An essential part of your horse’s health program is developing a deworming strategy. Your horse’s deworming schedule depends on your management system and the parasite loads of individual horses pastured together. Important considerations are the age and health status of treated horses. Types of dewormers are selected based on active ingredients which target each specific type of parasite. Typical deworming strategies target small strongyles, Ascarids (large roundworms), Gasterophilus (bots) and tapeworms.
Small strongyles are small (less than 1” in length) red worms that affect adult and young horses. These parasites occur in large numbers (30,000-100,000) in the large intestine. Larva live on the pasture and are easily killed by extreme climate, whether that be too hot, cold or dry. In Florida, the greatest number of infective larva are found from November to April. Pastures are essentially free of larva during the summer months. The opposite is true for northern climates. Small strongyles are known as “thieves” rather than “killers” as they typically cause ill-thrift, colic and diarrhea. Death from this type of parasite is rare.
Ascarids (Large Roundworms):
Ascarids are large (6”-9” in length) tan worms that primarily affect foals less than 6 months old. Most adult horses develop an immunity to Ascarids. When compared to small strongyles, Ascarid egg are extremely tough. Typically eggs can live one or more years on pasture, and are not affected by climate. Transmission occurs year round. Usually this year’s foal crop are infected with eggs passed during the previous year. Ascarids typically live in the small intestine. Clinical signs include weight loss, ill thrift, snotty nose and pneumonia like symptoms, colic with intestinal obstruction and death.
Horse bots are the larva of bot flies. Adult flies (Gasterophilius) lay their eggs on the hairs of almost any part of the body, but mostly forelimbs and shoulders. These eggs are stimulated to hatch by the animal’s licking, and make their way into the mouth. From there, the larva migrate into stomach and small intestine where they attach themselves. The larva develop for 8-10 months, after which they are passed in the feces, pupate and become adult flies. The larva causes damage when they attach to the stomach lining. This causes ulceration around the attachment site, as well as an immune reaction. In addition, horses can develop erosions at migration sites of the larva. Horse bots are difficult to diagnose via fecal examination. Presence of yellow egg packets on hair or signs of gastric inflammation during winter months are indicative of infestation. In Florida, transmission is thought to occur year round. Current recommendations are to treat at least once annually, at the end of bot fly season. In Florida, where the bot fly season is longer, additional treatments may be necessary.
Tapeworm larva develop within an intermediate host, the forage mite. Horses become infected while grazing and ingesting the forage mite. After ingestion, the larva hatch from the mite, and develop into adult tapeworms. The adult worms are usually found in the small intestine and cecum. There are usually no clinical signs if a horse has a light worm burden. However, with a heavy infestation horses may be thin, have a poor hair coat, and anemia. Heavy infestations have also been associated with GI ulceration, perforation and colic. Diagnosis can be made by identifying egg packets in feces- however these egg packets are only shed sporadically. It is difficult to identify tape worms on fecal float or fecal egg count. There are blood and saliva tests that can be used to detect presence of tapeworms, however, more frequently we use timed deworming to treat for tapeworms.
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