Anthelmintic resistance: what do we tell our clients?

Gastrointestinal parasites are one of the most common threats to equine health and well-being. However, anthelmintic resistance in horse parasites is increasing worldwide. With no new classes of anthelmintic expected on the equine market in the near future, this poses a serious threat to equine health.

Parasite–host relationships and the rationale for targeted anthelmintic use

Cyathostomins (small strongyles) are considered the most significant parasites affecting horses of all ages, due to their high prevalence, potential pathogenicity and ability to develop anthelmintic resistance. Whilst cyathostomin infections are often subclinical, they can result in signs of malaise, anorexia, colic and weight loss. More rarely, ‘larval cyathostominosis’ resulting from mass synchronous emergence of previously encysted larvae can cause rapid weight loss, diarrhoea, colic and death in up to 50% of cases. Individual horses differ markedly in their susceptibility to strongyle infections. On typical premises, 20% of horses shed 80% of the strongyle eggs, thereby providing the major contribution to pasture contamination. Once identified by faecal egg counts (FEC), these ‘high egg shedders’ can be targeted with anthelmintics during the grazing season to reduce pasture contamination. Conversely, ‘low egg shedders’ can be left untreated; the eggs passed in their faeces then contribute to the parasite population ‘in refugia’. […]