Managing anthelmintic resistance in cyathostomin parasites: Investigating the benefits of refugia-based strategies

Investigating Dirofilaria immitis isolates infecting domestic canines and their susceptibility/resistance patterns to macrocyclic lactones in the northern region of the Mississippi Delta area (southeast Missouri)

Previous reports of macrocyclic lactone (ML) resistance in Dirofilaria immitis, the parasitic nematode which causes heartworm disease, have mainly been from the southern Mississippi Delta region. Southeast Missouri (SEMO), forming the northern boundary of this region, has not previously been well studied. The area is an ideal propagation region for heartworm infection and possibly for the spread of ML resistance. To assess whether D. immitis isolates infecting domestic canines in SEMO exhibit evidence of resistance to MLs, domestic canines, presented to veterinary facilities testing positive for heartworms through antigen and microfilariae (MF) examination, were utilized in the study. Using a descriptive epidemiological cross-sectional study, from March 2021 through February 2022, blood sample collection from 96 canines living in SEMO testing positive for heartworms were analyzed. MiSeq technology was utilized to sequence specific genetic markers associated with susceptibility/resistance for MLs in D. immitis isolates. Genomic data revealed most D. immitis isolates had genotypic profiles consistent with resistance to MLs. Of the 96 samples tested, 91 (94.8%) had a resistant genotype, 4 (4.2%) had a mixed genotype, and 1 sample (1%) genotyped as susceptible. While detailed and reliable medical histories were not available for most canines, detailed medical history from 2 canines indicated evidence of phenotypic resistance that was consistent with their genotypes. However, in vivo preventive tests are needed to confirm a high frequency of phenotypic ML resistance in D. immitis from this region. Increasing resistance patterns to MLs indicate the approach to heartworm prevention/treatment protocol should be reconsidered. New measures may be required to stop heartworm disease.

First report of ivermectin resistance in cyathostomins (small strongyles) of horses in Argentina

In Argentina, as in the rest of the world, cyathostomins are the most common nematodes parasitizing horses. Control is based almost exclusively on the administration of benzimidazoles, pyrimidines, and macrocyclic lactones. However, intensive use of these drugs is resulting in the development of anthelmintic resistance (AR). For example, AR to benzimidazoles is currently distributed throughout Argentina, while incipient AR to pyrimidines (pyrantel embonate) is appearing in areas where this drug is used. Macrocyclic lactones and especially ivermectin, are by far the most used drugs by the vast majority of equine premises in the country. Although ivermectin has been used since 1982, its efficacy against equine strongylid parasites has remained very high until the present. In this study we report for the first time, the presence of a cyathostomin population with resistance to ivermectin in adult horses belonging to an equine premise located in central Argentina. Fecal egg count reduction tests (FECRT) were performed following the most recent guidelines of the World Association for the Advancement of Veterinary Parasitology (WAAVP) for the diagnosis of anthelmintic resistance (research protocol) and resistance was considered when the Upper 90% Credible Interval fell below the expected efficacy threshold of 99.9%. Calculations were carried out using two different online calculation interfaces suggested by WAAVP. For the 14-day post-treatment interval, ivermectin efficacy was 79.5% (90% Credible Interval: 68.1-88.8) and 79.3% (74.2-83.6.3%) with the two methods, respectively. At 19 days post treatment, fecal egg count reductions were 68.6% (50.5-83.1) and 68.4% (61.9-74.1), respectively. At both intervals, this cyathostomin population fullfilled the criteria for AR. These findings suggest dispersion of ivermectin resistant cyathostomins in Argentina. Given the widespread use of macrocyclic lactones, it is important that veterinarians and the equine industry promote a more selective and evidence-based use of these drugs and establish routine monitoring to determine anthelmintic field efficacy to detect treatment failures as early as possible and avoid potential health problems as well as further spread of resistant genes.

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.

