Filariasis: new drugs and new opportunities for lymphatic filariasis and onchocerciasis:

Metabolomics-based discovery of diagnostic biomarkers for onchocerciasis

Background

Development of robust, sensitive, and reproducible diagnostic tests for understanding the epidemiology of neglected tropical diseases is an integral aspect of the success of worldwide control and elimination programs. In the treatment of onchocerciasis, clinical diagnostics that can function in an elimination scenario are non-existent and desperately needed. Due to its sensitivity and quantitative reproducibility, liquid chromatography-mass spectrometry (LC-MS) based metabolomics is a powerful approach to this problem.

Methodology/Principal Findings

Analysis of an African sample set comprised of 73 serum and plasma samples revealed a set of 14 biomarkers that showed excellent discrimination between Onchocerca volvulus–positive and negative individuals by multivariate statistical analysis. Application of this biomarker set to an additional sample set from onchocerciasis endemic areas where long-term ivermectin treatment has been successful revealed that the biomarker set may also distinguish individuals with worms of compromised viability from those with active infection. Machine learning extended the utility of the biomarker set from a complex multivariate analysis to a binary format applicable for adaptation to a field-based diagnostic, validating the use of complex data mining tools applied to infectious disease biomarker discovery and diagnostic development.

Conclusions/Significance

An LC-MS metabolomics-based diagnostic has the potential to monitor the progression of onchocerciasis in both endemic and non-endemic geographic areas, as well as provide an essential tool to multinational programs in the ongoing fight against this neglected tropical disease. Ultimately this technology can be expanded for the diagnosis of other filarial and/or neglected tropical diseases.

Onchocerciasis, caused by the filarial parasite Onchocerca volvulus, afflicts millions of people, causing such debilitating symptoms as blindness and acute dermatitis. There are no accurate, sensitive means of diagnosing O. volvulus infection. Clinical diagnostics are desperately needed in order to achieve the goals of controlling and eliminating onchocerciasis and neglected tropical diseases in general. In this study, a metabolomics approach is introduced for the discovery of small molecule biomarkers that can be used to diagnose O. volvulus infection. Blood samples from O. volvulus infected and uninfected individuals from different geographic regions were compared using liquid chromatography separation and mass spectrometry identification. Thousands of chromatographic mass features were statistically compared to discover 14 mass features that were significantly different between infected and uninfected individuals. Multivariate statistical analysis and machine learning algorithms demonstrated how these biomarkers could be used to differentiate between infected and uninfected individuals and indicate that the diagnostic may even be sensitive enough to assess the viability of worms. This study suggests a future potential of these biomarkers for use in a field-based onchocerciasis diagnostic and how such an approach could be expanded for the development of diagnostics for other neglected tropical diseases.

Lymphatic filariasis and onchocerciasis

Lymphatic filariasis and onchocerciasis are parasitic helminth diseases that constitute a serious public health issue in tropical regions. The filarial nematodes that cause these diseases are transmitted by blood-feeding insects and produce chronic and long-term infection through suppression of host immunity. Disease pathogenesis is linked to host inflammation invoked by the death of the parasite, causing hydrocoele, lymphoedema, and elephantiasis in lymphatic filariasis, and skin disease and blindness in onchocerciasis. Most filarial species that infect people co-exist in mutualistic symbiosis with Wolbachia bacteria, which are essential for growth, development, and survival of their nematode hosts. These endosymbionts contribute to inflammatory disease pathogenesis and are a target for doxycycline therapy, which delivers macrofilaricidal activity, improves pathological outcomes, and is effective as monotherapy. Drugs to treat filariasis include diethylcarbamazine, ivermectin, and albendazole, which are used mostly in combination to reduce microfilariae in blood (lymphatic filariasis) and skin (onchocerciasis). Global programmes for control and elimination have been developed to provide sustained delivery of drugs to affected communities to interrupt transmission of disease and ultimately eliminate this burden on public health.

