Macrocyclic lactone resistance in Dirofilaria immitis: risks for prevention of heartworm disease

Genotyping USA laboratory-maintained isolates and European clinical isolates of Dirofilaria immitis to assess macrocyclic lactone susceptibility or resistance at predictive SNP sites using droplet digital PCR

Dirofilaria immitis is a parasitic nematode that causes cardiovascular dirofilariosis ("heartworm disease") primarily in canids. The principal approach for mitigating heartworm infection involves the use of macrocyclic lactone (ML) for prophylaxis. Recent research has substantiated the emergence of D. immitis displaying resistance to MLs in the USA. Numerous factors, such as the mobility of companion animals and competent vectors could impact the spread of drug resistance. Genomic analysis has unveiled that isolates resistant to ML exhibit unique genetic profiles when compared to their wild-type (susceptible) counterparts. Out of the ten single nucleotide polymorphism (SNP) markers validated in clinical samples of D. immitis from the USA, four have demonstrated their effectiveness in distinguishing between isolates with varying ML efficacy phenotypes. This study explores the potential of these confirmed SNPs for conducting surveillance studies. Genotypic analysis using SNP markers emerges as a valuable tool for carrying out surveys and evaluating individual clinical isolates. Two USA laboratory-maintained isolates (Berkeley, WildCat) and twenty-five random European clinical samples of either adult worms or microfilariae (mf) pools isolated from domestic dogs, were tested by droplet digital PCR (ddPCR)-based duplex assay. This approach elucidates genetic evidence pertaining to the development of drug resistance and provides baseline data on resistance related genotypes in Europe. The data on these clinical samples suggests genotypes consistent with the continued efficacy of ML treatment regimens in Europe. In addition, this assay can be significant in discriminating cases of drug-resistance from those possibly due to non-compliance to the recommended preventive protocols.

Benzimidazole F167Y polymorphism in the canine hookworm, Ancylostoma caninum: Widespread geographic, seasonal, age, and breed distribution in United States and Canada dogs

Surveillance data for Ancylostoma spp. and the A. caninum benzimidazole treatment resistance associated F167Y polymorphism using molecular diagnostics was obtained in a large population of dogs from the United States and Canada. Real-time PCR (qPCR) for Ancylostoma spp. and allele-specific qPCR detecting a single nucleotide polymorphism (SNP) F167Y was used in 262,872 canine stool samples collected between March and December of 2022. Ancylostoma spp. was found at an overall prevalence of 2.5% (6538/262,872), with the highest prevalence in the Southern US, 4.4% (4490/103,095), and the lowest prevalence in Canada 0.6% (101/15,829). The A. caninum F167Y polymorphism was found with the highest prevalence (13.4%, n = 46/343) in the Western US and the lowest in Canada at 4.1% (4/97). The F167Y polymorphism was detected every month over the 10-month collection period. Seasonal distribution showed a peak in June for both Ancylostoma spp. (3.08%, 547/17,775) and A. caninum F167Y (12.25%, 67/547). However, the A. caninum F167Y polymorphism prevalence was highest in September (13.9%, 119/856). Age analysis indicates a higher prevalence of both hookworm infections and occurrence of resistant isolates in puppies. The breeds with the highest F167Y polymorphism prevalence in Ancylostoma spp. detected samples were poodles (28.9%), followed by Bernese Mountain dogs (25%), Cocker spaniels (23.1%), and greyhounds (22.4%). Our data set describes widespread geographic distribution of the A. caninum benzimidazole resistance associated F167Y polymorphism in the United States and Canada, with no clear seasonality compared to the Ancylostoma spp. prevalence patterns. The F167 polymorphism was present in all geographic areas with detected hookworms, including Canada. Our study highlights that the F167Y polymorphism is represented in many dog breeds, including greyhounds.

Development of adult Dirofilaria immitis worms in the Rag2/Il-2rγ-/- mouse model

Dirofilaria immitis is the causative agent for one of the major parasitic infections in dogs. It is currently not possible to reliably diagnose the infection before the development of fertile adult female worms and the presence of microfilariae which takes six to 7 months. However, at this point adult worms already reside in the pulmonary arteries and can cause significant damage. Novel in vivo models may facilitate the development of new diagnostic tools and improve treatment options for both the early and late stages of D. immitis infections. In this paper, we aimed to increase the capabilities of recently published mouse models in which severely immune-deficient mice were shown to be susceptible to D. immitis. Our data shows that D. immitis may grow into fully developed mature male and female worms in C57BL/6 Rag2/Il-2rγ-/- mice with comparable growth rates to the natural canine host. The adult worms of D. immitis were shown to migrate into body cavities as well as the heart in this model. However, the presence of adult worms inside the heart of infected mice led to the development of caval syndrome in 36% of infected mice after five to 6 months. Overall, the current study complements recently published efforts to establish a D. immitis mouse model by extending the development of D. immitis into mature adult stages and will facilitate further preclinical research.

