comparable to that of dimethanesulphonate salt but higher than that of Melarsoprol (Mel B). However, the crude ethanolic extract of E. hirta was found to be the least toxic to the LLCMK2 compared to the fractionated forms.

Conclusions

Extracts from both plants possess antifilarial properties; however, the crude extract of E. hirta was found to be least toxic to LLCMK2.

Ivermectin resistance and overview of the Consortium for Anthelmintic Resistance SNPs

Ivermectin (IVM) has transformed nematode parasite control in veterinary medicine and the control of some nematode infections in humans, such as onchocerciasais, lymphatic filariasis in Africa and strongyloidiasis. Unfortunately, IVM resistance is now a serious problem for parasite control in livestock and there is a concern about resistance development and spread in nematode parasites of humans. IVM is believed to act by opening glutamate-gated chloride channels and GABA-gated channels in invertebrate neurons or muscle cells, leading to hyperpolarisation of the cells and to an inhibitory paralysis. However, in the filarial nematodes, it is not altogether clear that the effect of IVM is confined to these actions or even whether these are the most important. Alterations in some ligand-gated ion channel (LGIC) receptor subunits may play a role in the mechanisms of IVM resistance in some nematodes, but the evidence that changes in LGICs are the most important cause of IVM resistance in nematodes is far from clear. What is evident is that IVM is an excellent substrate for some ATP-binding cassette transporters, IVM selects for changes in expression levels of ABC transporters, such as P-glycoproteins, and that altered levels of some ABC transporters contribute to IVM resistance. In addition, there is growing evidence that IVM selects on β-tubulin, at least in some nematodes. Based on these various mechanisms, which contribute to IVM resistance, it may become possible to develop panels of molecular markers for IVM resistance in different nematode parasites. In order to stimulate the development of such markers, an international Consortium for Anthelmintic Resistance SNPs (CARS) has been developed to help coordinate marker development, advance our knowledge of helminth biology and possibly assist with the development of new anthelmintic molecules.

Dynamics of Onchocerca volvulus microfilarial densities after ivermectin treatment in an ivermectin-naïve and a multiply treated population from Cameroon

Background/Objective

Ivermectin has been the keystone of onchocerciasis control for the last 25 years. Sub-optimal responses to the drug have been reported in Ghanaian communities under long-term treatment. We assessed, in two Cameroonian foci, whether the microfilaricidal and/or embryostatic effects of ivermectin on Onchocerca volvulus have been altered after several years of drug pressure.

Methods

We compared the dynamics of O. volvulus skin microfilarial densities after ivermectin treatment in two cohorts with contrasting exposure to this drug: one received repeated treatment for 13 years whereas the other had no history of large-scale treatments (referred to as controls). Microfilarial densities were assessed 15, 80 and 180 days after ivermectin in 122 multiply treated and 127 ivermectin-naïve individuals. Comparisons were adjusted for individual factors related to microfilarial density: age and number of nodules.

Findings

Two weeks post ivermectin, microfilarial density dropped equally (98% reduction) in the ivermectin-naïve and multiply treated groups. Between 15 and 180 days post ivermectin, the proportion of individuals with skin microfilariae doubled (from 30.8% to 67.8%) in controls and quadrupled (from 19.8% to 76.9%) in multiply treated individuals but the mean densities remained low in both sites. In fact, between 15 and 80 days, the repopulation rate was significantly higher in the multiply treated individuals than in the controls but no such difference was demonstrated when extending the follow-up to 180 days. The repopulation rate by microfilariae was associated with host factors: negatively with age and positively with the number of nodules.

Conclusion

These observations may indicate that the worms from the multi-treated area recover mf productivity earlier but would be less productive than the worms from the ivermectin-naïve area between 80 and 180 days after ivermectin. Moreover, they do not support the operation of a strong cumulative effect of repeated treatments on the fecundity of female worms as previously described.

