Parasitism - Part I - The Opponents

by: Dr. Robin Winenger

The best defense is a good offense. One place to start is to first learn a little bit about the players on the other team - the nasty internal and external parasites that like to pester your horse. Once you learn about their weaknesses you can better defend your horses against them.

First up to the plate is the roundworm (Parascaris equorum). This parasite likes to inhabit your horse's small intestine. Unlike the roundworm species that infest dogs and cats, they are not passed from the mare through the milk or the placenta. Most commonly they are picked up in the environment by ingesting contaminated feedstuffs or licking contaminated surfaces. They can migrate through the liver and lungs and are coughed up and then swallowed. Roundworms are mainly a problem in foals and can cause respiratory signs as well as poor growth and a rough hair coat.

Second on the list are the intestinal threadworms (strongyloides sp.). They inhabit the small intestine as well and can be the cause of foal heat diarrhea. Like roundworms this parasite is almost exclusively a problem in foals. They can be passed through the mammary gland to the nursing foal or can penetrate the skin from a contaminated environment. It migrates through the body's tissues to the small intestine. Tracheal migration can also occur, causing coughing.

Third, is the group known as the large strongyles. They parasitize the large colon. The most notorious species, Strongylus vulgaris, is responsible for verminous arteritis, a disease where the worm is able to penetrate the intestinal wall into the arterial supply, causing clot formation that blocks blood flow to the intestines. This causes a lack of oxygen to the intestinal tissues, which leads to colic, shock, and death. Luckily, with the introduction of ivermectin, these parasites are practically extinct.

Next is the heavy-hitting large group of parasites called small strongyles (or cyathostomes). There are many species in this group, but they behave biologically the same and therefore are controlled in the same way. They tend to be more challenging to control because they have a hypo biotic stage, which means they can suspend development until grazing season.

This makes small strongyles very difficult to treat because deworming medications target mechanisms of growth. Usually small strongyles are transmitted seasonally through ingestion of contaminated pasture grasses or feedstuffs in contaminated stalls. They mature in the large colon and cecum. Control programs in adult horses are designed to primarily target small strongyles.

Now up are tapeworms. They reach sexual maturity in your horse's rectum where they lay eggs that pass in your horse's feces. Orbatid mites ingest the eggs and then attach to your horse's legs. Your horse ingests mite and egg when licking its legs to relieve the itching, repeating the process. Occasionally, they cause ulcerations and erosions around the junction of the small intestine with the cecum and the large intestine. This sometimes causes the intestine to fold in on itself like a telescope causing what we call medically an intussusception, which can only be corrected surgically. The vast majority of the time, however, tapeworms do not cause problems.

Sixth in the order are pinworms. They reach adulthood in the rectum and crawl out and lay their eggs, which they attach in a sticky sac to your horse's anus. This is very itchy and may explain why your horse rubs his backside on fence posts, breaking all his beautiful tail hair.

Seven and eight will be grouped together. They are the stomach and eye worms. They are transmitted through biting flies that like to land on your horse's eyes and nose, and any open wound. They are attracted to the fluid secretions or the blood and transmit the worm larvae through their fly saliva when they bite your horse. When the larvae are transmitted into an open wound and get under the skin, they cause the nasty skin lesions known as summer sores.

I mention botflies (or gastrophilus as the larvae are called) because most horse owners know about them. They rarely cause a problem for horses. The flies lay eggs on the horse's chin or legs, where they are licked off or migrate to your horse's gums. They encyst in the gums and erupt, sometimes leaving ulcers. They are then swallowed where they live in the stomach, to later be passed in the feces where they mature into the adult fly and start the life cycle over again.

We cannot forget to talk about the external parasites, like midges, horseflies, black flies, mites, and lice. Mostly, the parasites of this section cause problems by driving your horse mad or being the intermediate hosts to the parasites mentioned above. However, lice and mites can also cause external irritation and skin lesions. Lice can be biting or sucking species. Sucking species can cause blood loss anemia. Mites cause severe itching and skin lesions.

This is by no means a comprehensive list of all the parasites that can affect horses, but simply a touch on the ones you will likely encounter. In the November issue we will discuss control and prevention, including what dewormers are available and what protocols to use.