Equine Parasite Control Practices in United States

Until recently, parasite control in equids commonly relied on treatments at frequent, fixed intervals and little to no diagnostic surveillance of parasite burden. Due to the widespread occurrence of cyathostomin and ascarid resistance to anthelmintic treatment, the American Association of Equine Practitioners (AAEP) currently recommends an overall reduced therapy intensity that involves treating only those members of the herd with fecal egg counts (FEC) exceeding a certain threshold.¹

Anthelmintic treatment efficacy is evaluated routinely by performing FEC reduction tests. This approach aims to reduce overall fecal egg shedding within the population while minimizing the potential for parasite resistance through overtreatment. Reduced treatment intensity combined with the use of FEC is considered the gold standard for equine parasite control in North America. Similar treatment protocols have been adopted successfully in Europe.^2,3 In North America, however, few surveys have evaluated current parasite control strategies. Results of a 2013 study that focused on the Kentucky thoroughbred industry revealed that about 25% of respondents used FEC to determine treatment protocol, and horses of all age classes received an average of 5 to 7 anthelmintic treatments per year.⁴

In 2015, the National Animal Health Monitoring System (NAHMS) conducted an equine study to examine several aspects of husbandry management and health care in captive equids (including horses, ponies, donkeys, zebras, and hybrids).⁵ The survey included 2 questionnaires related to equine internal parasite control to determine parasite control practices and level of adherence to current AAEP guidelines.

EXTENT OF THE STUDY

Twenty-eight states were included in the study, based on highest equine density and proportion of farms with 5 or more equids. Overall, these states represented 70.9% of all US farms with 5 or more equids. Stratified random sampling was then used to select specific equine operations for the study.

Phase I of the study, performed from April through July 2015 via in-person interviews with equine operation personnel, included questions regarding the primary function of the equine operation and primary use of resident equids. Phase II, performed via mail from August 2015 through April 2016 or in person from May through October 2016, included questions on number, type, and age of resident equids. Questions regarding parasite control included reasons for deworming, deworming strategy for resident equids, number of treatments per equid per year for each anthelmintic category, use of FEC for each age category, additional parasite control strategies, and knowledge of, concern level for, and testing strategy for anthelmintic resistance. States were categorized by region (West, South Central, Northeast, and Southeast) to simplify statistical analysis and reporting of results.

In total, 1920 operations responded to the phase I questionnaire; 945 of those operations also participated in phase II of the study. A total of 380 operations responded to the questionnaire section on parasite control, including 82 West, 100 South Central, 100 Northeast, and 98 Southeast operations.

STUDY RESULTS

Frequency and Method of Parasite Control

Most (74.1%) operations treated juvenile equids, on average, fewer than 6 times per year and adults fewer than 4 times per year (80.2%). More than 90% of operations, regardless of region, used macrocyclic lactones to treat parasites in resident equids, while 40% to 50% and 30% to 40% of farms used benzimidazoles and pyrimidines, respectively.

The majority of boarding/training, breeding, and personal use equine facilities used raking or mowing of pastures and pasture rotation as methods for parasite control. Less than 40% and 20% of facilities, respectively, used manure removal and combination deworming.

Reasons for Deworming

Respondents indicated general prevention as their typical reason for deworming, regardless of facility type. Most prioritized tail rubbing, suggesting pinworm infection, as a more common reason for deworming than other signs such as thin body condition, worms in the manure, positive fecal test results, and colic. Small operations were more likely than large operations to use poor body condition and tail rubbing as indicators to deworm, and operations with equids for personal use were significantly more likely to cite tail rubbing and fecal tests as their reasons.

Use of Fecal Egg Counts

Overall, 21.8% of respondents performed FEC in resident equids; however, far fewer (4.2%) equine operations used FEC reduction tests to influence actual parasite control strategies. Respondents were significantly more likely to perform FEC on their farms upon veterinarian recommendation.

HOW VETERINARIANS CAN INCREASE COMPLIANCE

The results of the NAHMS Equine Study, the first of its scale since 1999, revealed that equine parasite control is generally uniform throughout the United States regardless of operation type or size. However, there is an overall lack of compliance with AAEP parasite control recommendations, as indicated by the majority of respondents failing to institute FEC or FEC reduction test—based treatment. These findings are similar to those in the 1999 survey, suggesting that the most recent AAEP guidelines for internal parasite control have not been widely implemented in the United States. These findings contrast with recent European studies, in which 50% to 60% of respondents routinely used FEC.^3,6

The survey results also suggest that a veterinarian’s recommendation significantly influences the use of FEC. Therefore, the authors encourage more communication between veterinarians and their clients to emphasize less frequent anthelmintic treatment and the benefits of FEC and FEC reduction tests when determining parasite control protocols. Finally, treatment should be tailored individually for each equine age category to account for age-specific variations in parasite population numbers and species.

References

• Nielsen MK, Mittel L, Grice A, et al. AAEP Parasite Control Guidelines. Lexington, KY: American Association of Equine Practitioners; 2013. aaep.org/sites/default/files/Guidelines/AAEPParasiteControlGuidelines_0.pdf. Accessed March 20, 2018.
• Hinney B, Wirtherle NC, Kyule M, Miethe N, Zessin KH, Clausen PH. A questionnaire survey on helminth control on horse farms in Brandenburg, Germany and the assessment of risks caused by different kinds of management. Parasitol Res. 2011;109(6):1625-1635. doi: 10.1007/s00436-011-2434-0.
• Stratford CH, Lester HE, Morgan ER, at al. A questionnaire study of equine gastrointestinal parasite control in Scotland. Equine Vet J. 2014;46(1):25-31. doi: 10.1111/evj.12101.
• Robert M, Hu W, Nielsen MK, Stowe CJ. Attitudes towards implementation of surveillance-based parasite control on Kentucky thoroughbred farms—current strategies awareness, and willingness-to-pay. Equine Vet J. 2015;47(6):694-700. doi: 10.1111/evj.12344.
• Nielsen MK, Branan MA, Wiedenheft AM, et al. Parasite control strategies used by equine owners in the United States: a national survey. Vet Parasitol. 2018;250:45-51. doi: 10.1016/j.vetpar.2017.12.012.
• Nielsen MK, Reist M, Kaplan RM, Pfister K, van Doorn DC, Becher A. Equine parasite control under prescription-only conditions in Denmark-awareness, knowledge, perception, and strategies applied. Vet Parasitol. 2014;204(1-2):64-72. doi: 10.1016/j.vetpar.2013.10.016.