Temperature-bounded development of Dirofilaria immitis larvae restricts the geographical distribution and seasonality of its transmission: case study and decision support system for canine heartworm management in Australia

Dirofilaria immitis is the causative agent of canine heartworm disease. We used the established heartworm development unit (HDU) principle to map the extrinsic incubation period (EIP) of D. immitis in Australia using historical weather data from 2013-2022. We found weather conditions suitable for EIP completion showed substantial seasonality and geographical variability. Whilst a considerable percentage of the Australian territory showed suitable weather conditions to always support EIP completion (17%), only 2.7% of the 2021 Australian human population lived in this region. Therefore, 97% of the population lived in an area that changed its EIP suitability within the study period. EIP completion is required prior to D. immitis transmission, meaning that infection risk of D. immitis is seasonal and location-dependent, being disrupted each year for most of the human population's dogs. We developed an online, open access tool allowing us to visualise EIP completion across Australia historically and in near real-time. We aim to support veterinarians to make risk-based recommendations for dirofilariosis prevention by using the tool, available at https://heartworm-mapping.adelaide.edu.au/shiny/.

Genome structure and population genomics of the canine heartworm Dirofilaria immitis

The heartworm, Dirofilaria immitis, is a filarial parasitic nematode responsible for significant morbidity and mortality in wild and domesticated canids. Resistance to macrocyclic lactone drug prevention represents a significant threat to parasite control and has prompted investigations to understand the genetic determinants of resistance. This study aimed to improve the genomic resources of D. immitis to enable a more precise understanding of how genetic variation is distributed within and between parasite populations worldwide, which will inform the likelihood and rate by which parasites, and in turn, resistant alleles, might spread. We have guided the scaffolding of a recently published genome assembly for D. immitis (ICBAS_JMDir_1.0) using the chromosomal-scale reference genomes of Brugia malayi and Onchocerca volvulus, resulting in an 89.5 Mb assembly composed of four autosomal- and one sex-linked chromosomal-scale scaffolds representing 99.7% of the genome. Publicly available and new whole-genome sequencing data from 32 D. immitis samples from Australia, Italy and the USA were assessed using principal component analysis, nucleotide diversity (Pi) and absolute genetic divergence (Dxy) to characterise the global genetic structure and measure within- and between-population diversity. These population genetic analyses revealed broad-scale genetic structure among globally diverse samples and differences in genetic diversity between populations; however, fine-scale subpopulation analysis was limited and biased by differences between sample types. Finally, we mapped single nucleotide polymorphisms previously associated with macrocyclic lactone resistance in the new genome assembly, revealing the physical linkage of high-priority variants on chromosome 3, and determined their frequency in the studied populations. This new chromosomal assembly for D. immitis now allows for a more precise investigation of selection on genome-wide genetic variation and will enhance our understanding of parasite transmission and the spread of genetic variants responsible for resistance to treatment.

Prevention and long-term outcomes of naturally occurring canine heartworm infection in primary care settings

Background

This study had two objectives: first, to examine the association between the history of heartworm preventive purchase compliance and the risk of positive heartworm tests, and second to preliminarily investigate the long-term cardiac outcomes of heartworm disease in dogs that had undergone successful adulticidal therapy.

Methods

A retrospective cohort study design was used for both analyses, using anonymous transaction data from Covetrus (retrospective analysis 1) and anonymized medical records from Banfield Pet Hospital (retrospective analysis 2), both including canine patients across the USA. The first analysis examined the relative risk (RR) of a positive heartworm test in dogs with lapses in heartworm preventive purchase history compared to dogs that had no history of a preventive purchase six to 24 months prior to the test. In the second analysis, a long-term evaluation of structured diagnostic codes pertaining to cardiac diseases and risk assessment of outcomes was performed in dogs that had previously been successfully treated for heartworm disease compared to dogs that never had a positive heartworm test.

