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Tag: Adrian Wolstenholme

Ghosts of symbionts past: The hidden history of the dynamic association between filarial nematodes and their Wolbachia endosymbionts

Donna Huber on October 1, 2025

graphical abstract

Many, but not all, parasitic filarial nematodes (Onchocercidae) carry intracellular, maternally-transmitted, alphaproteobacterial Wolbachia symbionts. The association between filarial nematodes and Wolbachia is often portrayed as mutualist, where the nematode is reliant on Wolbachia for an essential but unknown service. Wolbachia are targets for anti-filarial chemotherapeutic interventions for human disease. Wolbachia of Onchocercidae derive from four of the major supergroups (C, D, F and J) defined within the genus. We explored the evolutionary history of the filarial nematode-Wolbachia symbiosis in twenty-two nematode species, sixteen of which have current Wolbachia infections, by screening the nematode nuclear genome sequences for nuclear Wolbachia transfers, fragments of the Wolbachia genome that have been inserted into the nuclear genome. We identified Wolbachia insertions in five of the six species that have no current Wolbachia infection, showing they have previously had and have now lost Wolbachia infections. In currently-infected species we found a diversity of origins of the insertions, including many cases where they derived from a different supergroup to the current live infection. Mapping the origins of the insertions onto the filarial nematode phylogeny we derive a complex model of evolution of Wolbachia symbiosis. The history of association between Wolbachia and onchocercid nematodes includes not only cospeciation, as would be expected from a mutualist symbiosis, but also loss (in the five Wolbachia-free species), frequent symbiont replacement, and dual infection. This dynamic pattern is challenging to models that assume host-symbiont mutualism.

Emmelien Vancaester, Guy R Oldrieve, Alex Reid, Georgios Koutsovoulos, Dominik R Laetsch, Benjamin L Makepeace, Vincent Tanya, Sven Poppert, Jürgen Krücken, Adrian Wolstenholme, Mark Blaxter. G3 (Bethesda). 2025 Oct 1:jkaf226. doi: 10.1093/g3journal/jkaf226.

Evaluation of the in vitro susceptibility of various filarial nematodes to emodepside

Donna Huber on July 28, 2021

Filariae are vector-borne nematodes responsible for an enormous burden of disease. Human lymphatic filariasis, caused by Wuchereria bancrofti, Brugia malayi, and Brugia timori, and onchocerciasis (caused by Onchocerca volvulus) are neglected parasitic diseases of major public health significance in tropical regions. To date, therapeutic efforts to eliminate human filariasis have been hampered by the lack of a drug with sufficient macrofilaricidal and/or long-term sterilizing effects that is suitable for use in mass drug administration (MDA) programs, particularly in areas co-endemic with Loa loa, the causative agent of loiasis. Emodepside, a semi-synthetic cyclooctadepsipeptide, has been shown to have broad-spectrum efficacy against gastrointestinal nematodes in a variety of mammalian hosts, and has been approved as an active ingredient in dewormers for cats and dogs. This paper evaluates, compares (where appropriate) and summarizes the in vitro effects of emodepside against a range of filarial nematodes at various developmental stages. Emodepside inhibited the motility of all tested stages of filariae frequently used as surrogate species for preclinical investigations (Acanthocheilonema viteae, Brugia pahangi, Litomosoides sigmodontis, Onchocerca gutturosa, and Onchocerca lienalis), human-pathogenic filariae (B. malayi) and filariae of veterinary importance (Dirofilaria immitis) in a concentration-dependent manner. While motility of all filariae was inhibited, both stage- and species-specific differences were observed. However, whether these differences were detected because of stage- and/or species-specific factors or as a consequence of variations in protocol parameters among the participating laboratories (such as purification of the parasites, read-out units, composition of media, incubation conditions, duration of incubation etc.) remains unclear. This study, however, clearly shows that emodepside demonstrates broad-spectrum in vitro activity against filarial nematode species across different genera and can therefore be validated as a promising candidate for the treatment of human filariases, including onchocerciasis and lymphatic filariasis.

Marc P Hübner, Simon Townson, Suzanne Gokool, Senyo Tagboto, Mary J Maclean, Guilherme G Verocai, Adrian J Wolstenholme, Stefan J Frohberger, Achim Hoerauf, Sabine Specht, Ivan Scandale, Achim Harder, Martin Glenschek-Sieberth, Steffen R Hahnel, Daniel Kulke. Int J Parasitol Drugs Drug Resist. 2021 Jul 28;17:27-35. doi: 10.1016/j.ijpddr.2021.07.005.

