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Threadworms are generally found to cause problems in horse foals, donkeys and zebras. Threadworm infection is the first patent parasitic infection often observed in foals from dams who have not been dewormed. Transmammary Transmission most common – larvae are actually transmitted from mammary tissue of dam via mare's milk to foal where they then undergo mucosal migration in foal's small intestine and mature into adult worms by the time the foal is 8 – 14 days of age. This is why mares should be de-wormed in the month prior to their due date.
Threadworm Lifecycle
- Adult female Threadworms in the foal's small intestines produce larvated (embryonated) eggs.
- When foals are 10 – 14 days old, larvated eggs are passed in their feces and they mature rapidly in environment to infective larval stage.
- The infective larval stage can be ingested from the environment, from mare's milk or even occur via skin penetration! Tracheal migration of larvae can even occur as they make their way to the small intestine.
Signs of Threadworm Infestation
- Large numbers of adult worms can cause moderate inflammation of the anterior third of the foal's small intestine, can erode the intestinal mucosa and cause acute diarrhea known as 'foal heat diarrhea'.
- Foals may experience loss of appetite, weight loss and weakness.
- Infection in foals is generally self-limiting – adult worms will usually disappear by the time foals are 16 – 24 weeks of age.
- Intestinal infection rarely occurs in horses over 6 months of age. Diagnosis is usually made by finding larvated eggs on fecal float of foal's manure. Larvae can also be found in mare's milk 4 – 9 days after foaling.
Treatment
- Ivermectin is > 99‰ effective in killing infective larvae in mammary tissue of mare. This coincides with recommendations of deworming pregnant mares with Ivermectin in the month preceding foaling to prevent transmission of Threadworms.
- Foals will not need to start a deworming protocol until 6 – 8 weeks of age at which time they can receive Pyrantel Pamoate (Strongid) or Fenbendazole (Panacur) once a month until 6 months of age. At 6 months they can begin receiving Ivermectin, Strongid or Panacur once a month until they start on an Adult horse protocol at 1 year of age.
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