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.