Millions of Africans and thousands of Latin Americans are infected with Onchocerca volvulus, the filarial worm responsible for onchocerciasis. Since the mid-1990s, control programs rely on annual or six-monthly community treatments with the only safe drug available, ivermectin. If sustained for another 10–15 years, this strategy could lead to elimination of onchocerciasis. Unfortunately, there have been reports of low response to the drug in Ghanaian communities under long-term treatment. Here, we compared the response of O. volvulus to ivermectin between a Cameroonian population repeatedly treated and an ivermectin-naïve population. Skin parasite density was assessed before and 15, 80 and 180 days after treatment. Parasite density dropped equally in the two groups (∼98% reduction) by 15 days, in accordance with the expected effect of ivermectin at this time point. In the multi-treated subjects, the repopulation rate of the skin by microfilariae was higher than in the controls from 15 to 80 days after treatment but the microfilarial levels reached similar levels six months after treatment in the two groups. Thirteen years of large-scale treatments may have selected worms less sensitive to the drug. In addition, those treatments had little if any cumulative effect on skin parasite repopulation after an additional treatment.

The role of several ABC transporter genes in ivermectin resistance in Caenorhabditis elegans

The functions of nine ATP-binding cassette (ABC) transporter genes, mrp-1, mrp-4, mrp-6, pgp-2, pgp-3, pgp-4, pgp-5, haf-2 and haf-9, in an ivermectin (IVM) resistant strain of Caenorhabditis elegans were screened by comparing transcription levels between the resistant (IVR10) and wild-type (Bristol N2) strains, and by measuring the effects of RNA interference (RNAi) on the IVM resistant strain, on motility, pharyngeal pumping, egg production and death in the presence or varying concentrations of IVM (0-20 ng/ml). mRNA levels of mrp-1, 2, 4, 5, 6, 7, pgp-1, 2, 4, 12, 14, haf-1, 2 and 3 were significantly increased in IVR10 compared with the N2 strain. At 15 or 20 ng/ml IVM, down regulation of mrp-1, pgp-4, haf-2 and haf-9 significantly increased the effect of IVM to reduce egg production. At low to moderate IVM concentrations, down regulation of mrp-1 and haf-2 reduced the motility of C. elegans. However, at high IVM concentrations motility was increased by down regulation of transcription of pgp-3, pgp-4 and haf-9. Down regulation of expression of mrp-1, pgp-2 and pgp-5 resulted in reduced pharyngeal pumping in the presence of varying concentrations of IVM, while down regulation of mrp-6 and haf-2 increased pharyngeal pumping of the resistant strain irrespective of the IVM concentration used. Although the IVR10 strain was markedly resistant to IVM, compared with the unselected N2 strain, IVM led to the death of the C. elegans in a concentration dependent manner. However, differences in the IVM induced death rate, following RNAi, were not significantly different from the IVR10 strain without RNAi. The study shows that different ABC transporter genes may play a role in modulating the effects of IVM on pharyngeal pumping, motility and egg production, with down regulation of mrp-1 and haf-2 perhaps having the greatest effects. However, down regulation of expression of no individual ABC transporter gene profoundly affected the effect of IVM on mortality in the IVR10 strain. This suggests that some of these ABC transporter genes and their products may play a role in modulating the effects of IVM, but are not, individually, the critical gene responsible for IVM resistance. This study provides a model that may help to understand drug resistance in parasitic nematodes.

Single nucleotide polymorphisms in β-tubulin selected in Onchocerca volvulus following repeated ivermectin treatment: possible indication of resistance selection

The control of onchocerciasis or river blindness by mass treatment of the population with ivermectin (IVM) has been a great success until now, so that in certain foci its elimination has become feasible. However, after more than 20 years of repeated IVM mass treatment, the disease still persists in many endemic countries. Sub-optimal responses and genetic changes have been reported in Onchocerca volvulus populations under high IVM pressure but more work is needed to determine whether resistance is developing. The situation needs to be urgently clarified to preserve the achievements of onchocerciasis control programs. In this study, O. volvulus adult worms were collected from the same individuals, before IVM exposure and following three years of annual or three-monthly treatments at 150 μg/kg or 800 μg/kg. Four single nucleotide polymorphisms (SNPs) occurring in the β-tubulin gene of these parasites were investigated. We found changes in genotype frequencies in O. volvulus β-tubulin gene associated with IVM treatments. The SNP at position 1545 (A/G) showed a significant increase in frequency of the less common nucleotide in the female worms following treatment. After 13 three-monthly treatments, female worm homozygotes with the less common genotype, prior to treatment, increased in frequency. The selected homozygotes, as well as heterozygotes, appeared to be less fertile (without or with very few embryonic stages in their uteri) than the wild-type homozygotes. These results provide additional evidence for genetic selection and strengthen the warning that selection for IVM resistance may be occurring in some O. volvulus populations.