Lack of detectable short-term effects of a single dose of ivermectin on the human immune system

Donna Huber on June 5, 2021

Background: Ivermectin is widely used in human and animal medicine to treat and prevent parasite nematode infections. It has been suggested that its mode of action requires the host immune system, as it is difficult to reproduce its clinical efficacy in vitro. We therefore studied the effects of a single dose of ivermectin (Stromectol®-0.15 mg/kg) on cytokine levels and immune cell gene expression in human volunteers. This dose reduces bloodstream microfilariae rapidly and for several months when given in mass drug administration programmes.

Methods: Healthy volunteers with no travel history to endemic regions were given 3-4 tablets, depending on their weight, of either ivermectin or a placebo. Blood samples were drawn immediately prior to administration, 4 h and 24 h afterwards, and complete blood counts performed. Serum levels of 41 cytokines and chemokines were measured using Luminex® and expression levels of 770 myeloid-cell-related genes determined using the NanoString nCounter®. Cytokine levels at 4 h and 24 h post-treatment were compared to the levels pre-treatment using simple t tests to determine if any individual results required further investigation, taking p = < 0.05 as the level of significance. NanoString data were analysed on the proprietary software, nSolver™.

Results: No significant differences were observed in complete blood counts or cytokine levels at either time point between people given ivermectin versus placebo. Only three genes showed a significant change in expression in peripheral blood mononuclear cells 4 h after ivermectin was given; there were no significant changes 24 h after drug administration or in polymorphonuclear cells at either time point. Leukocytes isolated from those participants given ivermectin showed no difference in their ability to kill Brugia malayi microfilariae in vitro.

Conclusions: Overall, our data do not support a direct effect of ivermectin, when given at the dose used in current filarial elimination programmes, on the human immune system. Trial registration ClinicalTrials.gov NCT03459794 Registered 9th March 2018, Retrospectively registered https://clinicaltrials.gov/ct2/show/NCT03459794?term=NCT03459794&draw=2&rank=1 .

Natalie E Wilson, Barbara J Reaves, Adrian J Wolstenholme. Parasit Vectors. 2021 Jun 5;14(1):304. doi: 10.1186/s13071-021-04810-6.

Evaluation of changes in drug susceptibility and population genetic structure in Haemonchus contortus following worm replacement as a means to reverse the impact of multiple-anthelmintic resistance on a sheep farm

Donna Huber on February 18, 2021

A population of Haemonchus contortus that was highly resistant to benzimidazoles and avermectin/milbemycins with a subpopulation that was resistant to levamisole, was replaced with a susceptible laboratory isolate of H. contortus in a flock of sheep. The anthelmintic susceptibility and population genetics of the newly established population were evaluated for 3.5 years using in vivo, in vitro, and molecular methods. Successful replacement of the resistant population with a susceptible population was confirmed using phenotypic and genotypic measurements; larval development assay indicated full anthelmintic susceptibility; albendazole treatment yielded 98.7% fecal egg count reduction; pyrosequence genotyping of single nucleotide polymorphisms in positions 167 and 200 of the isotype-1 beta tubulin gene were present at 0.0 and 1.7%, respectively; microsatellite genotyping indicated the background haplotype was similar to the susceptible isolate; and haplotypes of the isotype-1 beta tubulin gene were similar to the susceptible isolate. To sustain the susceptibility of the new population, targeted selective treatment was implemented using albendazole. Surprisingly, within 1.5 years post-replacement, the population reverted to a resistant phenotype. Resistance to albendazole, ivermectin, and moxidectin was confirmed via fecal egg count reduction test, larval development assay, and pyrosequencing-based genotyping. Targeted selective treatment was then carried out using levamisole. However, within one year, resistance was detected to levamisole. Population genetics demonstrated a gradual change in the genetic structure of the population until the final population was similar to the initial resistant population. Genetic analyses showed a lack of diversity in the susceptible isolate, suggesting the susceptible isolate had reduced environmental fitness compared to the resistant population, providing a possible explanation for the rapid reversion to resistance. This work demonstrates the power of combining molecular, in vitro, and in vivo assays to study phenotypic and genotypic changes in a field population of nematodes, enabling improved insights into the epidemiology of anthelmintic resistance.

Melissa M George, Adriano F Vatta, Sue B Howell, Bob E Storey, Ciaran J McCoy, Adrian J Wolstenholme, Elizabeth M Redman, John S Gilleard, Ray M Kaplan. International Journal for Parasitology: Drugs and Drug Resistance. 15, April 2021, 134-143. https://doi.org/10.1016/j.ijpddr.2021.02.004

Comparative sequences of the Wolbachia genomes of drug-sensitive and resistant isolates of Dirofilaria immitis

