Comparative efficacy of moxidectin and ivermectin against hypobiotic and encysted cyathostomes and other equine parasites

BEVA primary care clinical guidelines: Equine parasite control

BACKGROUND

There is a lack of consensus on how best to balance our need to minimise the risk of parasite-associated disease in the individual horse, with the need to limit the use of anthelmintics in the population to preserve their efficacy through delaying further development of resistance.

OBJECTIVES

To develop evidence-based guidelines utilising a modified GRADE framework.

METHODS

A panel of veterinary scientists with relevant expertise and experience was convened. Relevant research questions were identified and developed with associated search terms being defined. Evidence in the veterinary literature was evaluated using the GRADE evidence-to-decision framework. Literature searches were performed utilising CAB abstracts and PubMed. Where there was insufficient evidence to answer the research question the panel developed practical guidance based on their collective knowledge and experience.

RESULTS

Search results are presented, and recommendation or practical guidance were made in response to 37 clinically relevant questions relating to the use of anthelmintics in horses.

MAIN LIMITATIONS

There was insufficient evidence to answer many of the questions with any degree of certainty and practical guidance frequently had to be based upon extrapolation of relevant information and the panel members' collective experience and opinions.

CONCLUSIONS

Equine parasite control practices and current recommendations have a weak evidence base. These guidelines highlight changes in equine parasite control that should be considered to reduce the threat of parasite-associated disease and delay the development of further anthelmintic resistance.

A systematic review and meta-analysis on the current status of anthelmintic resistance in equine nematodes: A global perspective

BACKGROUND

The intensive application of anthelmintics in equine has led to considerable resistance in cyathostomins and Parascaris equorum. It has been well documented that benzimidazole (BZ) and pyrantel resistance is widespread in cyathostomins and Parascaris equorum. Since no new classes of anthelmintic have been introduced in the last 40 years, it is critical to be aware of the current risk factors of anthelmintic application to avoid further resistance.

OBJECTIVE

To review the factors affecting the level of anthelmintics resistance in equine around the world, type of anthelmintics, mode of application, dosage, nematode species, and location of anthelmintics application were evaluated and summarized.

DESIGN/PROCEDURE

A systematic review and meta-analyses following the PRISMA Framework were conducted to identify, evaluate, and synthesize primary literature reporting the efficacy of anthelmintic drugs in equines. Information on the bibliographic data, anthelmintic drugs, animals, continents, parasite genera, type of anthelmintics, and dosage was collected. Nonparametric tests (Kruskal-Wallis and Mann-Whitney) were used in SPSS (v.27) to investigate the association between variables. Factors that have a significant impact on efficacy have been subjected to binary logistic regression. Six meta-analyses were conducted in Microsoft Excel (2021) to qualify current resistance issues of the three major anthelmintics classes.

RESULTS

The final database was composed of 60 articles published between 1994 and 2022 with a total of 11835 animals. Anthelmintic class as well as anthelmintic active principle selection did have a significant effect on resistance (P < 0.01), whilst no correlation of the type of anthelmintics, mode of application, and dosage with efficacy were found. Anthelmintics resistance in ascarid was significantly more severe than in strongyle (P < 0.01). Macrocyclic lactone (ML) class and the benzimidazole and probenzimidazole (BP) class have the lowest efficacy against ascarid and strongyle, respectively (67.83% and 69.85%). The effect of location (by continent) also had a significant influence on the resistance of the ML class (P < 0.01). The resistance of the BP class which is the most prevalently applied was demonstrated in all six continents. Binary logistic regression revealed that parasite genera and drug class independently influenced the presence of drug resistance. The forest plots included in this study did not show a significant difference over time.

CONCLUSION

Current evidence indicated that anthelmintics resistance of ML and BP class were common in ascarid and strongyle. A combination of anthelmintics may reduce anthelmintics resistance, but multi-drug resistance may be a concern. Customerised anthelmintics strategy could help reduce resistance.

Validation of a serum ELISA test for cyathostomin infection in equines

Cyathostomins are ubiquitous equine nematodes. Infection can result in larval cyathostominosis due to mass larval emergence. Although faecal egg count (FEC) tests provide estimates of egg shedding, these correlate poorly with burden and provide no information on mucosal/luminal larvae. Previous studies describe a serum IgG(T)-based ELISA (CT3) that exhibits utility for detection of mucosal/luminal cyathostomins. Here, this ELISA is optimised/validated for commercial application using sera from horses for which burden data were available. Optimisation included addition of total IgG-based calibrators to provide standard curves for quantification of antigen-specific IgG(T) used to generate a CT3-specific 'serum score' for each horse. Validation dataset results were then used to assess the optimised test's performance and select serum score cut-off values for diagnosis of burdens above 1000, 5000 and 10,000 cyathostomins. The test demonstrated excellent performance (Receiver Operating Characteristic Area Under the Curve values >0.9) in diagnosing infection, with >90% sensitivity and >70% specificity at the selected serum score cut-off values. CT3-specific serum IgG(T) profiles in equines in different settings were assessed to provide information for commercial test use. These studies demonstrated maternal transfer of CT3-specific IgG(T) in colostrum to newborns, levels of which declined before increasing as foals consumed contaminated pasture. Studies in geographically distinct populations demonstrated that the proportion of horses that reported as test positive at a 14.37 CT3 serum score (1000-cyathostomin threshold) was associated with parasite transmission risk. Based on the results, inclusion criteria for commercial use were developed. Logistic regression models were developed to predict probabilities that burdens of individuals are above defined thresholds based on the reported serum score. The models performed at a similar level to the serum score cut-off approach. In conclusion, the CT3 test provides an option for veterinarians to obtain evidence of low cyathostomin burdens that do not require anthelmintic treatment and to support diagnosis of infection.