Worm control practices used by Thoroughbred horse managers in Australia: A national survey

This study assessed worm control practices used by Australian Thoroughbred farm managers with an online questionnaire survey. The questionnaire comprised 52 questions (close-ended: 44; open-ended: 8) about farm demography and general husbandry practices, farm managers' knowledge of gastrointestinal nematodes (GIN) and their importance, diagnosis, worm control strategies and anthelmintics, anthelmintic resistance (AR) and grazing management. Following the pilot survey, the link for the questionnaire survey was sent to all (n = 657) registered members of the Thoroughbred Breeders Australia on 12th April 2020. The response rate for the questionnaire was 18.5% (122 of 675). The farm managers reported a good understanding of GIN and their importance in different age groups of horses as most respondents (70% of 122) perceived worm-related illness to be more important in young (i.e., foals, weanlings and yearlings) than adult (> 3 years old) horses. Although most respondents (93%, 113 of 122) used anthelmintics prophylactically to control GIN, only 15% (18 of 122) observed worm-related illness in their horses. Just under 40% of respondents were performing faecal egg counts, with less than 20% using the results of faecal egg counts to guide deworming decisions. The interval-based deworming strategy was the most common method (≥55% of 122 respondents) to control GIN in all age groups of horses. Macrocyclic lactones were the first choice of anthelmintics for all age groups of horses. Although the majority of respondents (88%, 107 of 122) perceived resistance in GIN against commonly used anthelmintics as an important issue in managing worms in horses, only 29% assessed the efficacy of anthelmintics and 91% (111 of 122) were unaware of AR on their properties. Grazing management practices, such as manure removal, were more frequently performed on smaller paddocks (<0.20 ha: 58%) than on larger paddocks (>0.20 ha: 18%). Multiple correspondence analyses showed that the likelihood of suboptimal worm control practices on small farms (n = ≤50 horses) was greater than that of medium (n = 51-100) and large (n = >100) farms. This study provides insights into the demography of Thoroughbred farms in Australia, husbandry practices used by stud managers and their knowledge about worms, control options and AR concerns, thereby paving the way for taking any initiatives to address the problem of AR in GIN of Australian Thoroughbred horses.

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.

Assessment of worm control practices recommended by equine veterinarians in Australia

This study aimed to assess Australian veterinarians' knowledge, perceptions and treatment strategies for worm control in horses with an online questionnaire. The questionnaire comprised 64 questions covering various aspects of: (i) veterinary practice; (ii) the veterinarian's knowledge of gastrointestinal nematodes (GINs) and the importance of parasites in different age groups of horses; (iii) the diagnosis and control of worms; (iv) anthelmintics and anthelmintic resistance (AR); (v) grazing management; and (vi) the means of communication and the discussion between veterinarians and their clients regarding worm control. Following a pilot survey, a link for the questionnaire survey was sent to all (n = 1,148) registered members of Equine Veterinarians Australia in April 2020. The response rate for the questionnaire was 10% (118 of 1,148). The findings of this study illustrate veterinarians' good understanding of aspects of equine parasites, including control. However, respondents mainly recommended frequent, interval-based prophylactic deworming in young horses, and only 40% (96 of 239) diagnosed GIN infections based on faecal egg count (FEC) results in all age groups of horses. Furthermore, only 27% (88 of 330) of the respondents made deworming decisions based on FECs. Most of the respondents recommended macrocyclic lactones (MLs) for all age groups of horses (71%, 481 of 677), and the most frequently used method to calculate the dose of anthelmintics was by estimating the weight of animals visually (53%, 63 of 118). Although the majority of respondents (97%, 115 of 118) perceived AR to be a critical issue in managing worms in horses, 58% (67 of 118) of them were unaware of the status of AR on their clients' properties. Forty-two percent (50 of 118) of the respondents perceived the presence of AR in worms, including pinworms (16%), strongylins (15%), species of Draschia and Habronema (6%), Strongyloides westeri (2%) and tapeworms (1%). Twenty-seven percent (32 of 118) of the respondents rarely discussed equine worm control practices with their clients. This study provides insights into the perception and worm control practices recommended by Australian veterinarians to manage equine parasites. The findings highlight the importance of continued education and awareness of AR, and the use of non-chemical methods as well as consideration of the legislation of prescription-only use of anthelmintics based on FECs to achieve sustainable control of GINs in Australian horses.