Results

83,478 unique patients were included in the first analysis. Compared to 32,413 dogs with no history of a heartworm preventive purchase, 44,410 dogs with lapses in monthly preventive purchases had a reduced risk of testing positive for heartworm disease (RR = 0.36, p < 0.0001). Dogs (n = 6,655) with lapses in injectable heartworm preventive administration had a decreased risk of a positive test versus dogs with no preventive purchases (RR = 0.15, p < 0.0001), as well as versus dogs with lapses in monthly heartworm preventive purchases (RR = 0.28, p = 0.0024). In the second analysis, 6,138 patients treated for heartworm infection were found to have significantly (p < 0.001) elevated risks of right heart failure (RR = 3.59), left heart failure (RR = 1.83), or cardiomyopathy (RR = 2.79) compared to 4,022,752 patients that never had a positive heartworm test.

Conclusion

This study highlights the importance of compliance with heartworm preventive guidelines, to reduce the risk of heartworm disease in dogs, which is not only a potentially life-threatening condition in the short-term but also associated with long-term negative cardiac outcomes.

The combined effect of bromadiolone and ivermectin (iBr) in controlling both rodents and their fleas

Rodent pests not only cause severe agricultural loss but also spread zoonotic pathogens to human beings. Anticoagulant rodenticides are widely used to decrease the population densities of rodents but often lead to the spillover of ectoparasites because fleas and ticks may gather on surviving rodents. Therefore, it is necessary to kill fleas and ticks before culling rodents to minimize the risk of pathogen transmission. In this study, we used a mixture of ivermectin (an antiparasitic drug) and bromadiolone (an anticoagulant rodenticide) to control both rodent and flea/tick abundances. We found that in a laboratory test, 0.01% ivermectin bait was not lethal for greater long-tailed hamsters after 7 days of treatment, while 0.1% ivermectin bait was lethal for approximately 33% of treated rodents. In a field test, bait containing 0.001%, 0.005%, 0.01%, and 0.05% ivermectin decreased the number of fleas per vole of Brandt's voles to 0.42, 0.22, 0.12, and 0.2, respectively, compared with 0.77 in the control group, indicating that 0.01% ivermectin bait performed best in removing fleas. In another laboratory test, bait containing a 0.01% ivermectin and 0.005% bromadiolone mixture caused the death of all voles within 6-14 days after the intake of the bait. In the field test, the bait containing 0.01% ivermectin and 0.005% bromadiolone reduced the average number of fleas per vole to 0.35, which was significantly lower than the 0.77 of the control group. Our results indicate that a 0.01% ivermectin and 0.005% bromadiolone mixture could be used to control both rodents and fleas to minimize the spillover risk of disease transmission when using traditional rodenticides.

Exploring multiplex qPCR as a diagnostic tool for detecting microfilarial DNA in dogs infected with Dirofilaria immitis: A comparative analysis with the modified Knott's test

Current recommendations to diagnose cardiopulmonary dirofilariosis in dogs caused by Dirofilaria immitis involves tandem antigen and circulating microfilariae tests. The modified Knott's test is an important tool in heartworm diagnosis, allowing identification of circulating microfilariae. However, the subjective nature of the modified Knott's test affects its accuracy and diagnostic laboratories usually do not provide a quantitative outcome. Quantitative enumeration of microfilariae enables clinicians to track treatment progress and acts as a proxy for detecting emerging macrocyclic lactone resistance. There is a need for better diagnostic tools suitable for routine use to efficiently and accurately quantify the presence of D. immitis microfilaremia. The aim of this study was to determine whether the quantitative modified Knott's test can be substituted by multiplex quantitative polymerase chain reaction (qPCR) targeting D. immitis and associated Wolbachia endosymbiont DNA in canine blood samples. To do this, genomic DNA samples (n = 161) from Australian dogs, collected as part of a previous 2021 study, were assessed in a TaqMan qPCR targeting DNA of D. immitis, Wolbachia sp. and Canis lupus familiaris. Of the 161 genomic DNA samples, eight were considered positive for D. immitis microfilariae. The qPCR assay demonstrated good efficiency (E = 90 to 110%, R2 > 0.94). Considering the performance and efficient use of bench time, this TaqMan qPCR assay is a suitable alternative to the modified Knott's test for quantitative enumeration of microfilariae (Cohen's kappa coefficient [κ]: κ = 1 using D. immitis qPCR marker, κ = 0.93 using Wolbachia qPCR marker). The qPCR data demonstrated a comparable result to that of the quantitative modified Knott's test in a 2022 survey of D. immitis in Australian dogs (n = 23) before and after macrocyclic lactone (ML) administration. Improving the detection and diagnosis of canine heartworm infections will assist veterinarians in better managing and controlling disease outcomes and will be valuable for tracking the spread of ML resistance in Australia.