Genotypic analysis of β-tubulin in Onchocerca volvulus from communities and individuals showing poor parasitological response to ivermectin treatment

Ivermectin (IVM) has been in operational use for the control of onchocerciasis for two decades and remains the only drug of choice. To investigate the parasitological responses and genetic profile of Onchocerca volvulus, we carried out a 21 month epidemiological study to determine the response of the parasite to IVM in 10 Ghanaian endemic communities. Onchocerca nodules were surgically removed from patients in three IVM response categories (good, intermediate and poor) and one IVM naïve community. DNA from adult worms was analyzed to determine any association between genotype and IVM response phenotypic. Embryogramme analysis showed significantly higher reproductive activity in worms from poor response communities, which had up to 41% of females with live stretched microfilaria (mf) in utero, despite IVM treatment, compared with good response communities, which had no intra-uterine stretched mf. β-tubulin isotype 1 gene has been shown to be linked to IVM selection in O. volvulus and also known to be associated with IVM resistance in veterinary nematodes. We have genotyped the full length genomic DNA sequence of the β-tubulin gene from 127 adult worms obtained from the four community categories. We found SNPs at 24 sites over the entire 3696 bp. Eight of the SNPs occurred at significantly higher (p < 0.05) frequencies in the poor response communities compared with the good response communities and the IVM naïve community. Phenotypic and genotypic analyses show that IVM resistance has been selected and the genotype (1183GG/1188CC/1308TT/1545GG) was strongly associated with the resistance phenotype. Since the region in the β-tubulin gene where these four SNPs occur is within 362 bp, it is feasible to develop a genetic marker for the early detection of IVM resistance.

Evidence for macrocyclic lactone anthelmintic resistance in Dirofilaria immitis

Reports of loss-of-efficacy (LOE) events in dogs infected with Dirofilaria immitis despite adherence to accepted prophylaxis regimens with a macrocyclic lactone anthelmintic are attracting considerable attention. It is crucially important to distinguish among several possible causes for these LOE reports, one of which is the evolution of resistance to these drugs in heartworms. We review here recent evidence at the molecular level that supports the hypothesis that parasites derived from LOE cases have experienced a strong selection event and that these populations are characterized by very high frequencies of single-nucleotide polymorphisms in a D. immitis gene encoding a P-glycoprotein transporter, comprised of homozygous guanosine residues at 2 locations ("GG-GG" genotype). Furthermore, an infected dog adopted to Canada from the southern United States harbored a microfilarial population that was insensitive to very high doses of macrocyclic lactones and was characterized by a high frequency of the GG-GG genotype associated with LOE cases. We propose that this case be defined as a drug-resistant heartworm infection and suggest that a simple assay for the existence of resistant parasites is a 7-day microfilariae suppression test, which can be performed in a veterinary clinic as part of an effort to document the geographic distribution of this phenotype.

P-glycoproteins and other multidrug resistance transporters in the pharmacology of anthelmintics: Prospects for reversing transport-dependent anthelmintic resistance

Parasitic helminths cause significant disease in animals and humans. In the absence of alternative treatments, anthelmintics remain the principal agents for their control. Resistance extends to the most important class of anthelmintics, the macrocyclic lactone endectocides (MLs), such as ivermectin, and presents serious problems for the livestock industries and threatens to severely limit current parasite control strategies in humans. Understanding drug resistance is important for optimizing and monitoring control, and reducing further selection for resistance. Multidrug resistance (MDR) ABC transporters have been implicated in ML resistance and contribute to resistance to a number of other anthelmintics. MDR transporters, such as P-glycoproteins, are essential for many cellular processes that require the transport of substrates across cell membranes. Being overexpressed in response to chemotherapy in tumour cells and to ML-based treatment in nematodes, they lead to therapy failure by decreasing drug concentration at the target. Several anthelmintics are inhibitors of these efflux pumps and appropriate combinations can result in higher treatment efficacy against parasites and reversal of resistance. However, this needs to be balanced against possible increased toxicity to the host, or the components of the combination selecting on the same genes involved in the resistance. Increased efficacy could result from modifying anthelmintic pharmacokinetics in the host or by blocking parasite transporters involved in resistance. Combination of anthelmintics can be beneficial for delaying selection for resistance. However, it should be based on knowledge of resistance mechanisms and not simply on mode of action classes, and is best started before resistance has been selected to any member of the combination. Increasing knowledge of the MDR transporters involved in anthelmintic resistance in helminths will play an important role in allowing for the identification of markers to monitor the spread of resistance and to evaluate new tools and management practices aimed at delaying its spread.

