Cyathostomins are the main digestive parasites found in equines: all equines with access to pastures are exposed to them. Most infested animals show no symptoms. However, in the event of heavy infestation, these parasites can cause diarrhoea, stunted growth and weight loss, potentially leading to death in young animals.

To limit economic losses, the measures taken to control these parasites are based mainly on the frequent use of antiparasitic drugs, which eliminate parasites in the host and can prevent reinfestation. Three classes of antiparasitic drugs are currently available: benzimidazoles (fenbendazole), tetrahydropyrimidines (pyrantel), and macrocyclic lactones (ivermectin and moxidectin).

The risk of resistance to antiparasitic drugs is particularly high on stud farms

“To avoid any consequences for the health and growth of their horses, breeders tend to systematically administer antiparasitic drugs, without first assessing the risk of infestation” explains Aurélie Merlin, research project manager in the Pathophysiology and Epidemiology of Equine Diseases Unit of ANSES’s Laboratory for Animal Health. “These practices often lead to the overuse of antiparasitic drugs, particularly on racehorse stud farms, thereby promoting the selection of parasites that are resistant to these products”. In this sector, frequent movements between countries, whether for breeding, training or racing, increase the risk of resistant parasites being introduced onto and spreading within and between farms.

An ivermectin-resistant cyathostomin population identified for the first time in France

ANSES's scientists conducted a study on a stud farm of thoroughbred racehorses, where the breeder suspected resistance to several antiparasitic drugs. The young horses on this farm had been successively treated every few weeks with a drug belonging to one of the three classes of antiparasitic agents authorised for the treatment of these worms in equines: fenbendazole, pyrantel, or ivermectin. After each treatment, cyathostomin eggs continued to be found in their dung, revealing resistance to these three antiparasitic drugs. “The existence of populations of worms resistant to fenbendazole or pyrantel is well known, but this is the first time that an ivermectin-resistant population of worms has been discovered in France” affirms Aurélie Merlin. This is also the first time that a population simultaneously resistant to all three classes of antiparasitic agents authorised in France has been discovered on the same farm.

The importance of the rational use of antiparasitic drugs

To assess the extent of resistance to antiparasitic drugs in France and to refine recommendations for owners, further studies need to be carried out on other farms and in other equines. Nevertheless, this example illustrates the need to use antiparasitic drugs more rationally. Indeed, it is possible to only treat those animals that shed a lot of eggs in order to reduce the overall level of infestation in pastures and protect the most susceptible animals: “Some horses are naturally capable of blocking infestation or living with it, without any consequences for their health or welfare and without any need to treat them” points out the scientist. “On the other hand, using antiparasitic drugs too often can not only promote resistance but can also disrupt the microbiota of equines and be toxic to both animals and the environment”.