Antiparasitic activity of ivermectin: Four decades of research into a "wonder drug"

Parasitic diseases still pose a serious threat to human and animal health, particularly for millions of people and their livelihoods in low-income countries. Therefore, research into the development of effective antiparasitic drugs remains a priority. Ivermectin, a sixteen-membered macrocyclic lactone, exhibits a broad spectrum of antiparasitic activities, which, combined with its low toxicity, has allowed the drug to be widely used in the treatment of parasitic diseases affecting humans and animals. In addition to its licensed use against river blindness and strongyloidiasis in humans, and against roundworm and arthropod infestations in animals, ivermectin is also used "off-label" to treat many other worm-related parasitic diseases, particularly in domestic animals. In addition, several experimental studies indicate that ivermectin displays also potent activity against viruses, bacteria, protozoans, trematodes, and insects. This review article summarizes the last 40 years of research on the antiparasitic effects of ivermectin, and the use of the drug in the treatment of parasitic diseases in humans and animals.

Droplet digital PCR as a tool to detect resistant isolates of Dirofilaria immitis

Prevention of canine heartworm disease, caused by Dirofilaria immitis, relies on macrocyclic lactones for which drug resistance is now a concern. Although genetic polymorphisms have been associated with resistance in D. immitis populations, the mechanism is still not well understood. The lack of reliable in vitro assays to detect resistance is a limitation for confirming resistance. Ten single nucleotide polymorphisms (SNPs) were previously clinically validated in D. immitis resistant isolates, using the MiSeq platform. This technique although useful for research studies is expensive and does not facilitate rapid detection of these markers in small numbers of clinical samples. We developed a droplet digital PCR protocol for detecting SNPs correlating with ML resistance. Specific primers and hydrolysis probes encompassing the wildtype and mutant alleles were designed to amplify the SNP targets from genomic DNA of different D. immitis isolates. Allele frequencies were determined and the suitability of the ddPCR assay was assessed and compared with MiSeq data. The ddPCR assay accurately detected and quantified alternate nucleotides in two isolates of reference, the ML-susceptible Missouri (MO) and ML-resistant JYD-34, at the previously identified SNP positions. The presence of the SNPs was also determined in additional isolates with known or putative susceptible or resistant phenotypes. We observed SNP1 and SNP2 are more predictive markers and appear suitable for rapid detection and monitoring of drug resistance. Our results suggested that ddPCR could be employed to distinguish infection due to actual genetic resistance from infection with susceptible parasites and also for rapid detection of isolates not only with ML susceptible and resistant genotypes but also mixed genotypes that correspond to heterogeneous isolates containing a mixed population of ML susceptible and resistant parasites. DdPCR may be a useful tool for conducting surveys, or assessments of individual isolates, for genetic evidence of resistance or developing resistance.

Development and validation of an LC-MS/MS method for the simultaneous quantification of milbemycin oxime and praziquantel in plasma: application to a pharmacokinetic study in cats

Introduction

Milbemycin oxime (MBO) and praziquantel (PZQ) have a broad spectrum of biological activity and are commonly used to treat the parasitic infection in the veterinary clinic. In this study, a fast and efficient LC-MS/MS method was established and validated for the simultaneous determination of MBO, PZQ, cis-4-hydroxylated-PZQ (C-4-OH-PZQ) and trans-4-hydroxylated-PZQ (T-4-OH-PZQ) and in cat plasma.

Methods

Extraction of analytes and internal standards from cat plasma by acetonitrile protein precipitation, allows rapid processing of large batches of samples. MBO, PZQ, C-4-OH-PZQ, T-4-OH-PZQ, and internal standard (IS) were eluted for 13.5 min on a C18 column with a 0.1% formic acid water/acetonitrile mixture as the mobile phase.

Results

Results showed that the method had good precision, accuracy, recovery, and linearity. The linearity range was 2.5-250 ng/mL for MBO, and 10-1000 ng/mL for PZQ, C-4-OH-PZQ, and T-4-OH-PZQ. The intra-day and inter-day precision CV values of the tested components were within 15%. The extraction recoveries of the four components ranged from 98.09% to 107.46%. The analytes in plasma remained stable for 6 h at room temperature, 26 h in the autosampler (4 °C), after freeze-thaw (-20°C) cycles, and 60 days in a -20°C freezer. Method sensitivity sufficed for assessing pharmacokinetic parameters of MBO, PZQ, C-4-OH-PZQ, and T-4-OH-PZQ in plasma samples with LLOQ of 2.5 ng/mL for MBO and 10 ng/mL for PZQ, C-4-OH-PZQ, and T-4-OH-PZQ.