Individual host factors associated with Onchocerca volvulus microfilarial densities 15, 80 and 180 days after a first dose of ivermectin

Reduction in Onchocerca volvulus skin microfilarial densities after treatment with ivermectin shows wide between-host variation. Data from two separate studies conducted in Cameroon on onchocerciasis patients treated for the first time with ivermectin were analyzed to identify host factors associated with microfilarial density at different time-points after treatment. In one site (Nkam valley), the dataset included 103 adult males for whom age, number of palpable onchocercal nodules and microfilarial densities on D0 (pre-treatment), D15, D80 and D180 were available. In the other site (Vina valley), analyses were conducted on 965 individuals of both sexes aged 5 years and over; in this dataset, available information included age, gender, exact dose of ivermectin received, onchocerciasis endemicity level in the village of residence and microfilarial densities on D0 and D180. Negative binomial regression models of microfilarial density at the different intervals post-treatment were fitted, using maximum likelihood, with the available independent variables. Gender and age were found to be associated with microfilarial density on D180. The initial microfilarial density influenced post-treatment densities at all the time-points. All other things being equal, microfilarial densities on D180 were higher in individuals harbouring a higher number of nodules or living in communities with high endemicity levels. This study demonstrates that O. volvulus microfilarial density measured after a first treatment with ivermectin, and thus probably the rate of skin repopulation by microfilariae (mf) varies according to several host factors. Should such factors also influence ivermectin efficacy after repeated treatment, then they should be taken into account to determine whether sub-optimal responses to treatment reported from various areas in Africa are actually due to parasite-related factors, particularly to the emergence of resistant populations.

The role of Brugia malayi ATP-binding cassette (ABC) transporters in potentiating drug sensitivity

ATP binding cassette (ABC) systems are a diverse group of proteins that have been identified in every organism, from bacteria to humans. Analysis of nematode genomes indicates that the number and arrangement of ABC systems are similar to other organisms, with the majority being ABC transporters. There are few functional studies of ABC transporters in parasitic nematodes; most reports have been on their identification or use as genetic markers to monitor drug resistance. In eukaryotes, some ABC transporters function in tissue defense by actively removing drugs, thus preventing their accumulation. The overexpression of ABC transporters that function as efflux pumps, such as P-glycoprotein (PGP) and the multidrug resistance associated protein (MRP) are known to confer resistance. Drug sensitivity can be restored by administration of PGP interfering or MDR reversal agents. The objective of this study was to determine if ABC systems in filarioid nematodes function similarly to those of other organisms. The relative expression of 33 ABC systems identified in Brugia malayi was quantified following exogenous exposure to the commonly used drug ivermectin (IVM). Following exposure of adults and microfilariae to IVM, there was a significant increase in the transcriptional profiles of a number of ABC systems, mostly within the PGP and MRP subgroups. Coadministration of PGP-interfering and MDR-reversal agents with IVM potentiated sensitivity to the drug in adults and microfilariae. The results suggest that B. malayi ABC transporters function similarly to those in other organisms and are a factor in determining drug sensitivity.

Phenotypic evidence of emerging ivermectin resistance in Onchocerca volvulus

Background

Ivermectin (IVM) has been used in Ghana for over two decades for onchocerciasis control. In recent years there have been reports of persistent microfilaridermias despite multiple treatments. This has necessitated a reexamination of its microfilaricidal and suppressive effects on reproduction in the adult female Onchocerca volvulus. In an initial study, we demonstrated the continued potent microfilaricidal effect of IVM. However, we also found communities in which the skin microfilarial repopulation rates at days 90 and 180 were much higher than expected. In this follow up study we have investigated the reproductive response of female worms to multiple treatments with IVM.

