Scientific name: Strongyloides westeri
Common name: Intestinal Threadworms

Description: Adult threadworms are thin, hair-like parasites that measure only 1/3 to 3/8 inch (8 to 9 mm) in length. Foals are especially vulnerable to intestinal threadworms. Foals become infected after nursing from infected mothers and ingesting larvae that are passed through the mare’s milk. Pregnancy stimulates larvae migration to the udder. Larvae can pass through the mare’s milk for the first 45 days after birthing. Left untreated, larvae can exist in the body tissue of broodmares for years. Consequently, these mares become a source of contamination for each foal crop.

Life Cycle: Threadworm larvae can be ingested through a contaminated feed source, or it can penetrate the skin. While foals can contract the parasite through nursing, other horses can ingest larvae while grazing or feeding. When larvae enter through the skin, they migrate to the lungs where they can cause bleeding and other respiratory problems before being coughed up and swallowed. Threadworm larvae mature in the small intestine. Adult females lay eggs that are passed out through manure.

Symptoms: Clinical signs of intestinal threadworms include diarrhea, respiratory distress, poor appetite, anemia, weakness and a slow growth rate. Dermatitis can result in areas where threadworms enter through the skin.

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