An anthelmintic is any drug that kills or expels parasitic worms (helminths) from a host. In horses, the term covers the paste and liquid dewormers used to control internal parasites, primarily strongyles (large and small), ascarids (roundworms), tapeworms, and bot fly larvae.
The major anthelmintic classes available for horses are benzimidazoles (fenbendazole, oxibendazole), macrocyclic lactones (ivermectin, moxidectin), and praziquantel, which is specifically active against tapeworms. Each class targets different parasite species and life stages, and resistance patterns differ: small strongyles have developed significant resistance to benzimidazoles in many populations, and ivermectin resistance in ascarids has been documented in horses treated repeatedly from a young age. This is why the current standard of care moves away from calendar-based rotational deworming toward fecal egg count-guided treatment, which doses only horses with demonstrated parasite burdens and reduces selection pressure for resistance.
Choosing the right anthelmintic requires knowing which parasites are present, what the local resistance profile is, and where the horse sits on the shedding spectrum (low, moderate, or high egg shedder). A fecal egg count done by a veterinarian provides this baseline. Heavy ascarid burdens in young horses should not be treated with macrocyclic lactones at full dose, as rapid kill of a large worm burden can cause intestinal impaction — a failure mode not present with slower-acting benzimidazoles. The interplay between anthelmintic selection, resistance management, and pasture management is the core of a modern parasite control program.
Further Reading
- Anthelmintic drugs — Wikipedia overview of drug classes used against parasitic helminths across species.
- Macrocyclic lactones in equine parasite control — Merck Veterinary Manual clinical reference on ivermectin and moxidectin pharmacology and use.