Methods and Findings

The parasitological responses to IVM in two hundred and sixty-eight microfilaridermic subjects from nine communities that had received 10 to 19 annual doses of IVM treatment and one pre-study IVM-naïve community were followed. Skin snips were taken 364 days after the initial IVM treatment during the study to determine the microfilaria (mf) recovery rate. Nodules were excised and skin snips taken 90 days following a second study IVM treatment. Nodule and worm density and the reproductive status of female worms were determined. On the basis of skin mf repopulation and skin mf recovery rates we defined three categories of response—good, intermediate and poor—and also determined that approximately 25% of subjects in the study carried adult female worms that responded suboptimally to IVM. Stratification of the female worms by morphological age and microfilarial content showed that almost 90% of the worms were older or middle aged and that most of the mf were produced by the middle aged and older worms previously exposed to multiple treatments with little contribution from young worms derived from ongoing transmission.

Conclusions

The results confirm that in some communities adult female worms were non-responsive or resistant to the anti-fecundity effects of multiple treatments with IVM. A scheme of the varied responses of the adult female worm to multiple treatments is proposed.

Onchocerciasis, commonly known as river blindness, is caused by the filarial nematode Onchocerca volvulus and is transmitted by a blackfly vector. Over 37 million people are thought to be infected, with over 90 million at risk. Infection predominantly occurs in sub-Saharan Africa. Foci also exist in the Arabian Peninsula and Central and South America. Ivermectin, the sole pharmaceutical available for mass chemotherapy, has been used on a community basis for annual or semi-annual treatment since 1987. Multiple treatments with ivermectin kill the microfilariae that are responsible for the pathology of onchocerciasis. More importantly, ivermectin suppresses the reproductive activity of the adult female worms, thus delaying or preventing the repopulation of the skin with new microfilariae and thereby reducing transmission. This study extends earlier reports of sub-optimal responses to ivermectin by examining repopulation levels of microfilaria one year after treatment, worm burdens per nodule, the age structure of adult female worms recovered from nodules, and the reproductive status of adult female worms 90 days after ivermectin treatment. In some communities which have shown a pattern of sub-optimal response to treatment, the data is consistent with an emergence of ivermectin non response or resistance manifested by a loss of the effect of ivermectin on the suppression of parasite reproduction.

Correlation between loss of efficacy of macrocyclic lactone heartworm anthelmintics and P-glycoprotein genotype

Macrocyclic lactone (ML) molecules have been used for heartworm control for more than 25 years. However, in recent years, there have been reports of loss of efficacy of ML heartworm preventatives against Dirofilaria immitis in some locations in the United States. Macrocyclic lactone resistance is a common problem in nematode parasites of livestock, and more recently, evidence of ivermectin resistance has been reported in the human filarial nematode Onchocerca volvulus. In this study, four D. immitis sample groups from the United States with different treatment histories were investigated for evidence of ML-driven genetic selection. DNA from individual adult worms and microfilariae was amplified by polymerase chain reaction to investigate a gene encoding a P-glycoprotein, a protein class known to be involved in ML pharmacology. A significant correlation of a GG-GG genotype with ivermectin response phenotype was found. Moreover, a significant loss of heterozygosity was found in a low responder group; loss of heterozygosity is commonly seen in loci when a population has been under selection. Further studies are required to confirm ML resistance in heartworm populations. However, the genetic changes observed in this study may be useful as a marker to monitor for ML resistance in D. immitis.

Filarial infections in travelers and immigrants

Filarial infections including loiasis, onchocerciasis, and lymphatic filariasis are important causes of morbidity in endemic populations worldwide, and they present a risk to travelers to endemic areas. Definitive diagnosis is complicated by overlap in the geographic distribution and clinical manifestations of the different filarial parasites, as well as similarities in their antigenic and nucleic acid composition. This has important implications for treatment, because the efficacies and toxicities of available antifilarial agents differ dramatically among filarial species. Recent advances, including the visualization of adult filarial worms in vivo by high-frequency ultrasound and the identification of the bacterial endosymbiont, Wolbachia, have greatly improved our understanding of the pathogenesis of filarial infection and have led to novel approaches to the diagnosis and treatment of travelers and immigrants from filarial-endemic regions.

