Anthelmintic efficacy against equine strongyles in the United States.

Equine strongyle parasites are ubiquitous in grazing equids across the world. Anthelmintic resistance is widely developed in cyathostomin populations, but very few surveys have evaluated anthelmintic efficacy in equine populations in the United States, and most of these are over 15 years old. The present study was carried out as part of the National Animal Health Monitoring Systems (NAHMS) Equine 2015-2016 study. The aims were to investigate anthelmintic treatment efficacy by means of the fecal egg count reduction test (FECRT) and identify parameters associated with decreased efficacy. Data were collected from equine operations in 28 states via questionnaires and fecal samples submitted for fecal egg count analysis. Participants were instructed to collect samples from six equids at the day of anthelmintic treatments and 14 days later, and they were asked to include an empty syringe with a legible label of the anthelmintic product used in the shipment. Overall, dewormer treatment was effective for 76.3% of operations (84.6% of animals). Macrocyclic lactone use was effective for 88.7% of operations (95.0% of animals) while pyrimidine/benzimidazole use was effective for 21.4% of operations (43.5% of animals). Univariate analysis revealed that overall, macrocyclic lactones exhibited significantly higher efficacy than the pyrimidine and benzimidazole drug classes (p < 0.0001). There were no statistically significant differences observed between geographic regions (West, South Central, North East, and Southeast). Body weight (p = 0.0355), amount of anthelmintic administered (p = 0.0119), and operation size (p = 0.0162) were statistically associated with anthelmintic efficacy, while anthelmintic treatment frequency in the previous 12 months was not (p = 0.7081). Multiple, mixed-effect logistic regression revealed that anthelmintic drug class (p < 0.0001) was the most impactful factor in predicting anthelmintic efficacy, after accounting for operation size, region and clustering of equids at the operation level. Pasture rotation (p = 0.0129) also demonstrated a significant effect using this model. These data document widespread occurrence of reduced anthelmintic efficacy of benzimidazole and pyrimidine products against strongyle infections in equids in the United States. Anthelmintic efficacy patterns were relatively uniform between the four studied regions, and some epidemiological factors were identified to be associated with anthelmintic efficacy against strongyle infections. This information can be useful in devising sustainable parasite control strategies in the future.

Risk factors associated with strongylid egg count prevalence and abundance in the United States equine population.

Equine strongyle parasites are considered ubiquitous in grazing equids across the world, and cyathostomin parasites are known pathogens causing well-described disease complexes in horses. Decades of intensive anthelmintic treatments have led to anthelmintic resistance in cyathostomins, and current recommendations are to lower treatment intensity and base control strategies on fecal egg count surveillance. Little is known about risk factors associated with strongyle parasite egg shedding patterns in the United States equine population, as the most recent national survey was conducted 20 years ago. The present study was carried out as part of the National Animal Health Monitoring Systems (NAHMS) Equine 2015-2016 study. The aims were to describe strongyle parasite egg shedding patterns in the United States equine population and identify risk factors associated with prevalence and egg count magnitude. Data were collected from equine operations in 28 states via questionnaires and fecal samples submitted to a parasitology research laboratory for fecal egg count analysis and the data gathered underwent comprehensive statistical analyses. Though region and season were related, overall, the summer months and the fall in the southeast tended to have the greatest odds of presence of strongyles eggs on a FEC. Generally, equids resident in the Western region (Arizona, California, Colorado, Montana, Oregon, and Wyoming) had significantly lower strongyle prevalence, no matter the season, as well as a markedly different distribution between strongyle egg shedding levels (p = 0.0005). Overall, egg counts were over-dispersed with about 27% of equids (95% Confidence Interval (CI): 20-34%) contributing 80% of the egg output. Pasture history was significantly associated with strongyle egg prevalence (p = 0.0003) and egg shedding levels (p = 0.0063) with daily access in the previous 30 days being associated with higher odds of presence and greater median egg count levels. Equid gender was significantly associated with strongylid presence (p = 0.0081) and egg count level (p = 0.0008), with male equids having significantly lower odds and median egg counts than female equids, and age was significantly negatively associated with strongylid prevalence (p < 0.0001). Time since last deworming was significantly positively associated with prevalence of strongyle eggs, and this was dependent on the class of dewormer used (p = 0.0086), with equids treated with macrocyclic lactone class of drugs having lower odds of strongyle egg presence at 120 days since the last deworming. These data provide useful insights into strongylid egg shedding patterns in the United States equine population, and they can help refine parasite control recommendations depending on region, pasture access, and age distribution.

Parasite control strategies used by equine owners in the United States: A national survey.

The widespread occurrence of anthelmintic resistance in equine parasites across the world has led to recommendations of fecal egg count-based parasite programs to reduce treatment intensity and thereby delay further development of resistance as much as possible. The most recent study describing equine parasite control in the United States was conducted 20 years ago, and little is known about current strategies employed. This study was part of the National Animal Health Monitoring Systems (NAHMS) Equine 2015 Study, and aimed to describe equine parasite control strategies in the U.S. and evaluate to which extent respondents were in compliance with current guidelines. The study was carried out in 28 states, representing 70.9% of all equine operations with at least five equids present. Two questionnaires were administered, either by mail or delivered in person by veterinary medical officers. Participants provided specific details of their operation and were asked questions about strategies for anthelmintic therapy and diagnostic testing. A total of 380 operations provided data regarding their parasite control practices. Most respondents dewormed 2-3 times a year with ivermectin being the most commonly used anthelmintic. About 22% of respondents used fecal egg counts (FEC) in some form, with less than 10% using them on a regular basis. Less than 5% made use of fecal egg count reduction tests (FECRT). These results suggest little change since the last nationwide survey was conducted in 1998, as the majority of respondents did not report using FECs. This is in stark contrast to recent European surveys, where 50-60% of respondents were using FECs routinely. However, the anthelmintic treatment intensity appears to have been lowered compared to 1998. Taken together, these results suggest a continuing need for education and outreach regarding sustainable parasite control.

Case-control study of a multistate equine herpesvirus myeloencephalopathy outbreak.

BACKGROUND: A large multistate outbreak of equine herpesvirus myeloencephalopathy (EHM) occurred in May 2011 among horses that participated in a competitive event. OBJECTIVE: To identify EHM risk factors among horses with a common exposure venue. ANIMALS: A total of 123 horses: 19 horses with EHM, 14 equine herpesvirus-1 cases with no reported neurologic signs, and 90 control horses. METHODS: EHM case survey data were compared with data from EHV-1 cases with no neurologic signs and healthy controls using univariable and multivariable methods. RESULTS: Significant factors associated with higher risk for EHM compared with EHV-1 cases with no neurologic signs were (1) greater number of biosecurity risks at the event, (2) female sex, (3) increasing number of classes competed in at the event, and (4) an interaction between sex and number of classes competed in. In the EHM versus controls comparison, in addition to sex and biosecurity risks, factors associated with higher EHM risk included EHV-1 vaccination in the 5 weeks before the event and increasing number of events attended in April 2011; zinc dietary supplementation was associated with decreased risk. An interaction between sex and the number of events attended in April 2011 also was significant. CONCLUSIONS AND CLINICAL IMPORTANCE: Findings from this study suggest that dietary zinc supplementation may be associated with decreased risk of EHM. Several factors were associated with increased risk of EHM. Additional investigations of factors associated with risk of EHM are warranted to evaluate the importance of these factors in this complex disease of horses.