Alaska Equine & Small Animal Hospital, LLC  

Internal parasites are the greatest single cause of colic in horses and can be attributed to many respiratory, digestive and performance problems. In addition, parasites can lower your horse’s resistance to disease and inhibit your horse’s absorption of valuable nutrients. There are more than 150 internal parasites that affect horses, but the most common are strongyles, ascarids, pinworms, tapeworms and bots. The most damaging are the larval stages of the strongyles and ascarids.

Many horses that have dangerous parasite levels appear to be healthy. Signs are occasionally present, however, especially in young horses.

Signs include: weight loss, cough and nasal discharge, lethargy, colic, loss of appetite, diarrhea, dull coat, tail rubbing and hair loss, and mouth lesions.

Prevention is the key.

Have a fecal test performed by your veterinarian. Though some infested horses may not be shedding eggs in the feces at the time of the test, eggs that are found will aid in the direction of treatment.

Utilize the appropriate drugs to kill the parasites.

Minimize pasture contamination. The infective third larval stage can "hibernate" in the winter and be ready to infect in the spring. Although horses exhibit fecal avoidance behavior (they won’t eat feed contaminated with feces), through the mechanical action of walking through the feces, rainfall, and farm equipment, the infective parasite larvae are spread to areas of pasture the horse will eat. Remove manure daily from stalls, and weekly from paddocks and pastures.

Large strongyles (bloodworms) are the most common and most destructive of all internal parasites in the horse. They can be seen in horses of all ages except in very young foals. The sexually mature strongyles are found in the large intestine, and the female lays eggs that are then passed in the manure. The eggs hatch into infective larvae and the horse subsequently ingests the infected hay and grass. The larval stages then travel through the blood vessels to reach the heart, liver and lungs, leaving in their way destructive scar tissue. The most destructive strongyles species, Strongylus vulgaris, migrates through the arties of the intestines causing thickening of the wall of the anterior mesenteric artery where it attaches to the aorta. This results in an aneurysm and blood clots that greatly reduce blood flow to a large portion of the intestines. Common signs of horses infested with large strongyles are periodic bouts of colic following feeding or exercising, which cause the blood clots to move into the small vessels of the intestines. Total blockage of the vessel results in death of the horse.

Small strongyles are increasing in species numbers due to drug resistance. Migration of these parasites is limited to the lining of the intestines where the small strongyles encyst. Colic can be induced when a large amount of encysted small strongyles emerge from the intestinal lining.

Life cycle: 8-10 weeks

Large roundworms (Parascaris equorum) are the largest internal parasite affecting the horse. They are commonly found in young horses, and usually not found in horses older than five years old. Immunity normally develops following exposure during adolescence. The female roundworm lays eggs in the intestine and the eggs pass out in the feces. The larvae develop within the eggs, which are ingested when feed and water are contaminated. The infective larvae hatch from the eggs in the intestine and subsequently burrow into the intestinal wall. The larvae then migrate through the liver, then through the lungs to the tiny air sacs (alveoli) in the lungs where they are coughed up and re-swallowed. The larvae then develop to sexual maturity in the small intestine and the cycle repeats.

In young horses, respiratory problems can be started and exacerbated as a result of roundworm migration. In addition, roundworm impactions (colic) can be seen in heavily infested young horses that are not on a routine deworming program.

Life cycle: 10-13 weeks

Pinworms are the least damaging of the internal parasites. Sexually mature female pinworms (Oxyuris equi) are found in the cecum, colon, large intestine and rectum of the horse. Some female worms that are full of eggs pass out in the feces. Others crawl out the rectum and deposit eggs on the surrounding skin, resulting in irritation and itching. For relief, the horse rubs his tail, causing breakage and loss of hair on the tail and around the anus. Occasionally, a secondary infection can result.

Life cycle: 5 months

Tapeworms, also known as flukes and flatworms, attach to the intestinal lining surrounding the ileocecal valve. Each tapeworm is made of segments called proglottids that develop from immature to mature, adult and gravid. Each segment contains both female and male organs. The gravid segment containing the fertile eggs slough off and pass out in the feces. An intermediate host such as an oribitad mite eats the eggs, which then undergo several developmental stages inside the mite to become the infective stage. The horse ingests the mite as it grazes. Heavy infestations can result in cecal hemorrhaging, cecal blockage, intestinal ulcers and intestinal rupture. In addition it is believed that tapeworm infestation can cause the gut to become hypermotile, or too active, which can result in the intestines telescoping into the cecum.

Eggs of the tapeworm are generally not seen on a fecal exam as the eggs are contained within the proglottid.

Life cycle: 2-5 months

Bots are the larvae of the botfly. The botfly lays yellow eggs of the hairs of the forelegs, shoulders and flanks of the horse. The larvae hatch and enter the horse’s mouth when the horse licks the eggs. The larvae then embed into the cheeks, gums and tongue. After a month, the larvae migrate to the stomach where they attach to the lining and cause inflammation and ulceration of the stomach. After 8-10 weeks, the larvae pass out in the feces. Once outside, the larvae hatch into adult botflies.

Life cycle: 8-10 weeks during the season

Deworming schedules are designed to rid the horse and further decrease the transmission of parasite eggs. Dewormers are rotated to limit the chance of the parasites developing resistance to the drug. At this time, equine internal parasites have not developed resistance to ivermectin and moxidectin, which further strengthens the case to rotate dewormers.

In addition, administering a daily dose of pyrantel (Strongid-C) is very effective in killing new incoming larvae before they invade the intestinal lining. This program is effective against strongyles, as well as ascarids in horses less than 10 months old. The key to effectively using a daily dewormer is first deworming the horse with ivermectin or moxidectin prior to starting the program. Also, don’t forget, horses still need to be dewormed on a rotational basis every 8 weeks for the parasites the Strongid-C does not affect.

Alaska Equine and Small Animal Hospital recommends starting in the spring with Panacur (fenbendazole) followed by Strongid (pyrantel pamoate) then Ivermectin for most horses in Southcentral Alaska. Rotations should occur every 8 weeks.

Chris Burns, DVM