Parasite faecal egg counts in equine veterinary practice

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.