Conclusion

In this study, a selective and sensitive LC-MS/MS method for the simultaneous quantification of MBO, PZQ, C-4-OH-PZQ, and T-4-OH-PZQ in cat plasma was developed and validated.This method had been successfully applied to evaluate the pharmacokinetics of MBO, PZQ, C-4-OH-PZQ, and T-4-OH-PZQ after a single oral administration of 8 mg MBO and 20 mg PZQ in cats.

Programmed cell death pathways as targets for developing antifilarial drugs: Lessons from the recent findings

More than half a century has passed since the introduction of the National Filariasis Control Program; however, as of 2023, lymphatic filariasis (LF) still prevails globally, particularly in the tropical and subtropical regions, posing a substantial challenge to the objective of worldwide elimination. LF is affecting human beings and its economically important livestock leading to a crucial contributor to morbidities and disabilities. The current scenario has been blowing up alarms of attention to develop potent therapeutics and strategies having efficiency against the adult stage of filarial nematodes. In this context, the exploration of a suitable drug target that ensures lethality to macro and microfilariae is now our first goal to achieve. Apoptosis has been the potential target across all three stages of filarial nematodes viz. oocytes, microfilariae (mf) and adults resulting in filarial death after receiving the signal from the reactive oxygen species (ROS) and executed through intrinsic and extrinsic pathways. Hence, it is considered a leading target for developing antifilarial drugs. Herein, we have shown the efficacy of several natural and synthetic compounds/nanoformulations in triggering the apoptotic death of filarial parasites with little or no toxicity to the host body system.

New paradigms in research on Dirofilaria immitis

BACKGROUND

Since the advent of ivermectin (along with melarsomine and doxycycline), heartworm has come to be viewed as a solved problem in veterinary medicine, diminishing investment into non-clinical research on Dirofilaria immitis. However, heartworm infections continue to pose problems for practitioners and their patients and seem to be increasing in frequency and geographic distribution. Resistance to preventative therapies (macrocyclic lactones) complicates the picture. The use of chemotherapy for other kinds of pathogens has benefitted enormously from research into the basic biology of the pathogen and on the host-pathogen interface. A lack of basic information on heartworms as parasites and how they interact with permissive and non-permissive hosts greatly limits the ability to discover new ways to prevent and treat heartworm disease. Recent advances in technical platforms will help overcome the intrinsic barriers that hamper research on D. immitis, most notably, the need for experimentally infected dogs to maintain the life cycle and provide material for experiments. Impressive achievements in the development of laboratory animal models for D. immitis will enhance efforts to discover new drugs for prevention or treatment, to characterize new diagnostic biomarkers and to identify key parasite-derived molecules that are essential for survival in permissive hosts, providing a rational basis for vaccine discovery. A 'genomics toolbox' for D. immitis could enable unprecedented insight into the negotiations between host and parasite that enable survival in a permissive host. The more we know about the pathogen and how it manipulates its host, the better able we will be to protect companion animals far into the future.

Fluralaner (Bravecto®) treatment kills Aedes aegypti after feeding on Dirofilaria immitis-infected dogs

BACKGROUND

Transmission of canine heartworm (Dirofilaria immitis) from infected to naïve dogs is dependent on successful mosquito feeding and survival.

METHODS

To determine whether treating heartworm-infected dogs with fluralaner (Bravecto^®) limits the survival of infected mosquitoes, and potentially the transmission of D. immitis, we allowed female mosquitoes to feed on microfilaremic dogs and evaluated mosquito survival and infection with D. immitis. Eight dogs were experimentally infected with D. immitis. On day 0 (~ 11 months post-infection), four microfilaremic dogs were treated with fluralaner according to label directions while the other four were non-treated controls. Mosquitoes (Aedes aegypti Liverpool) were allowed to feed on each dog on days -7, 2, 30, 56, and 84. Fed mosquitoes were collected, and the number of live mosquitoes determined at 6 h, 24 h, 48 h, and 72 h post-feeding. Surviving mosquitoes held for 2 weeks were dissected to confirm third-stage D. immitis larvae; PCR (12S rRNA gene) was performed post-dissection to identify D. immitis in mosquitoes.