Occurrence and risk factors associated with gastrointestinal parasitism in horses reared in different systems

To facilitate exploration of the immense range of gastrointestinal parasites in horses and the impact of parasitism on equine health, the present study aimed to evaluate the occurrence and risk factors associated with these infections in horses reared in different management systems in the state of Santa Catarina, Brazil. Samples were collected from 208 horses: 91 from extensive, 64 from semi-extensive, and 53 from intensive rearing systems. The identified helminths included those of the Strongylida order (80.29%), Parascaris equorum (3.36%), Oxyuris equi (4.33%), and Anoplocephala spp. (1.92%). By analyzing the coproculture results to differentiate among Strongylida order parasites, species such as Strongylus vulgaris, S. edentatus, S. equinus, Triodontophorus spp., and Trichostrongylus axei, in addition to members of the Cyathostominae subfamily, which include Gyalocephalus capitatus and Poteriostomum spp., were identified. The only positive sample of protozoa was that of Cryptosporidium spp. (13%). Regarding the rearing system, animals in the extensive system had a higher proportion of infected horses and a higher chance of infection than those in the other systems. For the variable co-grazing with cattle, only cyathostomins showed a significant difference with relatively low infection risk in co-grazing. Overall, the present study demonstrated a high occurrence of equine gastrointestinal parasites, especially those of the Strongylida order, with emphasis on small strongylids. In addition, analyzing factors associated with infection showed that management variables are important for controlling parasitism in horses.

Shortened strongylid egg reappearance periods in horses following macrocyclic lactone administration - The impact on parasite dynamics

Over the past three decades, equine strongylid egg reappearance periods (ERPs) have shortened substantially for macrocyclic lactone anthelmintics. The ERPs of ivermectin and moxidectin were originally reported in the 8-10 and 12-16 week ranges, respectively, but several recent studies have found them to be around 4-5 weeks for both actives. This loss of several weeks of suppressed strongylid egg output could have substantial implications for parasite control. This study made use of a computer simulation model to evaluate the impact of shortened ERPs on the anthelmintic performance of ivermectin and moxidectin against equine cyathostomins. The original ERPs were set to 7.1 and 15.4 weeks for ivermectin and moxidectin, respectively, while the reduced ERP was set to 4.6 weeks for both actives. Simulations were set to compare model outputs between original and reduced ERP scenarios and results expressed as percent increase in strongylid egg output, infective third stage larvae on herbage (L3h), and encysted early third stage larvae (EL3). For each drug, simulations were evaluated for two different treatment scenarios (2 and 4 treatments annually), two different age groups (yearlings and adults), and for four different climates (cold humid continental, temperate oceanic, humid subtropical, and hot/cold semi-arid). Across all simulations, there was a substantial increase of the three evaluated parameters. With the ivermectin simulations, all three parameters increased in the 100-300% range across climates, age groups and treatment intensities. The moxidectin simulations displayed a wider range of results with parameters increasing from a few hundred to several thousand percent. The increases were most pronounced for L3h in the two cooler climates, reaching as high as 6727%. Overall, the loss of anthelmintic performance was at a magnitude of 10 times larger for moxidectin compared to ivermectin. This performance loss was climate dependent, and was also affected by treatment intensity, but not by horse age. This is the first study to evaluate consequences of shortened ERPs in horses and demonstrated a substantial loss in anthelmintic performance resulting from this development. The results illustrate that anthelmintic efficacy is more than the percent reduction of fecal egg counts at 14 days post treatment, and that substantial anthelmintic performance can be lost despite FECRTs remaining at 100%.

Occurrence and control of equine strongyle nematode infections in Prince Edward Island, Canada

Widespread overuse of anthelmintics has produced a growing population of intestinal parasites resistant to control measures. A paradigm shift in equine parasite control is warranted to prevent continued resistance development and maintain equine health. Small strongyles, which are ubiquitous in horses, are currently the most important intestinal parasites of adult horses. Sustainable management programs consider the variation in egg shedding by individual horses, and varied risks associated with age, use, density, climate, and environment. To develop regional recommendations for Prince Edward Island (PEI), Canada, it is important to first characterize strongyle egg shedding patterns and parasite management practices in use. Study objectives were to conduct a cross-sectional observational survey and risk factor analysis of parasite control programs, strongyle egg shedding and Strongylus vulgaris serology. A total of 339 horses from 40 PEI farms were sampled. Mean farm size was 8 horses and ranged from 2 to 30. Mean horse age was 11.6 years (Std. Dev. =7.2) and ranged from 2 months to 32 years. Mean fecal egg count (FEC) was 322 eggs per gram (EPG) (Std. Dev. =648). On average, 32% (Std. Dev. =16%) of horses shed 80% of strongyle eggs across 32 eligible farms. When considering all horses (n = 313) as one large herd, 18.7% of horses shed 80% of strongyle eggs. Use of FEC was identified in 4.6% of horses at 15% (n = 6) of PEI farms. Reported deworming intervals included 37.4% (n = 123) every 2-3 months and 58% (n = 191) every 3-4 months. Positive S. vulgaris titers were identified in 60% of horses (n = 200). Univariate analysis revealed that months since last deworming, age, and body condition score (BCS) were associated with strongyle shedding. The estimated odds of being in the high FEC category (>500 EPG) was 1.4 (95% CI, 1.1-1.8) times higher when last deworming increased by one month. Under-conditioned (BCS <4.5) horses had 3.6 (95% CI, 1.2-10.6) times odds of being in the higher FEC category than over-conditioned horses. Non-racing horses had 5.4 times odds of having a positive S. vulgaris titer than racehorses. This cross-sectional observational study is the first to report on the occurrence, risk factors and control of equine strongyle nematode infections in PEI, Canada. We conclude that the 80:20 rule can be used to develop control recommendations in PEI. Very few farms in PEI currently use FEC to guide parasite management. These findings provide a basis for future client education and investigations aimed at providing region specific recommendations.

