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Endoparasites - Strongyloides

Strongyloides Strongyloides westeri: Equine intestinal threadworm.

General Description: Threadworms are hair-like and 8 to 9 mm. long. The only parasitic adult form is a parthenogenic female. Under favourable conditions, free-living males and females reproduce sexually outside the host.

Life Cycle: Infection with intestinal threadworm can be by ingestion of larvae or skin penetration and involves migration to the lungs. However, once infective larvae are in the lungs, they migrate up the trachea and are swallowed. They mature to adulthood in the small intestine, where females may lay eggs that do not require fertilization to develop. These eggs are passed in faeces and then hatch to yield first-stage larvae, which develop in the manner described as typical for roundworms, to become infective L3 larvae. This life cycle is termed homogonic. A homogonic life cycle involves a host and allows for a rapid increase in adult threadworm population when conditions outside the host are unsuitable for larval development. However, the adult threadworm in the intestine may also lay eggs which develop into a different kind of larvae. If the environmental conditions provide necessary levels of warmth and humidity, these larvae go through a series of moults on the pasture and develop to adult worms which can live outside the host. Males and females of this type mate. The fertilised eggs laid by free-living female worms eventually yield infective L3 larvae that are eaten by the host during grazing or actively penetrate through the skin. This kind of free-living cycle is termed heterogonic and requires warmth and humidity. The prepatent period is 5 to 7 days. Migrating larvae can cross the mammary gland to infect nursing foals, one of the most important means of transmission to young foals.

Location: Small intestine. Geographical Distribution: Worldwide.

Significance: Heavy threadworm infections are very important in foals. Control measures are important with older animals, primarily to protect young horses.

Effect on Host: Migration of larval Strongyloides through the lungs can cause severe haemorrhage and respiratory distress. Skin penetration may result in irritation and dermatitis. If adult worms are numerous, erosion of the intestinal mucosa causes mucosal sloughing and interference with digestion. A few infections lead to strong immunity to this species by 4 to 6 months of age. Foals with heavy worm burdens are characterised by acute diarrhoea, weakness, and emaciation. Older animals may harbour large worm burdens without clinical manifestations.

Diagnostic Information: Eggs and larvae may be identified in fresh faeces.

Control: Treatment with anthelmintics will lower environment contamination. Removal of faeces and provision of dry quarters will limit the free-living generation. Eradication is unlikely, especially as Strongyloides larvae migrating in the blood may enter muscle tissue and lie dormant in the hypobiotic state for long periods, during which time they are resistant to currently available drugs. Pre-foaling treatments to the mare with effective antiparasitic compounds may reduce transmammary infection.

Threadworm   Egg   Skin migration of larvae



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