Detection and discrimination of Loa loa, Mansonella perstans and Wuchereria bancrofti by PCR-RFLP and nested-PCR of ribosomal DNA ITS1 region

The ribosomal deoxyribonucleic acid (DNA) internal transcribed spacer region (ITS1) of two filarial nematodes, Loa loa and Mansonella perstans, was amplified and further sequenced to develop an species-specific polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) protocol for the differentiation of both species from Wuchereria bancrofti, three filarial nematodes with blood circulating microfilariae. The ITS1-PCR product digested with the restriction endonuclease Ase I generated an specific diagnostic pattern for each of the three species. Moreover, three new specific nested-PCRs, targeting the ITS1 region, for differential detection of L. loa, M. perstans and W. bancrofti were developed and used when the ITS1-PCR products were insufficient for the Ase I enzymatic digestion. These filarial species-specific molecular protocols were evaluated in forty blood samples from African adult immigrants attending in the Hospital Insular of Gran Canaria, Canarias, Spain.

Assay strategies for the discovery and validation of therapeutics targeting Brugia pahangi Hsp90

The chemotherapy of lymphatic filariasis relies upon drugs such as diethylcarbamazine and ivermectin that largely target the microfilarial stages of the parasite, necessitating continued treatment over the long reproductive life span of the adult worm. The identification of compounds that target adult worms has been a long-term goal of WHO. Here we describe a fluorescence polarization assay for the identification of compounds that target Hsp90 in adult filarial worms. The assay was originally developed to identify inhibitors of Hsp90 in tumor cells, and relies upon the ability of small molecules to inhibit the binding of fluorescently labelled geldanamycin to Hsp90. We demonstrate that the assay works well with soluble extracts of Brugia, while extracts of the free-living nematode C. elegans fail to bind the probe, in agreement with data from other experiments. The assay was validated using known inhibitors of Hsp90 that compete with geldanamycin for binding to Hsp90, including members of the synthetic purine-scaffold series of compounds. The efficacy of some of these compounds against adult worms was confirmed in vitro. Moreover, the assay is sufficiently sensitive to differentiate between binding of purine-scaffold compounds to human and Brugia Hsp90. The assay is suitable for high-throughput screening and provides the first example of a format with the potential to identify novel inhibitors of Hsp90 in filarial worms and in other parasitic species where Hsp90 may be a target.

Onchodermatitis

PURPOSE OF REVIEW

This review is timely because awareness of the burden of disease from onchodermatitis has increased significantly over recent years. Recent progress in the field is reviewed with emphasis on publications within the past 2 years.

RECENT FINDINGS

Advances have been made in understanding immunopathogenesis and in diagnosis and treatment. The World Bank/WHO African Programme for Onchocerciasis Control (APOC), which uses annual community-directed treatment with ivermectin (CDTI) via the Mectizan Donation Programme, now covers 19 African countries. Development of ivermectin resistance is a concern. Unlike ivermectin, which is a microfilaricide, doxycycline, which targets Wolbachia endosymbiotic bacteria, sterilizes adult female worms and has a macrofilaricidal effect. Moxidectin, which sterilizes or kills adult worms has started a phase III trial with ivermectin. Additional primary healthcare interventions have been successfully integrated with CTDI. In Latin America, transmission has been interrupted in half of the original endemic foci and Colombia is the first nation to have achieved countrywide interruption of transmission. The first report of elimination using ivermectin in an African setting is a milestone. Two African foci using vector control plus CDTI have reported vector elimination.

SUMMARY

Results of the longer-term impact of large-scale ivermectin distribution by the APOC are awaited. Research is needed into new drug targets within Wolbachia's metabolic pathways. Elimination of transmission of disease is on the horizon but more research is needed on when and where ivermectin treatment can be stopped.

Macrofilaricidal activity after doxycycline only treatment of Onchocerca volvulus in an area of Loa loa co-endemicity: a randomized controlled trial

BACKGROUND

The risk of severe adverse events following treatment of onchocerciasis with ivermectin in areas co-endemic with loiasis currently compromises the development of control programmes and the treatment of co-infected individuals. We therefore assessed whether doxycycline treatment could be used without subsequent ivermectin administration to effectively deliver sustained effects on Onchocerca volvulus microfilaridermia and adult viability. Furthermore we assessed the safety of doxycycline treatment prior to ivermectin administration in a subset of onchocerciasis individuals co-infected with low to moderate intensities of Loa loa microfilaraemia.