The first report of macrocyclic lactone resistant cyathostomins in the UK

In recent years, resistance to the benzimidazole (BZ) and tetrahydropyrimidine (PYR) anthelmintics in global cyathostomin populations, has led to reliance on the macrocyclic lactone drugs (ML-of which ivermectin and moxidectin are licensed in horses) to control these parasites. Recently, the first confirmed case of resistance to both ivermectin (IVM) and moxidectin (MOX) was reported in the USA in yearlings imported from Ireland. This suggests that ML resistance in cyathostomins has emerged, and raises the possibility that regular movement of horses may result in rapid spread of ML resistant cyathostomins. Resistance may go undetected due to a lack of surveillance for ML efficacy. Here, we report anthelmintic efficacies in cyathostomins infecting UK Thoroughbreds on four studs. Faecal egg count reduction tests (FECRT) were performed to define resistance (resistance = FECR <95% lower credible interval (LCI) < 90%). Stud A yearlings had FECRs of 36.4-78.6% (CI:15.7-86.3) after three IVM treatments, 72.6% (CI: 50.8-85.2) after MOX, and 80.8% (CI: 61.9-90.0) after PYR. Mares on stud A had a FECR of 97.8% (CI: 93.3-99.9) and 98% (95.1-99.4) after IVM and MOX treatment, respectively. Resistance to MLs was not found in yearlings or mares on studs B, C or D with FECR after MOX OR IVM treatment ranging from 99.8 to 99.9% (95.4-100); although yearlings on studs B, C and D all had an egg reappearance period (ERP) of six weeks for MOX and stud C had a four-week ERP for IVM. This study describes the first confirmed case of resistance to both licensed ML drugs on a UK Thoroughbred stud and highlights the urgent need for a) increased awareness of the threat of ML resistant parasites infecting horses, and b) extensive surveillance of ML efficacy against cyathostomin populations in the UK, to gauge the extent of the problem.

Species interactions, stability, and resilience of the gut microbiota - Helminth assemblage in horses

The nature and strength of interactions entertained among helminths and their host gut microbiota remain largely unexplored. Using 40 naturally infected Welsh ponies, we tracked the gut microbiota-cyathostomin temporal dynamics and stability before and following anthelmintic treatment and the associated host blood transcriptomic response. High shedders harbored 14 species of cyathostomins, dominated by Cylicocyclus nassatus. They exhibited a highly diverse and temporal dynamic gut microbiota, with butyrate-producing Clostridia likely driving the ecosystem steadiness and host tolerance toward cyathostomins infection. However, anthelmintic administration sharply bent the microbial community. It disrupted the ecosystem stability and the time-dependent network of interactions, affecting longer term microbial resilience. These observations highlight how anthelmintic treatments alter the triangular relationship of parasite, host, and gut microbiota and open new perspectives for adding nutritional intervention to current parasite management strategies.

Egg reappearance periods of anthelmintics against equine cyathostomins: The state of play revisited

Cyathostomins are the most common and highly prevalent parasites of horses worldwide. Historically, the control of cyathostomins has mainly relied on the routine use of anthelmintic products. Increasing reports on anthelmintic resistance (AR) in cyathostomins are concerning. A potential method proposed for detecting emerging AR in cyathostomins has been estimating the egg reappearance period (ERP). This paper reviews the data available for the ERP of cyathostomins against the three major classes of anthelmintics, macrocyclic lactones, tetrahydropyrimidines, and benzimidazoles. Published peer-reviewed original research articles were obtained from three databases (PubMed, CAB Direct and Web of Science) and were evaluated for their inclusion in a systematic review. Subsets of articles were then subjected to a review of ERP data. A total of 54 (of 134) studies published between 1972 and 2022 met the criteria for inclusion in the systematic review. Until the beginning of 2022, there was no agreed definition of the ERP; eight definitions of ERP were identified in the literature, complicating the comparison between studies. Additionally, potential risk factors for the shortening of the ERP, including previous anthelmintic use and climate, were frequently not described. Reports of shortened ERP for moxidectin and ivermectin are frequent: 20 studies that used comparable ERP definitions reported shortened moxidectin and ivermectin ERPs of 35 and 28 days, respectively. It is unclear whether the ERPs of these anthelmintics reduced to such levels are due to the development of AR or some biological factors related to horses, cyathostomin species, and/or the environment. The ERPs for other anthelmintics, such as fenbendazole and pyrantel, were frequently not reported due to established resistance against these drugs. Future research in horses is required to understand the mechanism(s) behind the shortening of ERP for cyathostomins. Based on this systematic review, we propose recommendations for future ERP studies.

Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition

Macrocyclic lactones have been the most widely used drugs for equine parasite control during the past four decades. Unlike ivermectin, moxidectin exhibits efficacy against encysted cyathostomin larvae, and is reported to have persistent efficacy with substantially longer egg reappearance periods. However, shortened egg reappearance periods have been reported recently for both macrocyclic lactones, and these findings have raised several questions: (i) are egg reappearance period patterns different after ivermectin or moxidectin treatment? (ii) Are shortened egg reappearance periods associated with certain cyathostomin species or stages? (iii) How does moxidectin's larvicidal efficacy affect egg reappearance period? To address these questions, 36 horses at pasture, aged 2-5 years old, were randomly allocated to three treatment groups: 1, moxidectin; 2, ivermectin; and 3, untreated control. Strongylid fecal egg counts were measured on a weekly basis, and the egg reappearance period was 5 weeks for both compounds. Strongylid worm counts were determined for all horses: 18 were necropsied at 2 weeks post-treatment (PT), and the remaining 18 at 5 weeks PT. Worms were identified to species morphologically and by internal transcribed spacer-2 (ITS-2) rDNA metabarcoding. Moxidectin and ivermectin were 99.9% and 99.7% efficacious against adults at 2 weeks post treatment, whereas the respective efficacies against luminal L4s were 84.3% and 69.7%. At 5 weeks PT, adulticidal efficacy was 88.3% and 57.6% for moxidectin and ivermectin, respectively, while the efficacy against luminal L4s was 0% for both drugs. Moxidectin reduced early L3 counts by 18.1% and 8.0% at 2 or 5 weeks, while the efficacies against late L3s and mucosal L4s were 60.4% and 21.2% at the same intervals, respectively. The luminal L4s surviving ivermectin treatment were predominantly Cylicocyclus (Cyc.) insigne. The ITS-2 rDNA metabarcoding was in good agreement with morphologic species estimates but suggested differential activity between moxidectin and ivermectin for several species, most notably Cyc. insigne and Cylicocyclus nassatus. This study was a comprehensive investigation of current macrocyclic lactone efficacy patterns and provided important insight into potential mechanisms behind shortened egg reappearance periods.