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.

Feasibility of selective anthelmintic therapy to horses in tropical conditions: the Cuban scenario

For the past several decades, selective anthelmintic therapy (SAT) has been recommended in temperate climate countries for controlling gastrointestinal parasites in horses. However, the feasibility of this approach in tropical climates remains unknown, given the very different parasite transmission patterns and a larger representation of working equids. The aim of this study was to evaluate the bio-economic feasibility of SAT in horses kept under tropical conditions of Camagüey, Cuba. Fecal egg counts were determined from 794 adult horses and used for evaluating three different putative treatment thresholds; > 500 strongylid eggs per gram of feces (EPG); > 1000 EPG; and treatments yielding > 80% of the total herd egg output. These scenarios were evaluated under three treatment frequencies (every 2, 6, and 12 months). The bio-economic feasibility of these nine possible selective anthelmintic therapy scenarios was evaluated taking into account the cost of parasitological tests, and the mean cost of anthelmintic treatment in Camagüey. The majority (96.7%) of the samples tested were positive for strongyle eggs, with a mean of 1549 EPG. The percentage of horses exceeding the cut-off points at the nine scenarios varied between 40.1 and 93.8%. All owners surveyed used extra-label anthelmintic product in their horses on a routine basis. The economic analysis demonstrated that selective therapy generally was much more costly than deworming the entire herd without determining egg counts. However, we consider that the deworming every 6 months of the horses that expel 80% of the eggs in the herd allows a reduction of the treatment intensity without increasing costs, and it can be considered as a viable selective deworming scheme under similar conditions.

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

Heartworm disease, caused by Dirofilaria immitis, can be lethal in dogs and cats. It is transmitted by mosquitoes, and occurs in many parts of the world. Prevention relies on macrocyclic lactones. Macrocyclic lactones used are ivermectin, selamectin, abamectin, eprinomectin, milbemycin oxime and moxidectin, administered at 30-day intervals during the transmission season. Some moxidectin formulations are long-acting injectables. In the USA, preventives are recommended throughout the year. Loss of efficacy of macrocyclic lactone preventives was reported in 2005 and proof of resistance in the USA was published a decade later. Understanding factors which promote resistance is important to maintain control. Factors important for resistance development are discussed. Better, inexpensive tests to confirm resistance are needed. Infection in animals under chemoprophylaxis per se does not imply resistance because lack of compliance in preventive use could be the reason. In vivo confirmation of resistance is expensive, slow and ethically questionable. A microfilariae suppression test can be a surrogate test, but requires a high dose of a macrocyclic lactone and repeated blood microfilaria counts 2-4 weeks later. DNA single nucleotide polymorphism markers have been successfully used. However, the specific genetic changes which cause resistance are unknown. Surveys to map and follow the extent of resistance are needed. Long acting mosquito repellants and insecticides can play a useful role. High dose rate formulations of moxidectin, coupled with mosquito biting mitigation may reduce transmission of resistant genotypes. Doxycycline, daily for 28 days, as anti-Wolbachia treatment, can reduce transmission and remove adult parasites. However, new classes of heartworm preventives are needed. While any preventive strategy must be highly effective, registration requirements for 100% efficacy may hinder development of useful new classes of preventives. Continued reliance on macrocyclic lactone preventives, when they do not work against resistant genotypes, will spread resistance, and allow for more disease.