METHODS

A double-blind, randomized, field trial was conducted of 6 weeks of doxycycline (200 mg/day) alone, doxycycline in combination with ivermectin (150 microg/kg) at +4 months or placebo matching doxycycline + ivermectin at +4 months in 150 individuals infected with Onchocerca volvulus. A further 22 individuals infected with O. volvulus and low to moderate intensities of Loa loa infection were administered with a course of 6 weeks doxycycline with ivermectin at +4 months. Treatment efficacy was determined at 4, 12 and 21 months after the start of doxycycline treatment together with the frequency and severity of adverse events.

RESULTS

One hundred and four (60.5%) participants completed all treatment allocations and follow up assessments over the 21-month trial period. At 12 months, doxycycline/ivermectin treated individuals had lower levels of microfilaridermia and higher frequency of amicrofilaridermia compared with ivermectin or doxycycline only groups. At 21 months, microfilaridermia in doxycycline/ivermectin and doxycycline only groups was significantly reduced compared to the ivermectin only group. 89% of the doxycycline/ivermectin group and 67% of the doxycycline only group were amicrofilaridermic, compared with 21% in the ivermectin only group. O. volvulus from doxycycline groups were depleted of Wolbachia and all embryonic stages in utero. Notably, the viability of female adult worms was significantly reduced in doxycycline treated groups and the macrofilaricidal and sterilising activity was unaffected by the addition of ivermectin. Treatment with doxycycline was well tolerated and the incidence of adverse event to doxycycline or ivermectin did not significantly deviate between treatment groups.

CONCLUSIONS

A six-week course of doxycycline delivers macrofilaricidal and sterilizing activities, which is not dependent upon co-administration of ivermectin. Doxycycline is well tolerated in patients co-infected with moderate intensities of L. loa microfilariae. Therefore, further trials are warranted to assess the safety and efficacy of doxycycline-based interventions to treat onchocerciasis in individuals at risk of serious adverse reactions to standard treatments due to the co-occurrence of high intensities of L. loa parasitaemias. The development of an anti-wolbachial treatment regime compatible with MDA control programmes could offer an alternative to the control of onchocerciasis in areas of co-endemicity with loiasis and at risk of severe adverse reactions to ivermectin.

TRIAL REGISTRATION

Controlled-Trials.com ISRCTN48118452.

Mitochondrial genes for heme-dependent respiratory chain complexes are up-regulated after depletion of Wolbachia from filarial nematodes

The filarial nematodes Brugia malayi, Wuchereria bancrofti and Onchocerca volvulus cause elephantiasis or dermatitis and blindness resulting in severe morbidity. Annually, 1.3 billion people are at risk of infection. Targeting the essential Wolbachia endobacteria of filarial nematodes with doxycycline has proven to be an effective therapy resulting in a block in embryogenesis, worm development and macrofilaricidal effects. However, doxycycline is contraindicated for a large portion of the at risk population. To identify new targets for anti-wolbachial therapy, understanding the molecular basis of the Wolbachia-filaria symbiosis is required. Using the B. malayi microarray we identified differentially expressed genes in the rodent filaria Litomosoides sigmodontis after depletion of Wolbachia which might have a role in symbiosis. The microarray data were filtered for regulated genes with a false discovery rate <5% and a > or = 2-fold-change. Most of the genes were differentially expressed at day 36 of tetracycline treatment, when 99.8% of Wolbachia were depleted. Several classes of genes were affected, including genes for translation, transcription, folding/sorting of proteins, motility, structure and metabolic and signalling pathways. Quantitative PCR validated 60% of the genes found to be regulated in the microarray. A nuclear encoded heme-binding protein of the globin family was up-regulated upon loss of Wolbachia. Interestingly, mitochondrial encoded subunits of respiratory chain complexes containing heme and riboflavin were also up-regulated. No change in the expression of these genes was seen in tetracycline treated Wolbachia-free Acanthocheilonema viteae. As Wolbachia synthesise heme and filaria do not, we hypothesise that without the endosymbionts no functional heme-containing enzymes can be formed, leading to loss of energy metabolism which then results in up-regulation of the mitochondrial encoded subunits in an attempt to correct the deviation from homeostasis. Our results support targeting the Wolbachia heme synthesis pathway for the discovery of new anti-filarial drugs.