Anthelmintic resistance in equine nematodes: Current status and emerging trends

Anthelmintic resistance is reported in equine nematodes with increasing frequency in recent years, and no new anthelmintic classes have been introduced during the past 40 years. This manuscript reviews published literature describing anthelmintic resistance in cyathostomins, Parascaris spp., and Oxyuris equi with special emphasis on larvicidal efficacy against encysted cyathostomin larvae and strongylid egg reappearance periods (ERP). Resistance to benzimidazoles and pyrimidines is highly prevalent in cyathostomin populations around the world, and macrocyclic lactone resistance has been documented in cyathostomins in recent years as well. Two recent studies have documented resistance to the larvicidal regimen of fenbendazole, whereas the larvicidal efficacy of moxidectin is variable, but with no evidence of a reduction from historic levels. In the 1990s, ERP estimates were 8-10 and 12-16 weeks for ivermectin and moxidectin, respectively, while several studies published after year 2000 found ERPs to be 5 weeks for both compounds. This is a clear change in anthelmintic performance, but it remains unclear if this is due to development of anthelmintic resistance or selection for other biological traits leading to a quicker resumption of strongylid egg shedding following anthelmintic treatment. Macrocyclic lactone resistance is common in Parascaris spp. around the world, but recent reports suggests that resistance to the two other classes should be monitored as well. Finally, O. equi has been reported resistant to ivermectin and moxidectin in countries representing four continents. In conclusion, multi-drug resistance is becoming the norm in managed cyathostomin populations around the world, and a similar pattern may be emerging in Parascaris spp. More work is required to understand the mechanisms behind the shortened ERPs, and researchers and veterinarians around the world are encouraged to routinely monitor anthelmintic efficacy against equine nematodes.

Integrative biology defines novel biomarkers of resistance to strongylid infection in horses

The widespread failure of anthelmintic drugs against nematodes of veterinary interest requires novel control strategies. Selective treatment of the most susceptible individuals could reduce drug selection pressure but requires appropriate biomarkers of the intrinsic susceptibility potential. To date, this has been missing in livestock species. Here, we selected Welsh ponies with divergent intrinsic susceptibility (measured by their egg excretion levels) to cyathostomin infection and found that their divergence was sustained across a 10-year time window. Using this unique set of individuals, we monitored variations in their blood cell populations, plasma metabolites and faecal microbiota over a grazing season to isolate core differences between their respective responses under worm-free or natural infection conditions. Our analyses identified the concomitant rise in plasma phenylalanine level and faecal Prevotella abundance and the reduction in circulating monocyte counts as biomarkers of the need for drug treatment (egg excretion above 200 eggs/g). This biological signal was replicated in other independent populations. We also unravelled an immunometabolic network encompassing plasma beta-hydroxybutyrate level, short-chain fatty acid producing bacteria and circulating neutrophils that forms the discriminant baseline between susceptible and resistant individuals. Altogether our observations open new perspectives on the susceptibility of equids to strongylid infection and leave scope for both new biomarkers of infection and nutritional intervention.

Precision and spatial variation of cyathostomin mucosal larval counts

Cyathostomins are pervasive parasites of equids across the world. Larval stages encyst in the mucosa of the cecum, ventral and dorsal colon and can induce an inflammatory response leading to larval cyathostominosis, a life-threatening generalized typhlocolitis. Mucosal digestion is the only gold standard procedure for identifying and quantifying all larval stages. There is a lack of standardization of this technique and several aspects are ambiguous, such as precision of the method and the possibility of spatial variation of mucosal larval counts. The aim of this study was to estimate precision for enumeration of early third stage larvae (EL3) and late third stage/fourth stage (LL3/L4) larvae and investigate spatial variation of encysted counts within large intestinal organs. Six naturally infected and untreated horses aged 2-5 years were euthanized as part of an anthelmintic efficacy study, and the cecum (Cec), ventral colon (VC) and dorsal colon (DC) were collected. Each organ was rinsed, weighed, and visually separated into 3 equally sized sections. Two 5% tissue samples were collected from each section, a total of six replicates per organ. The mucosae were digested, and 2% examined under the microscope for presence of EL3 and LL3/L4 stage larvae. Overall, 59 % of the harvested larvae were EL3s, and 41 % were LL3/L4s. The ventral colons represented 45 % of the total organ weight, and contributed 37 and 41 % of the EL3s and LL3/L4s harvested, respectively. The Cec, representing only 27 % of the weight contributed 23 % of EL3s and 47 % of LL3/L4s. The DC represented 28 % of the total organ weight, and 28 % and 12 % of the total EL3s and LL3/L4s, respectively. Coefficients of variation varied from 33 to 183 % for EL3 counts and 38-245% for LL3/L4 counts. There were no statistically significant associations between EL3 counts and either organ or location. For LL3/L4 counts there were no statistically significant differences between the three locations within organs (p = 0.1166), but the DC had significantly lower counts than the other two organs (p < 0.0001). Increasing the number of mucosal replicates from each organ improved estimation, but required a considerably increased workload. In conclusion, mucosal larval cyathostomin counts are highly variable, complicating their use for treatment efficacy estimation.