Control of nematode parasites with agents acting on neuro-musculature systems: lessons for neuropeptide ligand discovery

There are a number of reasons why the development of novel anthelmintics is very necessary. In domestic animals, parasites cause serious loss of production and are a welfare concern. The control of these parasites requires changes in management practices to reduce the spread of infection and the use of therapeutic agents to treat affected animals. The development of vaccines against parasites is desirable but their development so far has been very limited. One notable exception is the vaccination of calves against infection by Dictyocaulus viviparous (lungworm) which has proved to be very effective. In domestic animals, the total market for anti-parasitic agents (both ecto- and endo-parasites) is in excess of a billion U.S. dollars. In humans there are serious problems ofmorbidity and mortality associated with parasite infections. 1.6 billion People throughout the world are infected with ascariasis (Fig. 1A) and/or hookworm. Approximately one-third of the world's population is suffering from the effects of intestinal nematode parasites, causing low growth-rates in infants, ill-thrift, diarrhea and in 2% of cases, loss of life. Despite the huge number of affected individuals, the market for anti-parasitic drugs for humans is not big enough to foster the development of anthelmintics because most infestations that occur are in undeveloped countries that lack the ability to pay for the development of these drugs. The major economic motivator then, is for the development of animal anthelmintics. In both domestic animals and now in humans, there is now a level of resistance to the available anthelmintic compounds. The resistance is either: constitutive, where a given species of parasite has never been sensitive to the compound; or acquired, where the resistance has developed through Darwinian selection fostered by the continued exposure to the anti-parasitic drugs. The continued use of all anthelmintics has and will, continue to increase the level of resistance. Cure rates are now often less than 100% and resistance of parasites to agents acting on the neuromuscular systems is present in a wide range of parasites of animals and humans hosts. In the face of this resistance the development of novel and effective agents is an urgent and imperative need. New drugs which act on the neuromuscular system have an advantage for medication for animals and humans because they have a rapid therapeutic effect within 3 hours of administration. The effects on the neuromuscular system include: spastic paralysis with drugs like levamisole and pyrantel; flaccid paralysis as with piperazine; or disruption of other vital muscular activity as with ivermectin. Figure 1 B and C, illustrates an example ofa spastic effect oflevamisole on infectious L3 larvae of Ostertagia ostertagiae, a parasite of pigs. The effect was produced within minutes of the in vitro application oflevamisole. In this chapter we comment on the properties of existing agents that have been used to control nematode parasites and that have an action on neuromuscular systems. We then draw attention to resistance that has developed to these compounds and comment on their toxicity and spectra of actions. We hope that some of the lessons that the use of these compounds has taught us may to be applied to any novel neuropeptide ligand that may be introduced. Our aim is then is to provide some warning signs for recognized but dangerous obstacles.

An analysis of genetic diversity and inbreeding in Wuchereria bancrofti: implications for the spread and detection of drug resistance

Estimates of genetic diversity in helminth infections of humans often have to rely on genotyping (immature) parasite transmission stages instead of adult worms. Here we analyse the results of one such study investigating a single polymorphic locus (a change at position 200 of the beta-tubulin gene) in microfilariae of the lymphatic filarial parasite Wuchereria bancrofti. The presence of this genetic change has been implicated in benzimidazole resistance in parasitic nematodes of farmed ruminants. Microfilariae were obtained from patients of three West African villages, two of which were sampled prior to the introduction of mass drug administration. An individual-based stochastic model was developed showing that a wide range of allele frequencies in the adult worm populations could have generated the observed microfilarial genetic diversity. This suggests that appropriate theoretical null models are required in order to interpret studies that genotype transmission stages. Wright's hierarchical F-statistic was used to investigate the population structure in W. bancrofti microfilariae and showed significant deficiency of heterozygotes compared to the Hardy-Weinberg equilibrium; this may be partially caused by a high degree of parasite genetic differentiation between hosts. Studies seeking to quantify accurately the genetic diversity of helminth populations by analysing transmission stages should increase their sample size to account for the variability in allele frequency between different parasite life-stages. Helminth genetic differentiation between hosts and non-random mating will also increase the number of hosts (and the number of samples per host) that need to be genotyped, and could enhance the rate of spread of anthelmintic resistance.

