584
EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2022) 34 (11) 584-591 doi: 10.1111/eve.13548
Review Article Parasite faecal egg counts in equine veterinary practice
M. K. Nielsen* Department of Veterinary Science, M.H. Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky, USA *Corresponding author email:
martin.nielsen@
uky.edu
Keywords: horse; egg counts; strongyle; ascarid; interpretation; performance
Summary Widespread occurrence of anthelmintic resistance in ascarid and strongylid parasites has led to recommendations of routine treatment efficacy screening with the faecal egg count reduction test. Furthermore, faecal egg counts are used for classifying mature horses into low, moderate and high strongylid egg shedders, and to monitor for ascarids in foals, weanlings and yearlings. The plethora of egg counting techniques is surrounded by confusion and misconception regarding their use, diagnostic performance and interpretation. This article aims to explain general concepts of evaluating and assessing faecal egg count test performance and address some of the most common misconceptions. The multiplication factor, which is used to convert the number of eggs counted under the microscope to eggs per gram of faeces, is a theoretically derived number, which does not reflect sensitivity. Accuracy and precision are quantitative performance parameters, which are not synonymous. Accuracy is a measure of how close a technique measures to the true count, and has implications for the classification of strongylid egg shedders, but will not affect anthelmintic treatment efficacy evaluation or ascarid monitoring. Precision is a measure of how close repeated counts are to each other, and affects anthelmintic efficacy estimates and strongylid shedding classification. Faecal egg counts do not correlate with worm burdens and do not inform on parasitic involvement in disease. Techniques using test tubes with cover slips placed on top tend to perform with low accuracy and precision in comparison to techniques using counting chambers. Veterinarians should consider the performance of the technique employed for determining faecal egg counts, whether they offer these in house or use a referral diagnostic service. Furthermore, they should consider the training and experience of personnel and protocols in place for diagnostic quality assurance.
Introduction
With increasing levels of anthelmintic resistance being documented in strongylid and ascarid parasites, it has long been recognised that traditional approaches for equine parasite control are not sustainable (Kaplan & Nielsen, 2010). Cyathostomin parasites are now found widely resistant to both benzimidazole and pyrimidine products (Peregrine et al., 2014), and recent findings demonstrate clear evidence of resistance to the macrocyclic lactone class as well (Nielsen et al., 2020). Similarly, equine ascarids are reported widely resistant to macrocyclic lactones (Peregrine et al., 2014) with recent studies also demonstrating resistance to
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benzimidazoles and pyrimidines (Martin et al., 2018, 2021). All available evidence suggests that, once developed and established, anthelmintic resistance does not disappear from equine cyathostomin populations for the next several decades (Lyons et al., 2001, 2007). The most recent anthelmintic class to be introduced for use in equines was the macrocyclic lactones, which were launched four decades ago. Thus, since no new anthelmintic classes with new modes of action are to be expected in a foreseeable future, it is clear that a paradigm shift is needed for equine parasite control. The use of parasite faecal egg counts as a monitoring
tool has been recommended for several decades (Duncan & Love, 1991; Gomez & Georgi, 1991; Nielsen et al., 2014), but these recommendations have been intensified over the past decade. Guidelines for equine parasite control have been published in the USA (Nielsen et al., 2019) as well as in Europe (ESCCAP, 2019), and emphasise the importance of routine faecal egg count testing. In general, egg counts can be used for at least three different purposes; (1) drug efficacy testing with the faecal egg count reduction test (FECRT), (2) classification of adult horses into low, moderate and high strongylid shedding categories for a targeted parasite control approach and (3) monitoring for presence of ascarids and strongylids in foals, weanlings and yearlings. Given the global anthelmintic resistance situation, it is evident that routine resistance testing is now of paramount importance, as no equine anthelmintic product can be assumed to be free of drug resistance issues in equine nematodes of clinical importance. Despite this, several recent questionnaire surveys have demonstrated that FECRTs remain rarely used in equine veterinary practice (Bolwell et al., 2015; Elghryani et al., 2019; Nielsen et al., 2018a; Stratford et al., 2014; Tzelos et al., 2019). While it is concerning that veterinarians and horse owners do not appear more actively engaged in monitoring for anthelmintic resistance at this stage, we do expect this to change in the years to come. Clearly, a veterinarian is no longer able to responsibly make a recommendation of or prescribe an anthelmintic product without appropriate efficacy testing. Guidelines for conducting FECRTs are included in the
current equine parasite control guideline document (Nielsen et al., 2019), published by the American Association for Equine Practitioners (AAEP). However, new and updated FECRT guidelines are underway from the World Association for the Advancement of Veterinary Parasitology (WAAVP), and once these are approved and published, the AAEP guidelines will be amended accordingly. Although fundamental principles for conducting FECRTs will remain unchanged, these new guidelines will increase emphasis on
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