The Usual Suspects
The most significant internal parasites of the horse include a nasty collection of characters:
What are Small Strongyles?
Small strongyles, also known as cyathostomes, are a group of small parasites that reside in the large intestines of the horse. They are called small strongyles because the adults are only 1/4 to 1 inch in length. However, these small parasites can produce tremendous damage to the intestinal lining of the horse.
These worms become dangerous when the larvae enter the horse’s digestive system, burrow into the intestinal lining, and form cysts. Encysted small strongyles can remain in this state for up to three years.
Unfortunately, there’s no way of knowing how heavy an encysted small strongyle load a horse is carrying, since fecal analysis cannot measure worms in the encysted state. What’s worse, horses may not show outward sighs of a parasite problem until it is too late.
How Does It Affect Your Horse?
Encysted small strongyles can cause severe clinical signs and even death when thousands to millions of the fourth stage larvae (L4) emerge simultaneously from the intestine wall. The resulting damage to the intestinal mucosa can show up in the horse as:
In less severe cases, horses may exhibit decreased performance, poor food utilization, and a dull hair coat.
What Are Large Strongyles?In the horse there are three species of large strongyles that range in size from 1.5 to 2 inches in length. Rather than encysting like the small strongyles, the immature larval stages of the large strongyles are known for their ability to migrate through the organs and arteries of the abdominal cavity of the horse. If this migration damages the blood supply to the intestinal tract—a condition known as verminous arteritis—a portion of the intestines may become inflamed or die due to loss of blood supply. The resulting thromboembolic colic often results in death of the horse.
What Are Roundworms?
The roundworm, also called ascarid, is the largest parasite of horses, growing as large as 12 to 15 inches in length. Roundworm infection is common in foals and weanlings, and to a lesser extent in yearlings. Considering the large size of the roundworm and the relatively small size of the intestines in young horses, it is not uncommon for intestinal blockage or rupture to occur when large numbers of adult ascarids are present.
During the life cycle of the roundworm, the immature larvae migrate through the liver and lungs causing tissue damage and signs of respiratory disease including fever, cough and nasal discharge (also termed verminouos pneumonia). This tissue damage in the lungs may predispose the infected horse to secondary bacterial pneumonia.
The eggs produced by the adult roundworm are passed in the feces and can live for years in stalls, paddocks, and pastures. This increases the potential for spreading ascarid infection between horses and for re-infecting the original host animal.
What Are Pinworms?
The equine pinworm is a worm that resides freely in the colon and feeds on the contents. Because pinworms do not encyst in the intestinal wall or migrate through the body, they cause less damage to the horse than other internal parasites discussed in this module. However, female pinworms often deposit their eggs around the anus, which causes irritation. As a result, infected horses may have a rough tail head due to the rubbing of the irritated region against posts or barns.
What Are Tapeworms?
Tapeworms are flat, segmented worms that are transmitted to the horse by ingesting orbatid mites living on pastures that are infected with an immature stage of the tapeworm. Because this parasite is dependent on the orbatid mite, only horses located in areas where these mites are present are at risk for tapeworms. Equine tapeworms live in the small intestine or cecum. Tapeworms have powerful suckers that can cause thickening and ulceration of the intestinal lining. Severe cases can lead to intestinal blockage or colic.
What Are Bots (fly larvae)?
Bots are not worms, but actually the immature larval stage of botflies. Adult botflies are about the same size as bees and lay their eggs on the hairs of the front legs, throat, and chin of the horse. The eggs hatch when the horse licks or bites the area where they have been deposited. The larvae penetrate the mucous membranes of the mouth and migrate to the stomach where they attach. They will remain inside the horse for 8 to 10 months if left untreated in the fall of the year. In the spring, the bots detach, move through the intestines, and pass out in the feces.
On initial infection, bot larvae can cause small ulcers in the mouth. In large numbers, they can obstruct the outlet of the stomach or produce deep pits in the stomach wall that can perforate and cause peritonitis, a fatal condition.