Concern for Dirofilaria immitis and Macrocyclic Lactone Loss of Efficacy: Current Situation in the USA and Europe, and Future Scenarios

Dirofilaria immitis infection is one of the most severe parasitic diseases in dogs. Prevention is achieved by the administration of drugs containing macrocyclic lactones (MLs). These products are very safe and highly effective, targeting the third and fourth larval stages (L3, L4) of the parasite. Until 2011, claims of the ineffectiveness of MLs, reported as “loss of efficacy” (LOE), were generally attributed to owners’ non-compliance, or other reasons associated with inadequate preventative coverage. There was solid argumentation that a resistance problem is not likely to occur because of (i) the great extent of refugia, (ii) the complexity of resistance development to MLs, and (iii) the possible large number of genes involved in resistance selection. Nevertheless, today, it is unequivocally proven that ML-resistant D. immitis strains exist, at least in the Lower Mississippi region, USA. Accordingly, tools have been developed to evaluate and confirm the susceptibility status of D. immitis strains. A simple, in-clinic, microfilariae suppression test, 14-28 days after ML administration, and a “decision tree” (algorithm), including compliance and preventatives’ purchase history, and testing gaps, may be applied for assessing any resistant nature of the parasite. On the molecular level, specific SNPs may be used as markers of ML resistance, offering a basis for the validation of clinically suspected resistant strains. In Europe, no LOE/resistance claims have been reported so far, and the existing conditions (stray dogs, rich wildlife, majority of owned dogs not on preventive ML treatment) do not favor selection pressure on the parasites. Considering the genetic basis of resistance and the epizootiological characteristics of D. immitis, ML resistance neither establishes easily nor spreads quickly, a fact confirmed by the current known dispersion of the problem, which is limited. Nevertheless, ML resistance may propagate from an initial geographical point, via animal and vector mobility, to other regions, while it can also emerge as an independent evolutionary process in a new area. For these reasons, and considering the current chemoprophylaxis recommendations and increasing use of ML endectoparasiticides as a potential selection pressure, it is important to remain vigilant for the timely detection of any ML LOE/resistance, in all continents where D. immitis is enzootic.

Three-year study to evaluate an anthelmintic treatment regimen with reduced treatment frequency in horses on two study sites in Belgium

In the present study, an anthelmintic treatment regimen with reduced treatment frequency was evaluated in horses on two study sites in Belgium during three consecutive summer pasture seasons. Historically, the horses on both study sites were treated up to 6 times a year with ivermectin (IVM) or up to 4 times a year with moxidectin (MOX), and previous efficacy evaluations indicated a reduced egg reappearance period in some of the treated horses for both IVM (28 days) and MOX (42 days). In the present study, all horses were treated with IVM or MOX in the spring and in autumn. Faecal egg counts (FEC) were conducted every two weeks during the summer pasture season and whenever the individual FEC exceeded 250 eggs per gram of faeces, the specific horse was treated with pyrantel embonate. No increase in parasitic disease over the three-year period of the study was observed. The FEC data collected in the study as well as the age of the animals and local weather data were then imported into a cyathostomin life-cycle model, to evaluate long term effects of the newly applied treatment regimen on the selection pressure for anthelmintic resistance, and compare to the previous high frequency treatment regimen. The model simulations indicated that the whole-herd treatment regimen with at least 4 macrocyclic lactone treatments annually led 2-3 times faster resistance development than any of the alternative treatment regimens evaluated under the specific conditions of these two study sites. Further lowering the treatment frequency or applying even more selective treatments enhanced the delay in resistance development, but to a lesser extent.

Spread of anthelmintic resistance in intestinal helminths of dogs and cats is currently less pronounced than in ruminants and horses - Yet it is of major concern

Anthelmintic resistance (AR) has thus far only rarely been reported for intestinal helminths of dogs and cats, in contrast to parasites of livestock and horses. We highlight possible reasons for this striking and important discrepancy, including ecological, biological and genetic factors and/or intervention regimens of key intestinal helminths concerning both host groups. In view of the current knowledge related to the genetics, mechanisms and principles of AR development, we point at issues which in our view contribute to a comparatively lower risk of AR development in intestinal helminths of dogs and cats. Finally, we specify research needs and provide recommendations by which, based on the available information about AR in ruminant and equine helminths, the development of AR in dog and cat helminths may best be documented, prevented or at least postponed.

