Preventing and Managing Anthelmintic Resistance

Parasitic helminths (roundworms and tapeworms) are commonly found in the large intestines of horses and other equids. The most important helminths in horses are the cyathostomins. In horses that graze contaminated pasture and which are not treated with appropriate anthelmintics, large numbers of these worms can accumulate. These include immature larval stages that develop within cysts in the wall of the caecum and colon. A build up of larvae at these sites can lead to serious disease and death if the worms subsequently emerge synchronously. In the field, cyathostomins are controlled using anthelmintics, of which 3 classes are available for use in horses. Unfortunately, in the developed world, resistance to 2 of these classes is widespread in cyathostomins. Moreover, it is now apparent that resistance is emerging to the third class of anthelmintics, the macrocyclic lactones (MLs), ivermectin and moxidectin. Should ML resistance spread throughout the general population, there will be no options left for control of these parasites: no new anthelmintic classes are under development for use in horses in the short to medium term. [...]