Habronematidosis in Equids: Current Status, Advances, Future Challenges

Over the past few decades, among equine parasitoses caused by gastrointestinal nematodes, habronematidosis has been discontinuously studied worldwide. Habronematidosis is a parasitic disease distributed all over the world. It is caused by Habronema microstoma, Habronema muscae, and Draschia megastoma (Spirurida, Habronematidae), and it is maintained in the environment by muscid flies which act as intermediate hosts. At larval and adult stages these species live in the stomach of domestic and wild equids. However, the larvae can also be found on the skin, causing lesions known as “summer sores”, and occasionally on other body areas, such as ocular and genital mucosa (muco-cutaneous habronematidosis) and lung, liver, brain parenchyma. Depending on the parasite's developmental stage and localization site, clinical signs vary from mild to severe. Habronematidosis is responsible for significant economic losses, mostly when sport horses are affected, because their performances are impaired and the infection can be unaesthetic. We used three on-line databases for searching the articles on habronematidosis according to the selected inclusion criteria; a total of 250 contributions, published between 1911 and 2020 were analyzed. This review summarizes the key features of pathogenesis, epizootiology, diagnosis, treatment, and control of habronematidosis, and highlights the current knownledge about its geographical distribution and spread. Anthelmintic drugs are the most widely-used tools against habronematidosis; given the known risk of anthelmintic resistance in some nematodes affecting horses, this aspect should also be explored for habronematidosis. Dedicated research is essential to fill gaps of knowledge and increase the understanding of habronematidosis to maximize equine health, reduce economic losses and sanitary impact associated with this parasitic infection.

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

Strongyle Infection and Gut Microbiota: Profiling of Resistant and Susceptible Horses Over a Grazing Season

Gastrointestinal strongyles are a major threat to horses' health and welfare. Given that strongyles inhabit the same niche as the gut microbiota, they may interact with each other. These beneficial or detrimental interactions are unknown in horses and could partly explain contrasted susceptibility to infection between individuals. To address these questions, an experimental pasture trial with 20 worm-free female Welsh ponies (10 susceptible (S) and 10 resistant (R) to parasite infection) was implemented for 5 months. Fecal egg counts (FEC), hematological and biochemical data, body weight and gut microbiological composition were studied in each individual after 0, 24, 43, 92 and 132 grazing days. R and S ponies displayed divergent immunological profiles and slight differences in microbiological composition under worm-free conditions. After exposure to natural infection, the predicted R ponies exhibited lower FEC after 92 and 132 grazing days, and maintained higher levels of circulating monocytes and eosinophils, while lymphocytosis persisted in S ponies. Although the overall gut microbiota diversity and structure remained similar during the parasite infection between the two groups, S ponies exhibited a reduction of bacteria such as Ruminococcus, Clostridium XIVa and members of the Lachnospiraceae family, which may have promoted a disruption of mucosal homeostasis at day 92. In line with this hypothesis, an increase in pathobionts such as Pseudomonas and Campylobacter together with changes in several predicted immunological pathways, including pathogen sensing, lipid metabolism, and activation of signal transduction that are critical for the regulation of immune system and energy homeostasis were observed in S relative to R ponies. Moreover, S ponies displayed an increase in protozoan concentrations at day 92, suggesting that strongyles and protozoa may contribute to each other's success in the equine intestines. It could also be that S individuals favor the increase of these carbohydrate-degrading microorganisms to enhance the supply of nutrients needed to fight strongyle infection. Overall, this study provides a foundation to better understand the mechanisms that underpin the relationship between equines and natural strongyle infection. The profiling of horse immune response and gut microbiota should contribute to the development of novel biomarkers for strongyle infection.

Anthelmintic therapy of equine cyathostomin nematodes - larvicidal efficacy, egg reappearance period, and drug resistance

Cyathostomins are ubiquitous in grazing horses across the world, and anthelmintic resistance has been reported with increasing levels over past decades. The aims of the present study were (i) to investigate the efficacy against encysted larval stages of moxidectin (0.4 mg/kg) and fenbendazole (10 mg/kg daily for five consecutive days) and compare these regimens at 2 and 5 weeks post-treatment, (ii) to investigate individual cyathostomin species associated with shortened egg reappearance periods, and (iii) to document species exhibiting decreased susceptibility to the evaluated compounds. Thirty-six ponies were allocated to treatment groups with half euthanatized 2 weeks post-treatment, and the remainder necropsied after 5 weeks. Luminal and mucosal worm counts were conducted and strongyle egg counts were determined at weekly intervals. At 2 weeks, mean reductions of early L3s were 50.4% and 73.8% for fenbendazole and moxidectin, respectively. At 5 weeks, the respective efficacies were 51.3% and 71.8%. Two week efficacies against late L3s and L4s (LL3s/L4s) were 70.8% and 74.6% for fenbendazole and moxidectin, respectively, whereas very low numbers were found in all three groups at 5 weeks. None of the mucosal counts were significantly different between treatment groups. Fenbendazole and moxidectin reduced luminal worm counts by 93.2% and 98.3% at 2 weeks following administration, with moxidectin group adult counts being significantly lower than the other two groups (P < 0.0001). Both treatment groups had increased counts 3 weeks later (P = 0.0415). A moxidectin ERP of 4 weeks was associated with surviving luminal L4s, and adult species contributing to this were Cyathostomum catinatum, Cylicostephanus longibursatus, Cylicocyclus ashworthi and Cylicocyclus nassatus. This study documented (i) larvicidal efficacy of fenbendazole much lower than historical standards, (ii) survival of luminal immatures (L4) following moxidectin administration, and (iii) new information about cyathostomin species associated with these phenomena.

