AffectedAnimals:
Horses. Younger horses, as well as those that have not been previously exposed to parasites, are more susceptible to infection.

Overview:
Horses that graze in pastures often become infected with large strongyle worms. These harmful parasites, transmitted through the feces of other infected horses, can cause significant gastrointestinal damage and anemia. In addition, as the parasitic larvae migrate within the horse's abdomen, they cause damage to organs such as the liver and the pancreas, which can lead to pancreatitis. Horses can die from a severe large strongyle infection; however, the parasites can be eliminated by appropriate deworming medications.

To prevent infection with large strongyle worms, animal owners should ensure that pastures are not overgrazed. This is optimally achieved by allocating two to three acres of pastureland per horse. In addition, all horses should be on a regular deworming schedule.

Clinical Signs:
Diarrhea, weakness, pyrexia, or fever, anorexia, or loss of appetite, weight loss, abdominal discomfort, and anemia if there is significant blood loss. Severe infections with Strongylus vulgaris can cause colic, gangrenous enteritis, torsion or rupture of the intestines, and death.

Symptoms:
See Clinical Signs.

Description:
Infection with large strongyle worms occurs commonly among horses that graze in groups or on pastures previously grazed by infected horses. Transmission occurs when an infected horse passes the strongyle eggs in its feces, thereby contaminating the environment. Once the larvae are passed into the soil, within one to three weeks they mature to the infective stage of the larvae called an L3. Once swallowed by a grazing horse, the infective larvae, or L3, will continue on a migration path that varies with species. There are three major species of large strongyle worms: Strongylus vulgaris, Strongylus edentatus, and Strongylus equinus.

Strongylus vulgaris infective larvae shed their sheaths into the small intestine, and then enter the cecum, or colon, where they continue to develop. Next, they migrate through the large intestine into blood vessels, causing damage and inflammation along the way. Extensive damage may ensue when the formation of blood clots, called thrombi, occurs; the clots can block the flow of blood to parts of the intestines. This causes death of parts of the bowel due to the lack of oxygen and vital nutrients. Horses with Strongylus vulgaris often develop abdominal pain, anemia, and sometimes life-threatening colic from necrotic or ruptured intestines. After several months, the larvae continue to develop, return via the bloodstream to the cecum and colon, complete the maturation to adulthood, and begin producing eggs.

Strongylus edentatus and Strongylus equinus adults are more harmful to the horse and suck more blood than Strongylus vulgaris adults, but the larvae are not as dangerous. The infective larvae of Strongylus edentatus actually leave the large intestines by boring into the walls of the intestine and entering the veins that lead to the liver. In the liver the larvae develop, grow larger, and migrate about the liver for about eight weeks, causing damage and inflammation called hepatitis. They then leave the liver, travel throughout specific parts of the horse's abdomen, and return again in eleven months to the horse's cecum.

Strongylus equinus larvae leave the cecum and travel to the liver, pancreas, and other parts of the abdomen. They then return again after nine months to the large intestines. This irritation can lead to generalized inflammation of the pancreas, called pancreatitis, inflammation of the abdomen, called peritonitis, and inflammation of the liver. Infection with large strongyles alone is rare. More commonly, horses develop a mixed infection of both large and small strongyles.

Diagnosis:
Diagnosis of the presence of strongyles can be determined by a fecal flotation examination, but this test does not differentiate between large and small strongyles. However, a fecal culture will allow for the maturation of the larvae and the subsequent identification of the parasites infecting the horse. Horses usually do not get a strictly large or small strongyle infection; typically, they are infected with both.

Colic or abdominal distress due to Strongylus vulgaris infection may result from inflammation and obstruction of the cranial mesenteric artery. This artery can be palpated on rectal examination, and a veterinarian may be able to identify changes consistent with Strongylus vulgaris infestation

Prognosis:
With proper deworming schedules and appropriate use of deworming medications, horses usually have a good prognosis. It is important that all horses be put on a regular deworming schedule according to an examining veterinarian's recommendations.

Transmission or Cause:
Horses become infected when they ingest feces containing strongyles at the infective larval stage, or L3. Once the larvae are swallowed, they travel to the small intestine, cecum and colon. From here, the larvae ultimately develop into sexually mature adults and return to the intestines to produce eggs that are passed into the horse's feces -- into the environment, where they can infect other horses. The eggs can then develop into the infective stage of the larvae, or L3, over a period of one to three weeks. Under certain conditions, these eggs can survive in the environment for up to two years. In moist conditions, L3 larvae can survive for approximately five weeks to several months.

Treatment:
Successful treatment requires the use of a dewormer that kills both large and small strongyles. Many small strongyles are resistant to certain medications, including several in the benzimidazole family. Some common dewormers that have been used with success against large and small strongyles include pyrantel pamoate, ivermectin, and oxibendazole. Owners should consult a veterinarian about the best deworming medications, dosages, and schedules for all horses on a farm.

Prevention:
Owners should avoid overstocking pastures with horses; optimally, each horse should be designated two to three acres of land for grazing. The overgrazing of pastures causes horses to eat near areas containing manure, thereby increasing the infection rate. Even one to two cases of infestation with strongyle worms will make it necessary to deworm all animals sharing the land in order to prevent reinfection. Pasture hygiene is also critical. It is important to remove and compost manure to kill strongyle larvae. Also, fresh manure should not be spread throughout the pasture; this further infects the land.

The rotation of dewormers has been suggested as a means of helping prevent parasites' developing resistance medications. An examining veterinarian will be able to determine the type of eggs present in a pasture, and put all horses on a farm on a deworming schedule that is specifically tailored to the farm. There is no single deworming schedule that works for all regions and all conditions. Consequently, regular consultation with a veterinarian is recommended.