AffectedAnimals:
Horses.

Overview:
More than 40 different species of parasites that commonly infect horses are grouped under the term "small strongyles." These and another type of strongyle parasite, large strongyle worms, most often will infect a horse simultaneously. However, horses tend to be burdened by greater quantities of small strongyles, which also produce more eggs than the large strongyle worms.

Small strongyle larvae pose a significant health risk because they feed on the superficial lining of the large intestines, or the mucosa, where they cause damage that leads to the formation of ulcers, as well as severe inflammation of the cecum and colon. Larvae will migrate inside the intestinal wall and continue to develop into an adult, and live there within the mucosa in a dormant stage for an extended period of time.

Typically during the late winter and spring months, the small strongyles migrate out of the intestinal wall into the center of the intestines, and this massive movement of worms causes clinical signs such as diarrhea and colic. Severely affected horses and ponies may die from a small strongyle infection.

Clinical Signs:
Clinical signs include diarrhea, depression, progressive weight loss, emaciation, fever, colic or abdominal distress, and death in severely affected horses and ponies.

Symptoms:
See clinical signs.

Description:
There are more than 40 different species of small strongyles, or cyathostomes, and most of these parasites are smaller than the large strongyle worms. The smaller parasites, however, are no less harmful. Small strongyles can significantly damage a horse's cecum and colon by feeding on the superficial lining of the intestines called the mucosa. Ulcer formation and inflammation of the cecum and colon can occur when the small strongyle larvae move into the wall of the intestines. The tissues of the intestinal wall will encyst, or surround the larvae, protecting them so that they can live inside the mucosa for a prolonged period of time. It is the encysted larvae that cause continued irritation and damage to the intestines.

During late winter and spring, the small strongyle larvae living in the wall of an infected horse's cecum and colon come out of dormancy as mature adults. Large numbers of adult worms can be present within the intestines at once. The presence of numerous adult parasites within the intestines can cause clinical signs such as diarrhea and colic. In severely affected horses and ponies, death can result.

Diagnosis:
Often, diagnosis is based on the clinical signs of disease, and the horse's response to treatment.

A more definitive diagnosis requires identifying the small strongyle eggs within the feces, through a fecal flotation exam. Unfortunately, a horse may be harboring an infection for a significant period of time before the disease is detected through this test, because once a horse becomes infected, the small strongyle larvae will take six to 12 months to mature into adults and produce eggs.

It is not possible to distinguish a small strongyle from a large strongyle through a fecal flotation exam, so often a fecal culture may be performed as well. This procedure allows the larvae to develop, so that they can be evaluated under a microscope to determine whether they are small or large strongyles. Most horses have a mixed infection of both small and large strongyle parasites.

Prognosis:
With proper treatment and regular deworming, most horses can be effectively cleared of the parasites. A good deworming protocol should virtually eliminate the number of eggs passed in the soil.

Transmission or Cause:
Horses become infected with strongyles when they eat infective strongyle eggs passed in the feces of another horse with the infection. For the eggs to be infective, they must continue to develop for a period of one to three weeks; it is not until the eggs have developed into the infective larval stage, or L3, that they will cause infection in a horse that ingests them. Once ingested, the L3 larvae migrate to the cecum and colon, which make up the majority of a horse's large intestine.

Treatment:
Horses and ponies need to be dewormed every eight weeks with a dewormer that is effective against small strongyles. The inflammatory response of the large intestine to the parasites often makes deworming treatments less effective. Consequently corticosteroids, which reduce inflammation, may be prescribed. The examining veterinarian will determine a specific steroid dosage; incorrect dosages can cause significant side effects.

Many small strongyles are resistant to several benzimidazoles; at this time, medications such as pyrantel pamoate, ivermectin, oxibendazole, and piperazine have been shown to be effective in treating small strongyle infections. To prevent reinfection, all animals in the pasture must be dewormed. For a guideline of suggested worming protocols, see the encyclopedia section on deworming schedules.

Prevention:
Routine deworming every eight weeks is an essential preventive measure against a strongyle infection. Multiple horses should not share small sections of land because often they will overgraze and be forced to eat grass near or in feces that may contain the infective larvae. Removal of manure at least once a week will help reduce the spread of eggs into the soil. In addition, before manure is spread on land, it should be composted to eliminate larvae.