Effect of Moxidectin on Bed Bug Feeding, Development, Fecundity, and Survivorship

The common bed bug, Cimex lectularius L. (Hemiptera: Cimicidae), is a blood-feeding ectoparasite which experienced world-wide resurgence during recent decades. The control of bed bugs is often challenging, due to their cryptic nature and resistance to commonly used insecticides. In this study, we evaluated the effect of the antiparasitic drug moxidectin on bed bug survival, reproduction, and development. The LC50 (lethal concentration to kill half the members of a tested population) of moxidectin against bed bug male adults, female adults, and large nymphs were 52.7 (95% CI (confidence interval): 39.5-70.8), 29.3 (95% CI: 20.7-40.5), and 29.1 ng/mL (95% CI: 23.3-35.3), respectively. Moxidectin (≥ 25 ng/mL) reduced egg laying of bed bug females, but showed no significant effect on egg hatching. One time feeding on rabbit blood containing 20 and 40 ng/mL moxidectin showed no negative effects in bed bug feeding and blood meal ingestion, but significantly reduced digestion rates and nymph molting rates. Although moxidectin at concentrations of 20 and 40 ng/mL only caused moderate mortality in bed bugs, it significantly interrupted digestion, development, and oviposition of survived bed bugs for at least one week after feeding. Moxidectin is a promising supplement of the existing bed bug control materials if its use on humans can be approved in the future.

Strongyle egg reappearance period after moxidectin treatment and its relationship with management factors in UK equine populations

Parasitic nematodes, particularly cyathostomins, are ubiquitous in grazing horses world-wide. Considerable burdens of cyathostomin larvae can encyst in the large intestinal wall. The most recommended treatment against these pathogenic stages is moxidectin. Information is required on how effective moxidectin is against cyathostomin populations in different regions. The objectives here were to determine the efficacy of moxidectin treatment and estimate the strongyle egg reappearance period (ERP) after treatment in several equine populations, to confirm the type of strongyle nematodes present and to identify other (i.e. management) factors associated with shortened ERP. Eight yards were recruited and moxidectin in combination with praziquantel administered to all horses (n=261). Faecal egg count (FEC) analysis was performed at weeks 0, 2, 6, 10 and 12 after treatment to determine efficacy and ERP. The ERP was estimated using two previously published methods. Morphological identification of cultured third stage larvae from the sample population was compared to a Strongylus vulgaris-specific end-point PCR to examine the presence of S. vulgaris in samples before and after treatment. Strongyle egg shedding patterns were also compared to worm management practices at each site. At 2 weeks post-treatment, moxidectin was highly effective (faecal egg count reduction range, 99.9-100%). The strongyle ERP ranged from 6 weeks to >12 weeks depending on the calculation method applied. Only cyathostomin larvae were detected by morphological identification. The results from the coprocultures and PCR showed that S. vulgaris was absent before and after treatment. Analysis revealed that regular faecal removal from pasture was associated with lower average FEC and lower prevalence of egg shedding.

Ivermectin failure in the control of Oxyuris equi in a herd of ponies in France

Drug resistance in equine gastro-intestinal parasitic nematodes has been reported throughout the world. While the focus is usually put on cyathostomins, observations of macrocylic lactone failure against Oxyuris equi have accumulated over the last decade. Here we report the failure of ivermectin in the control of O. equi in an experimental Welsh pony herd. In a first trial, 6 ponies previously drenched with moxidectin and showing patent O. equi infections were administered ivermectin and subsequently monitored for O. equi egg excretion over one month. This trial demonstrated a failure of ivermectin to control O. equi egg excretion as half of ponies demonstrated recurrent egg excretion in the peri-anal region during 21days after treatment. One year later, six female Welsh ponies drenched with moxidectin demonstrated signs of itching and scratching in their peri-anal region with worms being found transiently in fecal materials three weeks later. Ponies were allocated to three treatment groups, i.e. ivermectin, pyrantel embonate and fenbendazole and monitored for egg excretion over five weeks. Fenbendazole and pyrantel embonate broke ivermectin suboptimal efficacy as soon as 8 and 14days respectively after treatment, while egg excretion remained constant throughout the 41-day long trial in the ivermectin-treated ponies. This is the first report of ivermectin failure against O. equi in France. In the absence of critical efficacy test, it remains unclear whether true resistance is at stake or if these observations confound a constitutive suboptimal efficacy of ivermectin against O. equi.

Anthelmintic resistance in equine nematodes

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Overuse of anthelmintics in horses has reduced disease, but led to widespread resistance.

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Benzimidazole, tetrahydropyrimidine and macrocyclic lactone resistance in cyathostomins.

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Ivermectin resistance common in Parascaris equorum.

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Good grazing management must be integrated with targeted anthelmintic treatments.

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Progress in novel diagnostics to define pre-patent worm levels is described.

Anthelmintics have been applied indiscriminately to control horse nematodes for over 40 years. Three broad-spectrum anthelmintic classes are currently registered for nematode control in horses: benzimidazoles (fenbendazole, oxibendazole), tetrahydropyrimidines (pyrantel) and macrocyclic lactones (ivermectin, moxidectin). Generally, control strategies have focused on nematode egg suppression regimens that involve the frequent application of anthelmintics to all horses at intervals based on strongyle egg reappearance periods after treatment. The widespread use of such programmes has substantially reduced clinical disease, especially that associated with large strongyle species; however, high treatment frequency has led to considerable selection pressure for anthelmintic resistance, particularly in cyathostomin species. Field studies published over the last decade indicate that benzimidazole resistance is widespread globally in cyathostomins and there are also many reports of resistance to pyrantel in these worms. Cyathostomin resistance to macrocyclic lactone compounds is emerging, principally measured as a reduction in strongyle egg reappearance time observed after treatment. Ivermectin resistance is a further concern in the small intestinal nematode, Parascaris equorum, an important pathogen of foals. These issues indicate that horse nematodes must now be controlled using methods less dependent on anthelmintic use and more reliant on management practices designed to reduce the force of infection in the environment. Such strategies include improved grazing management integrated with targeted anthelmintic administration involving faecal egg count (FEC)-directed treatments. The latter require that the supporting diagnostic tests available are robust and practically applicable. Recent research has focused on maximising the value of FEC analysis in horses and on optimizing protocols for anthelmintic efficacy testing. Other studies have sought to develop diagnostics that will help define levels of pre-patent infection. This review describes recent advances in each of these areas of research.