Filariasis: diagnosis and treatment

Filariasis is an infectious disease of the lymphatics and subcutaneous tissues caused by nematodes or filariae. Carried by mosquito vectors, this disease causes millions of people to suffer from lymphedema and elephantiasis, characteristics of filariasis infection. This disease can be diagnosed through the identification of microfilariae in blood or skin samples, antigen detection, radiographic imaging, or polymerase chain reaction. Mass drug administration by the World Health Organization has helped to diminish the incidence of filariasis. However, continued research on new drugs and vaccinations will be needed to control and reduce the microfilarial levels in the human population.

Phage WO of Wolbachia: lambda of the endosymbiont world

The discovery of an extraordinarily high level of mobile elements in the genome of Wolbachia, a widespread arthropod and nematode endosymbiont, suggests that this bacterium could be an excellent model for assessing the evolution and function of mobile DNA in specialized bacteria. In this paper, we discuss how studies on the temperate bacteriophage WO of Wolbachia have revealed unexpected levels of genomic flux and are challenging previously held views about the clonality of obligate intracellular bacteria. We also discuss the roles this phage might play in the Wolbachia-arthropod symbiosis and infer how this research can be translated to combating human diseases vectored by arthropods. We expect that this temperate phage will be a preeminent model system to understand phage genetics, evolution and ecology in obligate intracellular bacteria. In this sense, phage WO might be likened to phage lambda of the endosymbiont world.

Immunisation with a multivalent, subunit vaccine reduces patent infection in a natural bovine model of onchocerciasis during intense field exposure

Human onchocerciasis, caused by the filarial nematode Onchocerca volvulus, is controlled almost exclusively by the drug ivermectin, which prevents pathology by targeting the microfilariae. However, this reliance on a single control tool has led to interest in vaccination as a potentially complementary strategy. Here, we describe the results of a trial in West Africa to evaluate a multivalent, subunit vaccine for onchocerciasis in the naturally evolved host-parasite relationship of Onchocerca ochengi in cattle. Naïve calves, reared in fly-proof accommodation, were immunised with eight recombinant antigens of O. ochengi, administered separately with either Freund's adjuvant or alum. The selected antigens were orthologues of O. volvulus recombinant proteins that had previously been shown to confer protection against filarial larvae in rodent models and, in some cases, were recognised by serum antibodies from putatively immune humans. The vaccine was highly immunogenic, eliciting a mixed IgG isotype response. Four weeks after the final immunisation, vaccinated and adjuvant-treated control calves were exposed to natural parasite transmission by the blackfly vectors in an area of Cameroon hyperendemic for O. ochengi. After 22 months, all the control animals had patent infections (i.e., microfilaridermia), compared with only 58% of vaccinated cattle (P = 0.015). This study indicates that vaccination to prevent patent infection may be an achievable goal in onchocerciasis, reducing both the pathology and transmissibility of the infection. The cattle model has also demonstrated its utility for preclinical vaccine discovery, although much research will be required to achieve the requisite target product profile of a clinical candidate.

River blindness, or onchocerciasis, is caused by a parasitic worm (Onchocerca volvulus) that is transmitted by blood-feeding blackflies, which breed in fast-flowing rivers. More than 37 million people are infected and may experience visual impairment and/or severe dermatitis. Control of onchocerciasis is largely dependent on a single drug, ivermectin. Whilst this is extremely effective at killing the worms' offspring (microfilariae) and preventing symptoms, ivermectin does not eliminate the long-lived adult parasites or always stop transmission. Consequently, treatments must be repeated for many years, and drug resistance may be emerging. Against this background, a vaccine against onchocerciasis would provide an important additional tool to sustain effective control. In this study, we evaluated eight worm antigens as vaccine components in cattle, which are often parasitized by O. ochengi (the closest relative of O. volvulus) in West Africa. Twelve uninfected animals received all eight antigens and were exposed to natural transmission of O. ochengi alongside 13 unvaccinated cattle. After almost two years, 92% of vaccinated animals had acquired adult worms, but only 58% were positive for microfilariae; whereas 100% of unvaccinated animals harboured both parasite stages. This suggests that a vaccine against microfilariae to prevent development of disease in humans may be achievable.