Formulation of the nematophagous fungus Duddingtonia flagrans in the control of equine gastrointestinal parasitic nematodes

Equine gastrointestinal nematodiosis contributes to the lower productivity of these animals. There are growing reports of the emergence of nematodes resistant to the drugs used for decades in anthelmintic treatments. An alternative to the emergence of resistance may be the use of nematophagous fungi as a complementary method of treatment. Therefore, the objective was to evaluate the effects of the use of the product Bioverm® as a carrier of Duddingtonia flagrans, on pasture contamination level and equine parasitic burden. Sixteen mares were used, divided into two groups, one control and one treated, in which the treated animals received a dose of 1 g of Bioverm®, containing 10⁵ chlamydospores per gram of the commercial product for each 10 kg of body weight, per day, for six months. Efficacy was evaluated by count of eggs per gram of faeces (EPG), coproculture and larval count on pastures and its correlation with climate, as well as weight gain evaluation. During the study, a significant influence of the formulation on weight gain and EPG was observed. The recovery of larvae from coprocultures revealed the predominance of small over large strongyles. There was a significant difference (p < 0.05) between the averages of the number of larvae of small strongyles recovered in the pasture at a distance of 0-20 cm from the faeces of treated and control groups. A correlation was also observed between the number of larvae recovered from the pasture and the average temperature during the experimental period, mainly in August and September. The ingestion of Bioverm® enhances the biological control of gastrointestinal nematodes of pasturing horses.

The Use of Molecular Profiling to Track Equine Reinfection Rates of Cyathostomin Species Following Anthelmintic Administration

Simple Summary

Cyathostomins (small strongyles) are a multispecies group of intestinal parasites in horses and the main target of deworming efforts by horse owners. It is not known whether species of cyathostomins have individual responses to dewormers. The objective of this study was to identify differences between cyathostomin species in reemergence rates following commercial dewormer treatment. This study used gene sequencing to profile the presence/absence of cyathostomin species in fecal samples at 2-week intervals following deworming to determine how quickly each species reinfected horses. Moxidectin was found to be the most effective at slowing the overall reemergence of these parasites, followed by Ivermectin, then Pyrantel. Seven species were resistant to all three deworming products. This study demonstrates that dewormer sensitivity differs between cyathostomin species, which could lead to more targeted control measures.

Abstract

Cyathostomins are a multispecies parasite ubiquitous in Equids. Cyathostomins have developed resistance to all but one class of anthelmintics, but species-level sensitivity to anthelmintics has not been shown. This study measured reinfection rates of cyathostomin species following the administration of three commercial dewormers. Nine treated horses were compared with 90 untreated controls during June-September 2017–2019. Ivermectin (IVM) (n = 6), Moxidectin (MOX) (n = 8) or Pyrantel (PYR) (n = 8) were orally administered. Fecal samples were collected every 14 d for 98 d. Fecal egg count reductions (FECR) were calculated using a modified McMaster technique. Nineteen cyathostomin species were identified by 5.8S-ITS-2 profiling using amplicon sequencing. Data were analyzed in QIIME1 and R statistical software using presence/absence methods. MOX had the lowest numbers of species present over the time course, followed by PYR then IVM (7.14, 10.17, 11.09, respectively); however, FECR was fastest for PYR. The presence of seven species: Coronocyclus labiatus, Cyathostomum catinatum, Cyathostomum tetracanthum, Cylicocylus elongatus, Cylicodontophorus bicoronatus, Cylicostephanus minutus, and Cylicostephanus goldi were unaffected by treatment (p > 0.05) points to species-specific differences in dewormer sensitivity and environmental persistence. Identifying resistance patterns at the species level will enable mechanistic understandings of cyathostomin anthelmintic resistance and targeted approaches to control them.