Anthelmintic efficacy on UK Thoroughbred stud farms

Anthelmintic drugs have been applied indiscriminately to control horse nematodes for over 40 years. We undertook a comprehensive study to investigate efficacy of the four available broad-spectrum anthelmintic drugs on 16 Thoroughbred stud farms using the faecal egg count reduction test. Efficacy against strongyles was determined by calculating the percentage of reduction in faecal egg count between the group mean at Day 0 and Days 14-17 post-treatment and the 95% lower confidence intervals estimated by non-parametric bootstrapping. Individual strongyle faecal egg count reduction tests (n=429) were performed in which 179, 131, 89 and 30 horses were administered ivermectin, moxidectin, pyrantel and fenbendazole, respectively. Moxidectin was efficacious in all tests (faecal egg count reduction range: 99.8-100%; 95% lower confidence intervals range: 96.8-100%) and reduced efficacy of ivermectin (faecal egg count reduction range: 85.7-100%; 95% lower confidence intervals range: 65-100%) was observed in one group of yearlings. Reduced pyrantel efficacy was observed in five groups of yearlings (faecal egg count reduction range: 0-73%; 95% lower confidence intervals range: 0-59.5%), but pyrantel was found to be efficacious when administered to mares (faecal egg count reduction range: 98-99.4%; 95% lower confidence intervals range: 91.8-99.3%). Low efficacy of fenbendazole was always observed (faecal egg count reduction range: 0.4-41%; 95% lower confidence intervals not calculable). Two further methods for estimating efficacy were applied and outputs obtained using all methodologies were in agreement. Efficacy against Parascaris equorum was assessed on four farms: fenbendazole had acceptable efficacy (faecal egg count reduction range: 97.5-99.9%; 95% lower confidence intervals range: 96.3-99.1%), but reduced efficacy of ivermectin was observed (faecal egg count reduction range: 25.5-91.2%; 95% lower confidence intervals range: 6.7-82.4%). Strongyle faecal egg count were analysed at approximately 2 week intervals for up to 12 weeks after anthelmintic drug administration to determine the egg reappearance period for moxidectin, ivermectin and pyrantel. The egg reappearance period for all three anthelmintic drugs was shorter than previously observed. Overall, our results indicate that ivermectin and moxidectin administration provided acceptable efficacy at 14 days; however, egg reappearance period results suggest that these products are working less effectively than measured previously. As shortened egg reappearance period is believed to be an early indicator of resistance, this highlights the issue of impending multi-drug resistance in strongyles on stud farms.

A xenodiagnostic method using Musca domestica for the diagnosis of gastric habronemosis and examining the anthelmintic efficacy of moxidectin

Equine habronematidosis has a global distribution and is caused by three spirurid species, Habronema muscae, Habronema microstoma and Draschia megastoma. A case of cutaneous habronematidosis in a stallion in a stable in Dubai, UAE gave occasion to investigate the parasite situation on the farm. Patent H. muscae infections were diagnosed in 18 out of 49 horses in a stable in Dubai, UAE with a xenodiagnostic test using houseflies as indicator host. All horses in the stable were treated with a single dose of moxidectin administered orally as 2% gel in a dosage of 0.4 mg/kg body weight and the efficacy of this targeted treatment was studied. Habronema infection was terminated in all horses. A fly survey conducted at the farm prior and after treatment revealed two muscid species: Musca domestica and Stomoxys calcitrans. Only M. domestica caught at the farm showed a natural infection with Habronema larvae prior and shortly after anthelmintic treatment. Later, examination of flies caught at the farm in monthly intervals up to the end of observation (8 months after treatment) gave negative results. The absence of infection in the intermediate host was an indication of the eradication of stomach worms. The described xenodiagnostic test is a useful tool to diagnose H. muscae infections and can be used to evaluate the efficacy of nematocides in equines.

Resistance to avermectin/milbemycin anthelmintics in equine cyathostomins - current situation

Avermectins and milbemycins (AM) are potent compounds against all major nematode parasites, but their continuous usage has led to the development of widespread resistance in many of the important species of ruminant and equine parasites. The exception to this has been the cyathostomins, where AM resistance was recently first reported only after decades of drug exposure. Data from a Brazilian study suggests that AM resistance has developed in cyathostomins and reports of shortened egg reappearance periods after ivermectin treatment have been published recently from USA and Germany. Thus, AM resistance in cyathostomins is an emerging worldwide concern, but there is only limited amount data on the extent of this problem. To limit the development and spread of AM-resistant cyathostomins the equine industry must implement new strategies for worm control, and the veterinary parasitology community must develop and validate improved protocols for detecting anthelmintic resistance in the field.

Equine cyathostomins: a review of biology, clinical significance and therapy

The small strongyles of horses, also known as cyathostomins, are considered the most prevalent and pathogenic parasites of horses today. The clinical syndrome of larval cyathostominosis which occurs as a result of mass emergence of inhibited stages has a high fatality rate despite the best standard of care given to affected horses. Management of the challenge level of cyathostomins to prevent the syndrome is preferable. Many different management programmes have been tried over the past two decades, with mixed success. Programmes have relied heavily on repeated use of anthelmintic treatments throughout the life of a horse. The widespread incidence of resistance to certain anthelmintics is reducing these options. An understanding of the biology of cyathostomins, risk factors for infection and appropriate strategic use of still effective anthelmintics is essential for the future management of this parasite group. This review highlights the necessity to use currently available anthelmintics that are appropriately suited to the biology of cyathostomins, and to maintain heir efficacy through an appropriate treatment strategy.

