In the fall, as the temperatures drop, horses will get some relief as external parasites like flies, mosquitoes and ticks begin to die off. We tend to think of parasite control as something we do in the spring because as the temperature rises, we see and hear the flies and mosquitoes. But fall is also a critical time to take action against the unseen pests. Veterinarians recommend that horses should be dewormed in the fall after the first hard freeze. The reason for this is that many parasites – namely, the gasterophilus intestinalis commonly referred to as horse bot flies – move from the outside to the warm inside of the horse as the temperature drops.

During the late summer months and early fall, horse owners will notice that horses begin to get little yellow specks all over their coats. These are bot eggs. When the horse licks its coat, the eggs are ingested. The friction, warmth and moisture of the horse’s tongue causes the eggs to hatch. The larvae will then move into the horse’s warm stomach and cling there during the cold winter months. In the spring, they release from the stomach lining and pass through the rest of the digestive tract until they are passed out through the feces. There the larvae pupates and becomes an adult about three weeks later.

After a hard freeze, the population of external parasites left to continue laying eggs will be greatly reduced. Most owners opt to give their horses an oral dewormer that kills many of the larvae or worms inside the horse. Even the best worming medicines will not kill all of the parasites in the horse. If horses are not dewormed, parasite populations can grow so much that horses can get colic or lose unhealthy amounts of weight. A tell-tale sign of a horse afflicted with worms may be dramatic weight loss. Worms will not appear in manure until after a horse is dewormed.

Parasites can adapt to or become tolerant of deworming medicines if the same ones are used frequently. Some parasites do not respond to certain types of dewormers. For this reason, many veterinarians recommend alternating medicines such as Ivermectin in the fall and Fenbendazol in the spring. Praziquantel is effective against tapeworms. To get the most effective dewormer for a horse, the manure can be analyzed and a vet can recommend a successful treatment program.