Lymph scrotum: an unusual urological presentation of lymphatic filariasis. A case series study

Lymphatic filariasis (LF) causes a wide range of clinical signs and symptoms, including urogenital manifestations. Transmission control and disability/morbidity management/control are the two pillars of the overall elimination strategy for LF. Lymph scrotum is an unusual urological clinical presentation of LF with important medical, psychological, social and economic repercussions. A retrospective case series study was conducted on outpatients attended at the National Reference Service for Filariasis, in an endemic area for filariasis (Recife, Brazil), between 2000 and 2007. Over this period, 6,361 patients were attended and seven cases with lymph scrotum were identified. Mean patient age was 45 years (range, 26 to 64 years). Mean disease duration was 8.5 years (range, two to 15 years). All patients had evidence of filarial infection from at least one laboratory test (parasitological, antigen investigation or 'filarial dance sign' on ultrasound). Six patients presented histories of urological surgery. The authors highlight the importance of the association between filarial infection and the inadequate surgical and clinical management of hydrocele in an endemic area, as risk factors for lymph scrotum. Thus, filarial infection should be routinely investigated in all individuals presenting urological morbidity within endemic areas, in order to identify likely links in the transmission chain.

Functional conservation of the lipid II biosynthesis pathway in the cell wall-less bacteria Chlamydia and Wolbachia: why is lipid II needed?

Cell division and cell wall biosynthesis in prokaryotes are driven by partially overlapping multiprotein machineries whose activities are tightly controlled and co-ordinated. So far, a number of protein components have been identified and acknowledged as essential for both fundamental cellular processes. Genes for enzymes of both machineries have been found in the genomes of the cell wall-less genera Chlamydia and Wolbachia, raising questions as to the functionality of the lipid II biosynthesis pathway and reasons for its conservation. We provide evidence on three levels that the lipid II biosynthesis pathway is indeed functional and essential in both genera: (i) fosfomycin, an inhibitor of MurA, catalysing the initial reaction in lipid II biosynthesis, has a detrimental effect on growth of Wolbachia cells; (ii) isolated cytoplasmic membranes from Wolbachia synthesize lipid II ex vivo; and (iii) recombinant MraY and MurG from Chlamydia and Wolbachia exhibit in vitro activity, synthesizing lipid I and lipid II respectively. We discuss the hypothesis that the necessity for maintaining lipid II biosynthesis in cell wall-lacking bacteria reflects an essential role of the precursor in prokaryotic cell division. Our results also indicate that the lipid II pathway may be exploited as an antibacterial target for chlamydial and filarial infections.

New insights into the biology of filarial infections

Recent successes in the control of lymphatic filariasis and onchocerciasis need continuing research in order to sustain the achievements and to develop further tools to tackle the new questions that arise when only reduced infection prevalences prevail. In this regard, in a symposium held at the Xth European Multicolloquium of Parasitology (August 2008, Paris) questions such as the impact of filarial immunosuppression, and its lack following filarial control, on the outcome of co-infections were addressed, as were new approaches to treatment with promising drugs such as moxidectin or the antibiotic chemotherapy against Wolbachia endosymbionts in filariae. In particular, longer treatment courses of doxycycline could be carried out by community-directed treatment at high coverage, thus potentially allowing its use in restricted areas with suboptimal responses to ivermectin against onchocerciasis, or in areas with co-infection by loiasis where onchocerciasis or lymphatic filariasis need to be controlled. New, more potent drugs, or eventually vaccines, will be of importance because in many vector–filarial parasite relationships worldwide, transmission efficacy increases with low numbers of ingested microfilariae, and since ivermectin may render treated hosts more susceptible to new infection.

Filarial parasites in the postgenomic era

Filarial parasites are able to survive for many years in their host, with suppression of the host's immune response being one major survival strategy of the parasite. However, knowledge on molecules that induce these pathways is limited. Additionally, molecules that induce inflammation, thereby leading to severe pathology, such as elephantiasis, are also not fully identified. This article assesses the findings of a recently published analysis of stage-specific excretory-secretory proteins by sodium dodecylsulfate-polyacrylamide gel electrophoresis in combination with liquid chromatography-tandem mass spectrometry. In total, 228 proteins with known and unknown functions were identified and compared with genomic, expressed sequence tags and proteomic databases. We discuss the key findings of this article for implications on filarial parasitism, as well as for a potential use for new therapeutics.