RESULTS

Prior to treatment, 98.4%, 85.1%, 60.7%, and 40.3% of mosquitoes fed on microfilaremic dogs were alive at 6 h, 24 h, 48 h, and 72 h post-feeding, respectively. Similarly, mosquitoes fed on microfilaremic, non-treated dogs were alive 6 h post-feeding (98.5-100%) throughout the study. In contrast, mosquitoes fed on fluralaner-treated dogs 2 days after treatment were dead or severely moribund by 6 h post-feeding. At 30 and 56 days post-treatment, > 99% of mosquitoes fed on treated dogs were dead by 24 h. At 84 days post-treatment, 98.4% of mosquitoes fed on treated dogs were dead by 24 h. Before treatment, third-stage larvae of D. immitis were recovered from 15.5% of Ae. aegypti 2 weeks after feeding, and 72.4% were positive for D. immitis by PCR. Similarly, 17.7% of mosquitoes fed on non-treated dogs had D. immitis third-stage larvae 2 weeks after feeding, and 88.2% were positive by PCR. Five mosquitoes fed on fluralaner-treated dogs survived 2 weeks post-feeding, and 4/5 were from day 84. None had third-stage larvae at dissection, and all were PCR-negative.

CONCLUSION

The data indicate that fluralaner treatment of dogs kills mosquitoes and thus would be expected to reduce transmission of heartworm in the surrounding community.

Treatment of dogs with Bravecto® (fluralaner) reduces mosquito survival and fecundity

BACKGROUND

Mosquitoes serve as the vector of canine heartworm (Dirofilaria immitis), which represents a significant and persistent threat to canine health. A reduction in the longevity and/or reproductive success of mosquitoes that take a blood meal from fluralaner-treated dogs may consequently reduce the local transmission of heartworm and prevent new infections. A novel secondary effect of an oral formulation of the ectoparasiticide fluralaner (Bravecto®) against a laboratory strain of the mosquito Aedes aegypti, a potential major vector of canine heartworm, was investigated in this study.

METHODS

Six dogs were administered a single dose of fluralaner orally in the form of Bravecto® Chews (at the labeled fluralaner dose of 25 mg/kg body weight), while six control dogs received no treatment. Mosquitoes were fed on blood that was collected from each dog prior to treatment and weekly for 15 weeks post-treatment to assess the continued effects of fluralaner as its serum level decreased. Mosquito fitness was assessed by three parameters: rate of successful blood-feeding, survival, and egg laying.

RESULTS

Successful blood-feeding rate was similar between control and treatment groups. In the fluralaner treatment, mosquito survival was significantly reduced within the first 24 h after blood-feeding, for the first 12 weeks post-treatment of the dogs (efficacy range = 33.2-73.3%). Survival of mosquitoes up until a potentially heartworm-infective timepoint (14 days post-blood-feeding) was significantly reduced in the fluralaner-treated group at several timepoints (1, 2, 5, 11, 12, 13, 14, and 15 weeks post-treatment; efficacy range = 49.4-91.4%), but was less consistently reduced at the other timepoints. Egg laying by mosquitoes was almost completely suppressed for the first 13 weeks following treatment of the dogs with fluralaner (treatment efficacy ≥ 99.8%).

CONCLUSIONS

Mosquitoes fed blood from fluralaner-treated dogs experienced a significant reduction in survival and fecundity. These findings support the potential for a reduction in heartworm transmission directly by lethal effects on the vector and indirectly through a reduction of the local vector population when mosquitoes are exposed to animals treated with fluralaner.

A rodent model for Dirofilaria immitis, canine heartworm: parasite growth, development, and drug sensitivity in NSG mice

Heartworm disease, caused by Dirofilaria immitis, remains a significant threat to canines and felines. The development of parasites resistant to macrocyclic lactones (ML) has created a significant challenge to the control of the infection. The goal of this study was to determine if mice lacking a functional immune response would be susceptible to D. immitis. Immunodeficient NSG mice were susceptible to the infection, sustaining parasites for at least 15 weeks, with infective third-stage larvae molting and developing into the late fourth-stage larvae. Proteomic analysis of host responses to the infection revealed a complex pattern of changes after infection, with at least some of the responses directed at reducing immune control mechanisms that remain in NSG mice. NSG mice were infected with isolates of D. immitis that were either susceptible or resistant to MLs, as a population. The susceptible isolate was killed by ivermectin whereas the resistant isolate had improved survivability, while both isolates were affected by moxidectin. It was concluded that D. immitis survives in NSG mice for at least 15 weeks. NSG mice provide an ideal model for monitoring host responses to the infection and for testing parasites in vivo for susceptibility to direct chemotherapeutic activity of new agents.