Probable reason why small strongyle EPG counts are returning "early" after ivermectin treatment of horses on a farm in Central Kentucky

Critical tests were carried out in 2008 in four yearling horses (H-2, H-4, H-10, and H-11) born and raised together on a farm (MC) in Central Kentucky. These horses were treated intraorally with ivermectin paste at the dose rate of 200 microg/kg. The main interest was to try and determine more precisely, from posttreatment (PT) worm count data, the current activity of ivermectin against small strongyles in a horse herd. These horses had been treated repeatedly with this compound and counts of small strongyle eggs per gram of feces (EPGs) of these parasites have been returning sooner than previously in field tests (Lyons et al. Parasitol Res 103:209-215, 2008a). Data from the four horses revealed that a total of 3237 (nonfactored number) specimens of small strongyles was recovered from aliquot samples of feces passed PT and in the large intestinal contents at necropsy; all specimens were examined and identified. Thirteen species of adult small strongyles were recorded. Cylicocyclus (Cyc.) insigne was the predominant species. Three of the yearlings (H-2, H-4, and H-11) were necropsied at 6 days PT with ivermectin, and removals of small strongyles were: 50%, 80%, and 36% of fourth stages (L4), respectively, and 100%, 99%, and 100% of adults, respectively. As indicated, the only incomplete removal of adults from the three horses was for H-4. They consisted of two species: (1) young Cyc. insigne (those passed in the feces were fully developed); removal of this species was 89% and (2) fully developed Cylicostephanus longibursatus; 99% were removed. The fourth yearling (H-10), necropsied at 25 days PT, harbored 19,150 adult small strongyles in the large intestinal contents. Most of the species were Cyc. insigne; all were fifth stage but not sexually mature. Comparing the percentage of adult small strongyles found at necropsy relative to the total number present (those passed in the feces and at necropsy), only 0% to 1% were in the contents of the large intestines of the three horses at 6 days PT but in 26% for horse H-10 at 25 days PT. As mentioned earlier, only a few adults were found in one horse and several L(4) in the three horses at necropsy at 6 days PT. Therefore, in horse H-10, most adults found at 25 days PT presumably developed from "young" specimens not removed by ivermectin. Thus, data from the present critical tests indicate the probable cause of the "early" return of small strongyle EPG values after ivermectin treatment in the horses in field tests on Farm MC. It seems this was the result of incomplete removal of luminal specimens (L(4) and possibly young adults), some of which matured and began laying eggs by about 4 weeks PT (Lyons et al. Parasitol Res 103:209-215, 2008a). The research also showed that ivermectin was highly effective on adult small strongyles. At necropsy, the following other species of parasites (adult) were found, but none was recovered from the feces. These were (n = number of horses infected): (1) ascarids (Parascaris equorum-n = 1), (2) tapeworms (Anoplocephala perfoliata-n = 4), and (3) pinworms (Oxyuris equi-n = 3). Immature (L4) O. equi were present in two horses and removals were 0% in one horse and 39% in the other. Eyeworms (Thelazia lacrymalis) were found in one horse at necropsy. Even though a small number of horses were used in the present research, the commonality of their background made them ideal candidates as a group for this study. This aspect helps strengthen the validity of the interpretation of the findings.

Prescription-only anthelmintics--a questionnaire survey of strategies for surveillance and control of equine strongyles in Denmark

In 1999, legislation in Denmark made anthelmintic drugs available only by prescription, and prohibited their use for routine, prophylactic treatment. A questionnaire survey was conducted in 2004 to determine current strategies for surveillance and control of equine strongyles used in Danish equine veterinary practices. Eighty-seven of 170 (51.2%) registered equine veterinary practices responded. Ninety seven percent of the respondents used faecal egg counts for diagnosis and surveillance, and 41% used larval cultures. Logistic regression revealed that the use of larval cultures was positively correlated with numbers of employees (P = 0.013) and the proportion of equine caseload in the practice (P < 0.000). Performing faecal egg counts and treating horses was most frequent during spring, summer and early autumn. Veterinary practices made treatment decisions based on cut-off values ranging from 20 to 500 eggs per gram (EPG) faeces. Ages of horses, clinical suspicions of parasitic disease, or requests by the owners were the most important factors in the strategies for faecal sampling. Less commonly, sampling strategies included all horses on the premises and random sampling. Ninety five percent of the respondents reported treating certain groups of horses without prior faecal analysis, including horses with clinical signs of parasitic disease (77%), foals (84%), horses less than 3 years of age (52%), and pregnant mares (51%). The respondents regarded the cyathostomin group as the most prevalent cause of parasitic disease and ill-thrift, followed by large strongyles and Parascaris equorum. Sixty seven percent rotated regularly between drugs, while 11% performed routine screening for anthelmintic resistance. Results of this survey suggest that limiting equine anthelmintics to prescription-only availability has increased the level of strongyle surveillance. Veterinary practitioners play a central role in equine parasite management with indications of a lowered intensity of treatment. However, screening for anthelmintic resistance remains uncommon.

Advances in developing molecular-diagnostic tools for strongyloid nematodes of equids: fundamental and applied implications

Infections of equids with parasitic nematodes of the order Strongylida (subfamilies Strongylinae and Cyathostominae) are of major veterinary importance. In last decades, the widespread use of drugs against these parasites has led to problems of resistance within the Cyathostominae, and to an increase in their prevalence and intensity of infection. Novel control strategies, based on improved knowledge of parasite biology and epidemiology, have thus become important. However, there are substantial limitations in the understanding of fundamental biological and systematic aspects of these parasites, which have been due largely to limitations in their specific identification and diagnosis using traditional, morphological approaches. Recently, there has been progress in the development of DNA-based approaches for the specific identification of strongyloids of equids for systematic studies and disease diagnosis. The present article briefly reviews information on the classification, biology, pathogenesis, epidemiology of equine strongyloids and the diagnosis of infections, highlights knowledge gaps in these areas, describes recent advances in the use of molecular techniques for the genetic characterisation, specific identification and differentiation of strongyloids of equids as a basis for fundamental investigations of the systematics, population biology and ecology.