Donna Huber on September 15, 2020

The recent identification of isolates of D. immitis with confirmed resistance to the macrocyclic lactone preventatives presents an opportunity for comparative genomic studies using these isolates, and examining the genetic diversity within and between them. We studied the genomes of Wolbachia endosymbionts of five isolates of D. immitis maintained at the University of Georgia. Missouri and Georgia-2 are maintained as drug susceptible isolates, and JYD-27, Yazoo-2013 and Metairie-2014 are resistant to the macrocyclic lactone preventatives. We used whole genome amplification followed by Illumina-based sequencing from 8 to 12 individual microfilariae from each of the five isolates, obtaining a depth of coverage of approximately 40–75 fold for each. The Illumina sequences were used to create new genome assemblies for all the Wolbachia isolates studied. Comparisons of the Wolbachia sequences revealed more than 3000 sequence variations in each isolate. We identified 67 loci specific in resistant isolates but not in susceptible isolates, including 18 genes affected. Phylogenetic analysis suggested that the endosymbionts of the drug-susceptible isolates are more closely related to each other than to those from any of the resistant parasites. This level of variation in the Wolbachia endosymbionts of D. immitis isolates suggests a potential for selection for resistance against drugs targeting them.

Pei-Tsz Shin, Rodrigo de Paula Baptista, Connor M. O’Neill, Connor Wallis, Barbara J.Reaves, Adrian J. Wolstenholme. Veterinary Parasitology, Volume 286, October 2020, 109225. https://doi.org/10.1016/j.vetpar.2020.109225

Using Population Genetics to Examine Relationships of Dirofilaria Immitis Based on Both Macrocyclic Lactone-Resistance Status and Geography

Donna Huber on May 23, 2020

Prevention of infection with canine heartworm (Dirofilaria immitis) is based on the compliant administration of macrocyclic lactone (ML) drugs. Resistance to ML drugs is well documented in D. immitis; however, there remains a paucity of information on the spatial distribution and prevalence of resistant isolates. This project aims to improve understanding of ML-resistance by using a population genetic approach. We developed a large panel of microsatellite loci and identified 12 novel highly polymorphic markers. These 12, and five previously published markers were used to screen pools of microfilariae from 16 confirmed drug-susceptible, 25 confirmed drug-resistant, and from 10 suspected drug-resistant field isolates. In isolates where microfilarial suppression testing indicated resistance, Spatial Principal Component Analysis (sPCoA), Neighbor Joining Trees and Bayesian clustering all revealed high genetic similarity between pre- and post-treatment samples. Somewhat surprisingly, the Neighbor Joining tree and sPCoA generated using pairwise Nei’s distances did not reveal clustering for resistant isolates, nor did it reveal state-level geographic clustering from samples collected in Georgia, Louisiana or Mississippi. In contrast, Discriminant Analysis of Principle Components was able to discriminate between susceptible, suspected-resistant and resistant samples. However, no resistance-associated markers were detected, and this clustering was driven by the combined effects of multiple alleles across multiple loci. Additionally, we measured unexpectedly large genetic distances between different passages of laboratory strains that originated from the same source infection. This finding strongly suggests that the genetic makeup of laboratory isolates can change substantially with each passage, likely due to genetic bottlenecking. Taken together, these data suggest greater than expected genetic variability in the resistant isolates, and in D. immitis overall. Our results also suggest that microsatellite genotyping lacks the sensitivity to detect a specific genetic signature for resistance. Future investigations using genomic analyses will be required to elucidate the genetic relationships of ML-resistant isolates.

Julie Sanchez, Guha Dharmarajan, Melissa M. George, Cassan Pulaski, Adrian J. Wolstenholme, John S. Gilleard, Ray M. Kaplan. 2020 Veterinary Parasitology; 283:109125. doi: 10.1016/j.vetpar.2020.109125.

EAT-18 is an essential auxiliary protein interacting with the non-alpha nAChR subunit EAT-2 to form a functional receptor

Donna Huber on April 3, 2020

Nematode parasites infect approximately 1.5 billion people globally and are a significant public health concern. There is an accepted need for new, more effective anthelmintic drugs. Nicotinic acetylcholine receptors on parasite nerve and somatic muscle are targets of the cholinomimetic anthelmintics, while glutamate-gated chloride channels in the pharynx of the nematode are affected by the avermectins. Here we describe a novel nicotinic acetylcholine receptor on the nematode pharynx that is a potential new drug target. This homomeric receptor is comprised of five non-α EAT-2 subunits and is not sensitive to existing cholinomimetic anthelmintics. We found that EAT-18, a novel auxiliary subunit protein, is essential for functional expression of the receptor. EAT-18 directly interacts with the mature receptor, and different homologs alter the pharmacological properties. Thus we have described not only a novel potential drug target but also a new type of obligate auxiliary protein for nAChRs.

Shivani Choudhary, Samuel K. Buxton, Sreekanth Puttachary, Saurabh Verma, Gunnar R. Mair, Ciaran J. McCoy, Barbara J. Reaves, Adrian J. Wolstenholme, Richard J. Martin, Alan P. Robertson. PLoS Pathog. 2020 Apr 3;16(4):e1008396. doi: 10.1371/journal.ppat.1008396.

