Independent Development of Resistance to Main Classes of Anthelmintics by Gastrointestinal Nematodes of Ruminants and Horses.

Resistance to anthelmintics in gastrointestinal nematodes (GINs) is highly prevalent, as these parasites have been treated with anthelmintics for decades in ruminants and horses. Anthelmintics belong to different classes, each with a different mode of action. The most used are benzimidazoles and macrocyclic lactones and, to a lesser extent, levamisole and pyrantel in herbivores, as estimated from the literature. Combining these classes should be effective in controlling GIN. However, several farmers' practices tend to promote GIN resistance. Therefore, it is unclear whether the use of anthelmintic associations is a sustainable solution for controlling resistance in natural conditions. It is not easy to establish the association of anthelmintic resistances on farms since rarely several anthelmintics and their combinations are used on a single farm. Composed probability calculations were employed when literature data indicated the presence of resistance (to benzimidazoles, levamisole, or macrocyclic lactones) in several ruminant GIN cases. The efficacy of different anthelmintics (benzimidazoles, pyrantel, or macrocyclic lactones) was evaluated in terms of the correlation between faecal nematode egg counts in horses in the available literature. No associations of anthelmintic resistance were found between the different classes of anthelmintics in either ruminants or horses. However, the association between anthelmintic resistance in GIN may appear in the long term. It is presumed that combining drugs may reduce the development of resistance and allow better control of infection on farms where resistance is already established to a low level.