Abstract
Against a global background of increasing anthelmintic resistance in parasites and a domestic over-reliance on anthelmintics to control gastrointestinal parasites, little is currently known about the resistance status of equine parasites in New Zealand. The objective of the study was to investigate the efficacy of ivermectin in yearling Thoroughbreds. Data were collected from six stud farms from November 2014 to January 2015. Yearlings were selected based on not being treated with anthelmintics for a minimum of six weeks previously and on having a preliminary screening strongyle faecal egg count (FEC) of 25 eggs per gram (EPG) of faeces or greater. On Day 0, faeces were collected and yearlings were treated with ivermectin (a minimum of 200μg/kg). On Days 7, 14 and 21 faeces were again collected from the yearlings, with follow-up samples take on either Day 28, 35 or 42 depending on the stud farm. In total, six stud farms and 117 yearlings (median 23 yearlings; range 5 to 27 yearlings per stud) were eligible for the study. The mean FEC on Day 0 was 609.6 EPG (range 100 to 2000). The majority of horses (113/117) had zero egg counts 7-days after treatment. One and five horses had positive FEC 14 and 21days' post-treatment, respectively. The efficacy of ivermectin ranged from 99.8% to 100% on Day 7 to between 98.5% and 100% on Day 21. On one stud, the efficacy on day 14 was 94.4%. There was reduced efficacy (<90%) on three studs 28 to 42days' post-treatment suggesting shortened egg reappearance period (ERP). On one stud farm (n=7), egg counts remained zero up to 42days' post-treatment. The FECRT observed could be interpreted as consistent with the continued efficacy of ivermectin in the egg-laying adult stages. However, shortened ERP after ivermectin treatment is a concern and current practices for parasite control in horses in New Zealand are arguably not sustainable.
Highlights
Introduction For the last40 years, horses have been treated regularly with anthelmintics for the purpose of controlling gastrointestinal parasites and to reduce reinfection by keeping the numbers of infective (L3) parasites on pasture to a minimum (Nielsen et al, 2014a)
On Day 0, faeces were collected from eligible horses, followed by treatment with ivermectin (Eqvalan®, Merial Ancare, New Zealand) at a minimum of the standard recommended dose rate of 200 micrograms per kilogram bodyweight overseen by a veterinarian contracted to the project or the stud farm’s own veterinarian
Despite horses being selected based on a preliminary positive faecal egg count (FEC), 5% (2/39), 33% (4/12), 17% (2/12) and 10% (3/30) yearlings from Stud 2, 3, 4, and 5, respectively, did not have positive FEC at Day 0
Summary
Introduction For the last40 years, horses have been treated regularly with anthelmintics for the purpose of controlling gastrointestinal parasites and to reduce reinfection by keeping the numbers of infective (L3) parasites on pasture to a minimum (Nielsen et al, 2014a). The frequent treatment of horses and lack of refugia have selected for parasites that are resistant to anthelmintics (Kaplan and Nielsen, 2010; Nielsen et al, 2014a; Reinemeyer, 2012; Traversa et al, 2012). The most widely used method for determining if an anthelmintic treatment is effective is the faecal egg count reduction test (FECRT) (Coles et al, 1992; Coles et al, 2006; Kaplan and Nielsen, 2010). Screening faecal egg counts (FEC) may be used before the initial treatment to ensure that the egg counts are high enough for treatment, as it can be hard to interpret the results in horses with low FEC (Kaplan et al, 2004)
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