Treatment and follow-up of clinical cyathostominosis in horses

The results of the treatment with moxidectin or ivermectin of 20 horses with clinical cyathostominosis were studied during a 3-week observation period. Both treatments were effective in completely eliminating larvae from the faeces within 1 or 2 weeks, but no significant improvement in body weight or clinical parameters could be demonstrated over the observation period. The poor short time results of the treatment support the need for an adequate prevention of cyathostominosis in horses.

Occurrence of gastrointestinal parasites in horses in metropolitan Perth, Western Australia

OBJECTIVE

To assess the occurrence of gastrointestinal parasites in horses in Perth. To apply polymerase chain reaction (PCR) for the identification of some species of encysted larval cyathostomes.

DESIGN

Between February and September of 2000, the gastrointestinal tracts of 29 horses submitted to a local knackery and Murdoch University Veterinary hospital in Perth were examined post mortem for the presence of gastrointestinal parasites.

PROCEDURE

The gastrointestinal tract was divided into six sections, which were screened for the presence of parasites such as Gasterophilus sp, Anoplocephala sp and Parascaris equorum. Samples of contents were taken for worm counts.

RESULTS

Cyathostomes were found in 28 of the 29 horses. Eighteen species of gastrointestinal helminths were identified. Twelve of these were cyathostomes, with the four most common species being Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus and Cylicostephanus goldi. The large strongyle, Triodontophorus serratus, was found in three of the horses but species of Strongylus were not found.

CONCLUSIONS

In contrast to a study conducted on horses from this region in 1985, cyathostomes were the most common gastrointestinal parasites found. The four most common cyathostome species found in the present study correlated well with findings of studies in other locations. The high number of cyathostomes may be due to the increase in resistance to anthelmintics among the species, and to improper anthelmintic use. The apparent reduction in number of large strongyles may be due to the widespread use of ivermectin, which is very effective against these parasites, and also possibly because some larvae may not have been detected.

Characterisation of IgG(T) serum antibody responses to two larval antigen complexes in horses naturally- or experimentally-infected with cyathostomins

Cyathostomins are the most common parasitic nematodes of horses. Larval stages, which inhabit the intestinal wall, are particularly pathogenic and can cause severe colitis and colic. Despite their clinical importance, diagnostic techniques for the prepatent stages do not exist. A method that could estimate mucosal infection intensity would have a major impact on the control and diagnosis of cyathostominosis. Here, serum IgG(T) responses to two larval antigen complexes of 25 and 20 kDa were quantified in horses with experimental infections, natural infections and in horses that presented with clinical larval cyathostominosis. In experimentally-infected animals, anti-25 kDa complex IgG(T) levels correlated positively with field exposure and with early third stage larval (r(s)=0.74, P=0.015) and total mucosal parasite (r(s)=0.78, P=0.010) burdens. In naturally exposed horses whose parasite burdens were quantified upon post-mortem examination, antigen-specific IgG(T) responses were significantly higher in infected than in uninfected horses (P=0.0001 and 0.002, for anti-25 and anti-20 kDa responses, respectively). In these animals, anti-25 kDa IgG(T) levels correlated positively with mucosal and lumenal burdens (P<0.05). IgG(T) responses to the 20 kDa antigen complex correlated positively with lumenal burdens (P=0.0043). In cases of larval cyathostominosis, antigen-specific IgG(T) levels were significantly higher than in uninfected ponies (P=0.002 and 0.0035, for anti-25 and anti-20 kDa responses, respectively). These results provide evidence that these two complexes contain antigens with potential as markers for prepatent cyathostomin infection.

Sustainable use of anthelmintics in grazing animals

It is suggested that the major factor in avoiding the development of anthelmintic resistance is the percentage of worms that do not encounter the anthelmintics (worms in refugia). This in turn is determined by the numbers of larvae on pasture, the percentage of animals treated and whether any stages in the host can avoid the action of anthelmintic. To maintain anthelmintic efficacy the percentage of worms in refugia must be sufficiently large. In cattle, this should involve treating only first-year animals and using a different pasture each year for calves. For sheep, only animals that have to be treated should be dosed with anthelmintic and clean grazing strategies that involve the use of anthelmintics should be avoided. For horses, reliance should be placed on the removal of faeces from pasture and only treating when the animals' condition requires it. Without a change in anthelmintic use there is the likelihood of increasing numbers of cases for which no anthelmintic is effective and animal welfare may be compromised.

Impact of management interventions on helminth levels, and body and blood measurements in working donkeys in South Africa

The aim of the study was to determine the effect of alternative management interventions on levels of nematodes and the condition of working donkeys in South Africa. Twenty-four adult donkeys (Equus asinus) within an area of 200km radius were randomly allocated to eight paddocks. Two replicates each of three management interventions together with a control group were tested in a 16-month study. The interventions included monthly removal of feces from paddocks where the donkeys grazed, a pre-winter moxidectin treatment, and a combination of a pre-winter moxidectin treatment and monthly fecal removal. The influence of the different interventions on the nematode fecal egg counts, animal live weights, body condition scores and general blood chemistry were compared. In addition, herbage samples were collected from the pastures in each paddock to determine the number of third-stage larvae (L(3)) per kg dry matter. At the end of the study worm recoveries and counts were performed on eight of the animals following euthanasia. The cyathostomes represented the largest portion of the helminth species composition in both the fecal egg counts and larval cultures. Monthly fecal removal alone did not significantly reduce the L(3) on pasture and consideration of more frequent removal is discussed. Pre-winter moxidectin treatment resulted in a 100% reduction in fecal egg counts, an average egg reappearance period of 42-55 days, a reduced average egg count for up to 8 months, and reduced total helminth burdens in all the treated donkeys. It also resulted in improved live weights, hemoglobin concentration, packed cell volumes and to some extent body condition score of the donkeys.