Reduced Efficacy of Fenbendazole and Pyrantel Pamoate Treatments against Intestinal Nematodes of Stud and Performance Horses

Nematodes are an important cause of disease and loss of performance in horses. Changes in the parasitic fauna of horses have occurred in the past few decades, making cyathostomins the major parasites in adult horses, while large strongyles have become less prevalent. Parascaris spp. remains the most important parasite infecting foals and weanlings. Anthelmintic resistance is highly prevalent in cyathostomins and Parascaris spp. worldwide and it must be factored into treatment decisions. To assess anthelmintic efficacy in Northern Italy, we sampled 215 horses from 17 sport and horse-breeding farms. Fecal egg count reduction tests (FECRT) were used to assess anthelmintic efficacy. Copromicroscopic analysis was performed using MiniFLOTAC before treatment with fenbendazole, pyrantel pamoate or ivermectin, and repeated 14 days post-treatment. Strongyle-type eggs were detected in 66.91% of horses (CI95% 61.40-73.79%), while Parascaris spp. was detected in 2.79% (CI95% 1.94-5.95%). Reduced efficacy against cyathostomins was observed for fenbendazole in 55.56% of the treated animals (CI95% 41.18-69.06%), and for pyrantel pamoate in 75% of animals (CI95% 30.06-95.44%). Ground-based actions must be set in place to promote the uptake of state-of-the-art worm control plans that will prevent clinical disease while minimizing the selection pressure of resistant parasites.

Preliminary Data from Six Years of Selective Anthelmintic Treatment on Five Horse Farms in France and Switzerland

Simple Summary

Today, anthelmintic resistance (AR) of small strongyles (cyathostomins) against all presently available anthelmintics for equids poses on many horse farms worldwide huge problems with regard to an efficient and satisfactory parasite control. Therefore, alternative parasite control schemes are urgently needed. The so-called selective anthelmintic or targeted selective treatment (SAT) is one of the concepts considered to delay or even to overcome this challenging AR-situation. In the present field study, all 93 equids (90 horses, 3 ponies) from five horse riding farms in France and Switzerland were regularly sampled (spring and autumn) and the feces were analyzed for a period of six years. From a total of 757 fecal samples, only 263 (34.7%) had a fecal egg count ≥200 Eggs per Gram (EpG) (threshold) and consequently needed an anthelmintic treatment. A long-term reduction in the number of anthelmintic treatments can be expected on a herd and on the individual horse level, respectively, when comparing to a conventional (or strategic) twice per year treatment regime.

Abstract

Anthelmintic resistance (AR) of small strongyle populations (cyathostomins) against products of the benzimidazole and tetrahydropyrimidine classes occurs now worldwide and there is an increasing number of reports also regarding macrocyclic lactones. Consequently, and in order to maintain an appropriate horse parasite control, alternative control schemes must be evaluated under field conditions. Here we present a six-year field study on the administration of the so-called selective or targeted selective anthelmintic treatment (SAT) concept. In this study on five horse farms in France and Switzerland, 757 fecal samples from 93 equids (90 horses, 3 ponies) have been taken twice a year (between early and late spring and between early and late autumn) from autumn 2014 to spring 2020 and processed by a McMaster technique. From a total of 757 samples, only 263 (34.7%) had a fecal egg count ≥200 EpG and needed an anthelmintic treatment. This small number of fecal samples ≥200 EpG demonstrates the considerable potential for a long-term reduction of the number of anthelmintic treatments and the anthelmintic pressure by using the SAT-programme.

Monitoring equine ascarid and cyathostomin parasites: Evaluating health parameters under different treatment regimens

BACKGROUND

Strongylid and ascarid parasites are omnipresent in equine stud farms, and ever-increasing levels of anthelmintic resistance are challenging the industry with finding more sustainable and yet effective parasite control programs.

OBJECTIVES

To evaluate egg count levels, bodyweight and equine health under defined parasite control protocols in foals and mares at two Standardbred and two Thoroughbred stud farms.

STUDY DESIGN

Longitudinal randomised field trial.

METHODS

A total of 93 foals were enrolled and split into two treatment groups, and 99 mares were enrolled and assigned to three treatment groups. All horses underwent a health examination, and episodes of colic or diarrhoea were recorded at each faecal collection date. Bodyweights were assessed using a weight tape, and mares were body condition scored. Group A foals (FA) were dewormed at 2 and 5 months of age with a fenbendazole/ivermectin/praziquantel product, while group B foals (FB) were dewormed on a monthly basis, alternating between the above-mentioned product and an oxfendazole/pyrantel embonate product. Group A mares (MA) were dewormed twice with fenbendazole/ivermectin/praziquantel, group B mares (MB) were dewormed with the same product, when egg counts exceeded 300 strongylid eggs per gram, and group C mares (MC) were dewormed every 2 months, alternating between the two products. Health data were collected monthly for 6 months (foals) and bimonthly for 13 months (mares). Data were analysed with mixed linear models and interpreted at the α = 0.05 significance level.