Macrocyclic lactone anthelmintic-induced leukocyte binding to Dirofilaria immitis microfilariae: Influence of the drug resistance status of the parasite

Donna Huber on April 26, 2019

The macrocyclic lactone anthelmintics are the only class of drug currently used to prevent heartworm disease. Their extremely high potency in vivo is not mirrored by their activity against Dirofilaria immitis larvae in vitro, leading to suggestions that they may require host immune functions to kill the parasites. We have previously shown that ivermectin stimulates the binding of canine peripheral blood mononuclear cells (PBMCs) and polymorphonuclear leukocytes (PMNs) to D. immitis microfilariae (Mf). We have now extended these studies to moxidectin and examined the ability of both drugs to stimulate canine PBMC and PMN attachment to Mf from multiple strains of D. immitis, including two that are proven to be resistant to ivermectin in vivo. Both ivermectin and moxidectin significantly increased the percentage of drug-susceptible parasites with cells attached at very low concentrations (<10 nM), but much higher concentrations of ivermectin (>100 nM) were required to increase the percentage of the two resistant strains, Yazoo-2013 and Metairie-2014, with cells attached. Moxidectin increased the percentage of the two resistant strains with cells attached at lower concentrations (<10 nM) than did ivermectin. The attachment of the PBMCs and PMNs did not result in any parasite killing in vitro. These data support the biological relevance of the drug-stimulated attachment of canine leukocytes to D. immitis Mf and suggest that this phenomenon is related to the drug resistance status of the parasites.

Tessa Berrafato, Ruby Coates, Barbara J. Reaves, Daniel Kulke, Adrian J. Wolstenholme. 2019. Int J Parasitol Drugs Drug Resist.; 10:45-50. doi: 10.1016/j.ijpddr.2019.04.004.

Efficacy and side effects of doxycycline versus minocycline in the three-dose melarsomine canine adulticidal heartworm treatment protocol

Donna Huber on December 27, 2018

Abstract

Background: The American Heartworm Society currently recommends the use of a monthly macrocyclic lactone, a 28-day course of 10 mg/kg doxycycline BID, and the 3-dose protocol of melarsomine dihydrochloride for the treatment of canine heartworm disease. Doxycycline is necessary for the reduction of the bacterium Wolbachia, found in all heartworm life-stages. Previous price increases and decreasing availability prompted us to evaluate alternative tetracycline antibiotics, i.e. minocycline, for the reduction of Wolbachia during canine heartworm treatment.

Methods: Thirty-two heartworm-positive dogs were randomized to receive 10 mg/kg or 5 mg/kg of either doxycycline or minocycline for 28 days BID, for a total of 8 dogs per experimental group. All dogs received 6 months of Heartgard Plus® (ivermectin/pyrantel) and the 3-dose protocol of 2.5 mg/kg melarsomine dihydrochloride. Blood samples were collected prior to the initiation of treatment, every 7 days throughout tetracycline treatment, and then monthly thereafter until the dog tested negative for the presence of heartworm antigen. DNA was isolated from circulating microfilarial samples and qPCR was performed on each sample.

Results: A greater number of dogs in the 10 mg/kg doxycycline and minocycline treated groups experienced gastrointestinal side effects as compared to the 5 mg/kg doxycycline and minocycline treated groups. All eight dogs in the 10 mg/kg doxycycline-treated group tested negative for the presence of Wolbachia DNA by 28 days post-tetracycline treatment. A total of two dogs in both the 5 mg/kg doxycycline- and 10 mg/kg minocycline-treated groups and three dogs in the 5 mg/kg minocycline-treated group remained positive for the presence of Wolbachia DNA by the end of tetracycline treatment.

Conclusions: No lung pathology was assessed in this clinical trial, therefore the clinical effect of the remaining Wolbachia DNA in the 10 mg/kg minocycline-, 5 mg/kg doxycycline- and 5 mg/kg minocycline-treated groups cannot be determined. Owner compliance in the proper administration of these tetracyclines may be impacted by the increased severe gastrointestinal side effects reported for the 10 mg/kg doxycycline- and minocycline-treated groups. We recommend that veterinarians prescribe the recommended 10 mg/kg doxycycline for canine heartworm treatment and reduce the dosage to 5 mg/kg in cases of severe gastrointestinal side effects in order to improve owner compliance in administration of medications.

Molly D. Savadelis, Katherine M. Day, Jenna L. Bradner, Adrian J. Wolstenholme, Michael T. Dzimianski, and Andrew R. Moorhead. 2018. Parasites & Vectors; 11:671. https://doi.org/10.1186/s13071-018-3264-z