Faecal worm egg count analysis for targeting anthelmintic treatment in horses: points to consider

Equine gastrointestinal nematodes are ubiquitous; in horses that graze contaminated pasture and that are not treated appropriately, large numbers of worms can accumulate, which can lead to serious clinical disease. Nematode control has traditionally followed interval treatment regimens, which involve regular anthelmintic administration to all horses based on the strongyle egg reappearance periods of each drug, usually defined around the time of licensing. Interval treatment programmes have resulted in substantial reductions in large strongyle disease, but have made major contributions to the development of anthelmintic resistance, particularly in cyathostomins. Cyathostomin resistance to 2 of the 3 available anthelmintic classes is widespread, and resistance to both classes in single populations is not uncommon. Reduced efficacy of the most commonly used macrocyclic lactone anthelmintics, as measured by shortened egg reappearance periods after treatment, is emerging in cyathostomins. Macrocyclic lactone resistance is also now commonly reported in Parascaris equorum on stud farms. Faecal worm egg counts (FWEC) are increasingly being used as part of targeted approaches to parasite control, whereby only those horses with moderate to high FWEC within a group are treated with an anthelmintic. The objective of this approach is to reduce environmental contamination, while leaving a proportion of the worm population in some horses unexposed to selection pressure for anthelmintic resistance. This article reviews recent findings in equine parasitology research that will underpin guidelines for control, with a particular focus on how to optimise the value of FWEC methodologies and anthelmintic efficacy analyses.