Genetic polymorphism, constitutive expression and tissue localization of Dirofilaria immitis P-glycoprotein 11: a putative marker of macrocyclic lactone resistance

BACKGROUND

Dirofilaria immitis causes dirofilariosis, a potentially fatal condition in canids. Dirofilaria infections can be prevented with a macrocyclic lactone (ML) prophylactic regimen. However, some D. immitis isolates have become resistant to MLs. Genetic changes on the P-glycoprotein 11 gene, encoding an ABCB transporter, have been linked to the ML-resistant phenotypes and have been proposed as markers of drug resistance. However, nothing is known about the expression and the localization of this transporter in D. immitis, despite its strong link to ML-resistant phenotypes.

METHODS

We examined the clinically validated D. immitis P-glycoprotein 11 (DimPgp-11) single nucleotide polymorphism (SNP) via MiSeq analysis in three ML-susceptible isolates (Missouri, MP3 and Yazoo) and two ML-resistant isolates (JYD-34 and Metairie), and correlated the data with previously published MiSeq results of USA laboratory-maintained D. immitis isolates. The level of the expression of the DimPgp-11 messenger RNA transcript was analyzed by droplet digital PCR (ddPCR) and compared in the USA laboratory-maintained isolates, namely the ML-susceptible Missouri and Berkeley isolates, the putative ML-susceptible Georgia III and Big Head isolates and the ML-resistant isolate JYD-34. The immunolocalization of DimPgp-11 was visualized in the microfilaria (mf) life stage of the Missouri isolate using confocal microscopy.

RESULTS

The results confirmed that the SNP found on DimPgp-11 is differentially expressed in the USA laboratory-maintained isolates. The ML-susceptible isolates had an alternate allele frequency of between 0% and 15%, while it ranged between 17% and 56% in the ML-resistant isolates. The constitutive expression of DimPgp-11 was similar in the Berkeley, Georgia III and Big Head isolates, while it was significantly decreased in the ML-resistant JYD-34 isolate (P < 0.05), when compared to the ML-susceptible Missouri isolate. The DimPgp-11 protein was distinctly localized within the excretory-secretory (ES) duct, pore cells and the excretory cell and, more faintly, along the mf body wall.

CONCLUSION

Our data confirm that genetic polymorphism of DimPgp-11 is associated with ML resistance in USA laboratory-maintained D. imminits isolates. A link between DimPgp-11 and ML resistance in D. immitis is further supported by the lower protein expression in the ML-resistant JYD-34 isolate when compared with the ML-susceptible Missouri isolate. Interestingly, DimPgp-11 is strategically located surrounding the ES pore where it could play an active role in ML efflux.

Exploration of the sensitivity to macrocyclic lactones in the canine heartworm (Dirofilaria immitis) in Australia using phenotypic and genotypic approaches

Canine heartworm disease is a potentially deadly cardiopulmonary disease caused by the mosquito-borne filarial nematode Dirofilaria immitis. In Australia, the administration of macrocyclic lactone (ML) drugs has successfully reduced the prevalence of D. immitis infection. However, the recent re-emergence of D. immitis in dogs in Queensland, Australia and the identification of ML-resistant isolates in the USA poses an important question of whether ML-resistance has emerged in this parasite in Australia. The aim of this study was to utilise phenotypic and genotypic approaches to examine the sensitivity to ML drugs in D. immitis in Australia. To do this, we surveyed 45 dogs from Queensland and New South Wales across 3 years (2019-2022) for the presence of D. immitis infection using an antigen test, quantitative Modified Knott's test, and qPCR targeting both D. immitis and the D. immitis symbiont Wolbachia. A phenotype observed by utilising sequential quantification of microfilariae for 23/45 dogs was coupled with genetic testing of filtered microfilariae for SNPs previously associated with ML-resistance in isolates from the USA. Sixteen (16/45) dogs tested positive for D. immitis infection despite reportedly receiving 'rigorous' heartworm prevention for 12 months prior to the study, according to the owners' assessment. The phenotype and genotypic assays in this study did not unequivocally demonstrate the presence of ML-resistant D. immitis in Australia. Although the failure of 16 dogs to reduce microfilaremia by >90% after ML treatment was considered a suspect phenotype of ML-resistance, no genotypic evidence was discovered using the genetic SNP analysis. The traditional quantitative Modified Knott's test can be substituted by qPCR targeting D. immitis or associated Wolbachia endosymbiont DNA for a more rapid measurement of microfilariae levels. More definitive phenotypic evidence of resistance is critically needed before the usefulness of SNPs for the detection of ML-resistance in Australia can be properly assessed.