RESULTS

There were no significant bodyweight differences between foal groups, but MA mares were significantly lighter than the other two groups. Very few health incidents were recorded. Foals in group FA had significantly higher ascarid and strongylid egg counts, whereas no significant differences were observed between mare groups.

MAIN LIMITATIONS

Study duration limited to one season.

CONCLUSIONS

Anthelmintic treatment intensity was lowered from the traditional intensive regimes without measurable negative health consequences for mares and foals.

Multiple resistance in equine cyathostomins: a case study from military establishments in Rio Grande do Sul, Brazil

Semi-intensive equine breeding system favors gastrointestinal nematode infections. The treatment of these infections is based on the use of anthelmintics. However, the inappropriate use of these drugs has led to parasitic resistance to the available active principles. The objective of this study was to evaluate the efficacy of the main classes of antiparasitic (ATP) used in control in adult and young animals, including: benzimidazoles (fenbendazole), pyrimidines (pyrantel pamoate), macrocyclic lactones (ivermectin and moxidectin), as well as the combination of active ingredients (ivermectin + pyrantel pamoate). The study was carried out in two military establishments, located in Rio Grande do Sul (RS), from January to December, 2018. The intervals between the treatments of the animals were performed from 30 to 90 days. Coproparasitological evaluations were determined by the egg count reduction in the faeces. Cyatostomine larvae were identified in pre and post-treatment cultures. The results demonstrated the multiple parasitic resistance of cyathostomins to fenbendazole, moxidectin in young animals, and to fenbendazole, pyrantel pamoate in adult animals. Thus, it is necessary to define or diagnose parasitic resistance to assist in the creation of prophylactic parasitic control, using suppressive treatment with ATP associated with integrated alternatives. The progress of parasitic resistance can be slowed.

Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand

Climate change is likely to influence livestock production by increasing the prevalence of diseases, including parasites. The traditional practice of controlling nematodes in livestock by the application of anthelmintics is, however, increasingly compromised by the development of resistance to these drugs in parasite populations. This study used a previously developed simulation model of the entire equine cyathostomin lifecycle to investigate the effect a changing climate would have on the development of anthelmintic resistance. Climate data from six General Circulation Models based on four different Representative Concentration Pathways was available for three New Zealand locations. These projections were used to estimate the time resistance will take to develop in the middle (2040–49) and by the end (2090–99) of the century in relation to current (2006–15) conditions under two treatment scenarios of either two or six yearly whole-herd anthelmintic treatments. To facilitate comparison, a scenario without any treatments was included as a baseline. In addition, the size of the infective and parasitic stage nematode population during the third simulation year were estimated. The development of resistance varied between locations, time periods and anthelmintic treatment strategies. In general, the simulations indicated a more rapid development of resistance under future climates coinciding with an increase in the numbers of infective larvae on pasture and encysted parasitic stages. This was especially obvious when climate changes resulted in a longer period suitable for development of free-living parasite stages. A longer period suitable for larval development resulted in an increase in the average size of the parasite population with a larger contribution from eggs passed by resistant worms surviving the anthelmintic treatments. It is projected that climate change will decrease the ability to control livestock parasites by means of anthelmintic treatments and non-drug related strategies will become increasingly important for sustainable parasite control.

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The development of anthelmintic resistance under climate change was simulated.

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Climate can become more suitable for parasite development, increasing population size.

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The time resistance took to develop was linked to changes in parasite population size.

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Non-drug related strategies will become increasingly important for parasite control.

What Modeling Parasites, Transmission, and Resistance Can Teach Us

Veterinarians and farmers must contend with the development of drug resistance and climate variability, which threaten the sustainability of current parasite control practices. Field trials evaluating competing strategies for controlling parasites while simultaneously slowing the development of resistance are time consuming and expensive. In contrast, modelling studies can rapidly explore a wide range of scenarios and have generated an array of decision support tools for veterinarians and farmers such as real-time weather-dependent infection risk alerts. Models have also been valuable for predicting the development of anthelmintic resistance, evaluating the sustainability of current parasite control practices and promoting the responsible use of novel anthelmintics.