The large strongyles (bloodworms) are the most common and most destructive of all of the equine parasites. These worms may be found in horses of all ages, except for young foals. The three main species of large strongyles are Strongylus vulgaris, Strongylus edentatus, and Strongylus vulgaris. For the most part, the mature strongyles are found in the large intestine. The most damaging species, Strongylus vulgaris, migrates via the arteries of the intestine, causing thickening of the wall of the anterior mesenteric artery, where it meets the aorta. This results in either a thrombus (blood clot) or aneurysm (ballooning of the vessel), leading to decreased blood flow to, and infarction of the large bowel. This damage can result in bouts of colic, in which the severity is dependent on the extent of damage. Death may occur.
Small Strongyles
There are about 40 species of small strongyles (cyathostomes). These worms do not migrate past the intestinal lining. When these encysted strongyles emerge back into the intestine, they may induce colic.
Roundworms
Parascaris equorum is the largest intestinal parasite infecting horses. Adult roundworms can reach up to 15 inches in length, and about a 1/2 inch in diameter!! These worms are usually found in younger horses, and usually never in horses older than 5 years. Adult worms develop in the intestine and eggs are passed in the feces. Eggs which hatch in the intestine, release larvae which penetrate the gut wall and migrate to the liver. From the liver, these larvae migrate to the lungs and into the lung alveoli, where they are coughed up and swallowed, reaching the small intestine once again, to complete their maturation. The prepatent period is between 8-10 weeks. In heavy infestations with roundworms, impaction colics may be seen.
Tapeworms
Otherwise known as Anoplocephala perfiolata, the equine tapeworm lives in the junction between the small and large intestines, known as the ileo-cecal junction. Eggs are passed the manure and eaten by the orbatid mite. While grazing, horses accidently eat this mite, ingesting the infective stages of the tapeworm. The adult tapeworms attach to the lining of the ileo-cecal junction. The presence of these worms at the ileo-cecal junction may cause diarrhea, poor condition and unthriftiness. More severe complications of tapeworm infection include impaction colic, spasmodic colic, and some intussusceptions.
Bots
The most common species of bot is Gastrophilus intestinalis. Bots are not actually worms, but rather fly larvae, which develop from eggs laid by the brown bot fly. The eggs are deposited on the hair of the forelegs, shoulders and flanks. The eggs are then ingested by the horse and embed themselves in the oral gums of the horse. They then migrate to the stomach and become attached to the stomach lining, causing irritation, inflammation, and ulceration. The bots can develop here for 8-10 months, at which point they are passed out in the manure and develop into an adult fly, starting the lifecycle over again. The brown bot fly is most popular in late summer and fall.
Pinworms
Otherwise known as Oxyuris equi, pinworms are found in the cecum, large intestine and rectum. Female worms are full of eggs which pass out in the feces. Other worms may crawl out of the anus and deposit their eggs on the surrounding skin, causing intense itching and irritation. Horses infected with pinworms usually rub their rump/base of tail, on fences and posts, causing a scarce appearance to the base of their tail, which may become infected. Other than irritation, these worms do not cause much damage to the gastrointestinal tract. The pinworms lifecycle is completed in approximately 5 months.
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