Scabies: molecular perspectives and therapeutic implications in the face of emerging drug resistance

Limited effective treatments, coupled with recent observations of emerging drug resistance to oral ivermectin and 5% permethrin, raise concerns regarding the future control of scabies, especially in severe cases and in endemic areas where repeated community treatment programs are in place. There is consequently an urgent need to define molecular mechanisms of drug resistance in scabies mites and to develop and assess alternative therapeutic options, such as tea tree oil, in the event of increasing treatment failure. Molecular studies on scabies mites have, until recently, been restricted; however, recent advances are providing new insights into scabies mite biology and genetic mechanisms underlying drug resistance. These may assist in overcoming many of the current difficulties in monitoring treatment efficacy and allow the development of more sensitive tools for monitoring emerging resistance.

P-glycoprotein-like protein, a possible genetic marker for ivermectin resistance selection in Onchocerca volvulus

Ivermectin (IVM) is the only safe drug for mass-treatment of onchocerciasis. IVM resistance has been reported in gastrointestinal nematode parasites of animals. A reduction in response to IVM in Onchocerca volvulus could have significant consequences for the onchocerciasis control programs. We have found evidence that, in O. volvulus, repeated IVM treatment selects for specific alleles, of P-glycoprotein-like protein (PLP), a half-sized ABC transporter. In this study, O. volvulus samples were derived from a clinical trial in Cameroon, in which patients were sampled before, and following 3 years (1994-1997) of IVM treatments. There were four treatment groups: 150 microg/kg (1 x p.a. or 4 x p.a.) and 800 microg/kg (1 x p.a. or 4 x p.a.). DNA of O. volvulus macrofilariae was genotyped over a 476bp region of the PLP gene and at two control genes. Of the six polymorphic positions found in the PLP amplicon, three of them showed significant selection after 4 x p.a. treatment with IVM (total of 13 IVM treatments) in female worms, and one of the same single nucleotide polymorphisms (SNPs) showed significant selection in the male worms. One of the selected SNPs in the female worms caused an amino acid coding change in the putative protein sequence. We found a clear selection of some genotypes, a high SNPs association and a loss of polymorphism following 4 x p.a. treatment with IVM. These PLP SNPs and genotypes could be useful markers to follow selection for IVM resistance in the field.

P-glycoprotein selection in strains of Haemonchus contortus resistant to benzimidazoles

Anthelmintic resistance in parasitic nematodes of livestock is a chronic problem in many parts of the world. Benzimidazoles are effective, broad-spectrum anthelmintics that bind to and selectively depolymerise microtubules. Resistance to the benzimidazoles, however, developed quickly and is caused by genetic changes in genes encoding beta-tubulins, subunits of microtubules. In Haemonchus contortus, resistance to avermectins has been correlated with genetic changes at a gene encoding a P-glycoprotein, a cell membrane transport protein that has a very high affinity for ivermectin. The substrate specificity of P-glycoprotein is very broad, and resistance to benzimidazoles can be modulated by lectins specific for P-glycoprotein. We investigated the possibility that genetic changes in P-glycoprotein might be correlated with benzimidazole resistance in nematodes. An analysis of restriction fragment length polymorphisms of a P-glycoprotein gene from a sensitive and a cambendazole-selected strain of H. contortus, derived from the sensitive strain, showed a significant difference in allele frequencies between strains. The frequency of one allele in particular increased substantially. The same allele was also found at a high frequency in an independently derived thiabendazole-selected field isolate. We present genetic evidence of selection at a P-glycoprotein locus during selection for benzimidzole resistance in H. contortus.

Markers for benzimidazole resistance in human parasitic nematodes?

