Horses have many different intestinal parasites and the goal is not to eliminate every parasite but just to control the level of parasites enough to prevent clinical disease. We were so successful at almost doing so that the only parasites remaining are resistant. Now we realize that leaving a few susceptible parasites to compete with the resistant ones is a good idea.

The equine roundworm infects horses when they swallow the infectious eggs and the larvae migrate through the horse's liver and lungs. Migration through the lungs can lead to disease, especially in foals. Large strongyles were the major intestinal parasite problem in horses until ivermectin was introduced to the market in the 80s and now large strongyles are rarely a problem. However, small strongyles are the most serious intestinal parasite at this time as they encyst in the wall of the intestine and most regular dewormers are not effective at killing the encysted larvae. Pinworms do not cause pathology in horses but can cause irritation around the anal area that cause the horse to rub the tail and can lead to skin infections. Bots are commonly in the horse's stomach and can cause gastric irritation and ulcers. Bot fly eggs are commonly seen as small yellow specks on the horse's lower legs and are swallowed by the horse while licking. Horses can also have tapeworms and all of these parasites need to be treated, but you cannot just go into the feed store and choose a deworming product.

All of these parasites require specific dewormers and should be used at specific times. Many of the dewormers are not as effective as their advertising indicates.