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Deworming in the First Year of Life Freya Stein, DVM, McKinlay & Peters Equine Hospital
One of the most impactful myths of foal management is that a foal with worms will always look “wormy,” i.e. pot-bellied with a dull, shaggy hair coat and poor growth or weight loss. In fact, it has been demonstrated that healthy, shiny, robust foals can have very high parasite egg Freya Stein DVM, counts. Please do not depend on your foal’s appearance to determine your deworming protocol! Foals require a slightly different approach to deworming than adults mainly due to their lack of immunity against intestinal parasites. Over the first few years of life, horses gradually build enough immunity to parasites to keep infestation to a dull roar (which still requires human intervention to prevent a higher incidence of parasite-associated disease). During their first year of life, however, foals are quite susceptible to heavy infestations of gastrointestinal parasites and their associated ill effects.
The two main genera of worms infecting foals are Parascaris species (aka round worms, ascarids – long, thick white worms as adults) and large and small strongyles (aka blood worms – small, thin red worms as adults). After ascarid larvae are ingested from the environment, they migrate through the intestinal wall and into blood vessels, eventually migrating through the liver and lungs. The larvae are then coughed up, swallowed and returned to the GI tract for maturation. Respiratory infections and inflammation can result from larval migration through the respiratory tract as larvae drag bacteria from the intestines and liver through sensitive lung tissue. The most notorious effects of both ascarids and strongyles are gastrointestinal. Inflammation and damage to the lining of the intestines can result in the poor body condition seen in stereotypically “wormy” foals, but the pathology can have more devastating consequences. Migrating strongyle larvae can cause significant damage to the gastrointestinal vasculature, potentially preventing lack of blood flow to sections of intestine, ultimately resulting in bowel necrosis and severe colic. Ascarids are responsible for intestinal blockages, as adult worms can reach high numbers in the small intestine and tend to “ball up” and cause strong spasmodic intestinal activity. These worm impactions may require surgical correction, with a variable prognosis for survival. A little over
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