Benzimidazole (BZ) resistance is widespread and appears to be readily selected in a variety of nematode parasites of animals. There have been reports of a lack of efficacy of BZ anthelmintics against soil transmitted nematode parasites of humans. However, resistance to BZs in nematodes of humans has not been confirmed. It is difficult to perform tests to confirm anthelmintic resistance in humans for a variety of technical and ethical reasons. The use of anthelmintic drugs for the control of helminth parasites in people is increasing massively as a result of numerous programmes to control gastrointestinal nematode parasites in children, the Global Program for the Elimination of Lymphatic Filariasis and other programmes. Many of these programmes are dependent on BZ anthelmintics and this will increase the pressure for resistance development to BZ anthelmintics in nematode parasites of people. We need to perform monitoring for anthelmintic resistance in these programmes and we need new tools to make that monitoring sensitive, inexpensive and practical. There is a real need for DNA-based markers for BZ resistance in nematode parasites of humans. We have a reasonable understanding of the molecular mechanisms and genetics of BZ resistance in some nematode parasites of animals and similar mechanisms are likely to prevail in nematodes of humans. Based on the likelihood that similar single nucleotide polymorphisms (SNPs) will be involved in BZ resistance in human, as in animal nematode parasites, rapid SNP assays have been developed for possible BZ resistance development in Wuchereria bancrofti.

ABC transporters and beta-tubulin in macrocyclic lactone resistance: prospects for marker development

Macrocyclic lactones (MLs) are highly lipophilic anthelmintics which are known to bind to and open ligand-gated ion channels. However, these anthelmintics, and particularly the avermectin members of the ML class of endectocides, are potent substrates for ABC transporters and these transporters may regulate drug concentration in both the host and the parasite. There is accumulating evidence that ivermectin (IVM), and to a lesser extent moxidectin (MOX), selects for certain alleles of P-glycoprotein and other ABC transporter genes, selects for constitutive overexpression of some of these gene products, and induces overexpression of some P-glycoproteins in nematodes. However, such mechanisms of ML resistance do not easily lend themselves to the identification of SNP markers for resistance because of the diversity of ABC transporters in nematodes, the apparent diversity of effects of different MLs, and because regulatory elements for ABC transporter gene expression are not well understood in nematodes. Another non ligand-gated ion channel gene which appears to be under IVM selection, at least in Onchocerca volvulus and Haemonchus contortus, is beta-tubulin, and a simple genetic test for this selection has been described in O. volvulus. However, further work is required to elucidate a reliable marker associated with this gene in H. contortus or other parasitic nematodes of livestock. The possible involvement of ABC transporter genes and beta-tubulin in ML resistance provides a start in developing our understanding of this phenotype and markers for its detection in field populations of parasitic nematodes. However, more work is required before these leads can provide practical SNP markers for ML resistance.

Genetic selection of low fertile Onchocerca volvulus by ivermectin treatment

BACKGROUND

Onchocerca volvulus is the causative agent of onchocerciasis, or "river blindness". Ivermectin has been used for mass treatment of onchocerciasis for up to 18 years, and recently there have been reports of poor parasitological responses to the drug. Should ivermectin resistance be developing, it would have a genetic basis. We monitored genetic changes in parasites obtained from the same patients before use of ivermectin and following different levels of ivermectin exposure.

METHODS AND FINDINGS

O. volvulus adult worms were obtained from 73 patients before exposure to ivermectin and in the same patients following three years of annual or three-monthly treatment at 150 microg/kg or 800 microg/kg. Genotype frequencies were determined in beta-tubulin, a gene previously found to be linked to ivermectin selection and resistance in parasitic nematodes. Such frequencies were also determined in two other genes, heat shock protein 60 and acidic ribosomal protein, not known to be linked to ivermectin effects. In addition, we investigated the relationship between beta-tubulin genotype and female parasite fertility. We found a significant selection for beta-tubulin heterozygotes in female worms. There was no significant selection for the two other genes. Quarterly ivermectin treatment over three years reduced the frequency of the beta-tubulin "aa" homozygotes from 68.6% to 25.6%, while the "ab" heterozygotes increased from 20.9% to 69.2% in the female parasites. The female worms that were homozygous at the beta-tubulin locus were more fertile than the heterozygous female worms before treatment (67% versus 37%; p = 0.003) and twelve months after the last dose of ivermectin in the groups treated annually (60% versus 17%; p<0.001). Differences in fertility between heterozygous and homozygous worms were less apparent three months after the last treatment in the groups treated three-monthly.

CONCLUSIONS

The results indicate that ivermectin is causing genetic selection on O. volvulus. This genetic selection is associated with a lower reproductive rate in the female parasites. We hypothesize that this genetic selection indicates that a population of O. volvulus, which is more tolerant to ivermectin, is being selected. This selection could have implications for the development of ivermectin resistance in O. volvulus and for the ongoing onchocerciasis control programmes.