Mosquito-borne diseases

Despite centuries of control efforts, mosquito-borne diseases are flourishing worldwide. With a disproportionate effect on children and adolescents, these conditions are responsible for substantial global morbidity and mortality. Malaria kills more than 1 million children annually, chiefly in sub-Saharan Africa. Dengue virus has expanded its range over the past several decades, following its principal vector, Aedes aegypti, back into regions from which it was eliminated in the mid-20th century and causing widespread epidemics of hemorrhagic fever. West Nile virus has become endemic throughout the Americas in the past 10 years, while chikungunya virus has emerged in the Indian Ocean basin and mainland Asia to affect millions. Japanese encephalitis virus, too, has expanded its range in the Indian subcontinent and Australasia, mainly affecting young children. Filariasis, on the other hand, is on the retreat, the subject of a global eradication campaign. Efforts to limit the effect of mosquito-borne diseases in endemic areas face the twin challenges of controlling mosquito populations and delivering effective public health interventions. Travelers to areas endemic for mosquito-borne diseases require special advice on mosquito avoidance, immunizations, and malaria prophylaxis.

Functional analysis of the cathepsin-like cysteine protease genes in adult Brugia malayi using RNA interference

Background

Cathepsin-like enzymes have been identified as potential targets for drug or vaccine development in many parasites, as their functions appear to be essential in a variety of important biological processes within the host, such as molting, cuticle remodeling, embryogenesis, feeding and immune evasion. Functional analysis of Caenorhabditis elegans cathepsin L (Ce-cpl-1) and cathepsin Z (Ce-cpz-1) has established that both genes are required for early embryogenesis, with Ce-cpl-1 having a role in regulating in part the processing of yolk proteins. Ce-cpz-1 also has an important role during molting.

Methods and Findings

RNA interference assays have allowed us to verify whether the functions of the orthologous filarial genes in Brugia malayi adult female worms are similar. Treatment of B. malayi adult female worms with Bm-cpl-1, Bm-cpl-5, which belong to group Ia of the filarial cpl gene family, or Bm-cpz-1 dsRNA resulted in decreased numbers of secreted microfilariae in vitro. In addition, analysis of the intrauterine progeny of the Bm-cpl-5 or Bm-cpl Pro dsRNA- and siRNA-treated worms revealed a clear disruption in the process of embryogenesis resulting in structural abnormalities in embryos and a varied differential development of embryonic stages.

Conclusions

Our studies suggest that these filarial cathepsin-like cysteine proteases are likely to be functional orthologs of the C. elegans genes. This functional conservation may thus allow for a more thorough investigation of their distinct functions and their development as potential drug targets.

Filarial nematodes are an important group of human pathogens, causing lymphatic filariasis and onchocerciasis, and infecting around 150 million people throughout the tropics with more than 1.5 billion at risk of infection. Control of filariasis currently relies on mass drug administration (MDA) programs using drugs which principally target the microfilarial life-cycle stage. These control programs are facing major challenges, including the absence of a drug with macrofilaricidal or permanent sterilizing activity, and the possibility of the development of drug-resistance against the drugs available. Cysteine proteases are essential enzymes which play important roles in a wide range of cellular processes, and the cathepsin-like cysteine proteases have been identified as potential targets for drug or vaccine development in many parasites. Here we have studied the function of several of the cathepsin-like enzymes in the filarial nematode, B. malayi, and demonstrate that these cysteine proteases are involved in the development of embryos, show similar functions to their counterparts in C. elegans, and therefore, provide an important target for future drug development targeted to eliminate filariasis.

Onchocerciasis Control: Vision for the Future from a Ghanian perspective

Since 1987 onchocerciasis control has relied on the donation of ivermectin (Mectizan(R), Merck & Co., Inc.) through the Mectizan Donation Programme. Recently, concern has been raised over the appearance of suboptimal responses to ivermectin in Ghana - highlighting the potential threat of the development of resistance to ivermectin. This report summarises a meeting held in Ghana to set the research agenda for future onchocerciasis control. The aim of this workshop was to define the research priorities for alternative drug and treatment regimes and control strategies to treat populations with existing evidence of suboptimal responsiveness and define research priorities for future control strategies in the event of the development of widespread ivermectin resistance.