Vector-borne and other pathogens of potential relevance disseminated by relocated cats

Large populations of unowned cats constitute an animal welfare, ecological, societal and public health issue worldwide. Their relocation and homing are currently carried out in many parts of the world with the intention of relieving suffering and social problems, while contributing to ethical and humane population control in these cat populations. An understanding of an individual cat’s lifestyle and disease status by veterinary team professionals and those working with cat charities can help to prevent severe cat stress and the spread of feline pathogens, especially vector-borne pathogens, which can be overlooked in cats. In this article, we discuss the issue of relocation and homing of unowned cats from a global perspective. We also review zoonotic and non-zoonotic infectious agents of cats and give a list of practical recommendations for veterinary team professionals dealing with homing cats. Finally, we present a consensus statement consolidated at the 15th Symposium of the Companion Vector-Borne Diseases (CVBD) World Forum in 2020, ultimately to help veterinary team professionals understand the problem and the role they have in helping to prevent and manage vector-borne and other pathogens in relocated cats.

Graphical Abstract

Dirofilaria immitis: Genotyping Randomly Selected European Clinical Samples and USA Laboratory Isolates with Molecular Markers Associated with Macrocyclic Lactone Susceptibility and Resistance

Dirofilaria immitis is a parasitic nematode and causes dirofilariosis, a potentially fatal pulmonary infection which primarily infects canids. Dirofilariosis infections are controlled via prophylactic macrocyclic lactone (ML) regimens. Recent evidence has confirmed the development of ML-resistant isolates in the USA, which are genetically distinct from wildtype populations. Single nucleotide polymorphisms (SNP) associated with ML-resistant phenotypes were clinically validated in USA populations. In this study, 3 USA laboratory-maintained isolates (Berkeley, Georgia II, and WildCat) and 11 randomly selected European clinical samples from 7 hosts were analyzed. The samples tested were fresh microfilaria (mf) in blood or adult worms preserved in ethanol. The samples underwent MiSeq sequencing of the top 9 SNP associated with ML resistance. The results provide the first genotypic analysis of the three USA laboratory-maintained isolates and any European samples. The European clinical samples show no genomic evidence of ML resistance based on the 9 SNP. The early adoption of genotyping of clinical D. immitis samples could provide an early indication of the potential development of ML resistance and aid to distinguish clinical cases of heartworm infection due to ML resistance from those due to a lack compliance with the recommended treatments, as has been seen in North America.

De Novo Assembly of the Dirofilaria immitis Genome by Long-Read Nanopore-Based Sequencing Technology on an Adult Worm from a Canine Cardiopulmonary Dirofilariosis Case

Simple Summary

Dirofilaria immitis is a zoonotic parasite that infects canids and other vertebrates. We expanded the use of long-read nanopore-based sequencing technology by performing genomic de novo assembly of a D. immitis specimen retrieved from a canine cardiopulmonary dirofilariasis case by using the ONT MinION platform. We also identified loci previously characterized as being associated to macrocyclic lactone resistance selection pressure. The identification of a resistant zoonotic parasite alerts for the overuse of macrocyclic lactone in the region.

Abstract

Dirofilaria immitis is a zoonotic parasitic nematode that infects domestic and wild canids, among its vertebrate hosts. The genetic analysis of D. immitis nowadays transcends the need for genetic taxonomy of nematodes, such as the study of resistance to macrocyclic lactone. We expanded the use of long-read nanopore-based sequencing technology on nematodes by performing genomic de novo assembly of a D. immitis specimen retrieved from a canine cardiopulmonary dirofilariasis case using the ONT MinION platform, followed by the study of macrocyclic lactone resistance. The assembled genome of D. immitis consists of 110 contigs with an N50 of 3687191. The genome size is 87899012 and contains a total of 9741 proteins; 6 ribosomal RNAs, with three belonging to the small subunit (18S) and three to the large subunit (28S); and 73 tRNAs. Subsequent analysis of six loci previously characterized as being associated to macrocyclic lactone resistance selection pressure showed that four have a genotype associated with either some loss of efficacy or the resistance phenotype. Considering the zoonotic potential of D. immitis, the identification of a resistant parasite alerts for the overuse of macrocyclic lactone in the region, which poses a potential risk to both veterinary and human public health.