Threadworm - Donkey
Strongyloides westeri
Introduction
Strongyloides westeri is a common intestinal parasite of equids (donkeys, horses, zebras and their hybrids) and rarely in pigs. It has a worldwide distribuion. It is a slender, cylindrical worm 6-9 mm long. The body of the female is composed of an intertwined intestine and uterus, forming a thread like appearance, hence the common name 'threadworm'.
Lifecycle
This nematode has an unusual life cycle, which involves both free-living and parasitic stages, both of which are capable of reproducing. Free living male and female adult worms are able to reproduce sexually, and more than one free-living generation may pass outside of the host. Adult female worms are usually embedded in the mucosa of the small intestine where they reproduce asexually (parthenogenesis) releasing embryonated eggs. Embryonated eggs or first stage larvae (L1) pass in faeces. These L1 stage then moult to L2 stage, which can either develop directly into infective L3 or into free-living adults. Only adult female worms form the parasitic stage while the male larvae develops into free-living males only. The eggs laid by the free-living females develop into infective L3 capable of infecting the animals. Environmental conditions (adequate warmth and humidity) seem to dictate the development of the free-living generation and their survival.
Donkeys become infected by infective third stage larvae through ingestion from the ground or milk or through skin penetration. Parasitic L3 passing in the milk of the dam is the primary source of infection for foals. Parasitic L3 are stored in body tissues of the dam and only mobilised during lactation. Parturition seems to trigger the migration of the larvae to the mammary glands of the dam.
Once the larvae are ingested or penetrate through the skin or oral mucosa, they migrate via the circulatory system to the lungs, and undergo pulmonary migration, penetrating the alveoli and travelling up the respiratory tract to be coughed up and swallowed into the digestive tract. Once in the small intestine the L3 burrow into the epithelium and develop into mature adult stage completing the cycle.The prepatent period is between 8-14 days. Larvae that have migrated to other body tissues become dormant, especially in adult animals, until they are mobilised into the mammary glands after parturition in dams.
Signalment
These worms usually only cause a clinically significant infection in foals under six months of age. This will often be the first parasitic infection that a foal is exposed to, and as such they have very little immunity. Infection tends to disappear in horses spontaneously after this age, due to acquired immunity, but it is not known if the same is true for donkeys. Adult horses rarely show any clinical signs or are of a concern although low level of infections may be found. Threadworm associated clinical signs or its effect on adult donkeys is not reported so far.
Diagnosis
Clinical signs
Low level infections may be asymptomatic. Pulmonary symptoms such as coughing may be seen due to the pulmonary migration process. Dermal symptoms such as skin lesions and irritation may also be seen due to penetration of the skin by the infective L3 stage. If large numbers of larvae are ingested or if the foal is immuno-compromised severe enteral signs such as diarrhoea may result. Damage due to burrowing adult females may range from inflammation to complete sloughing off of the gut wall. Malabsorption, electrolyte imbalances and dehydration may all contribute to an infection that is potentially fatal if left untreated.
Laboratory tests
- Patent infections in foals are diagnosed by examining the faeces for the presence of emryonated eggs or larvae. Eggs are rarely detected in faeces as they hatch within the host or as soon as they are passed in faeces.
- Larvae may be detected in a jenny’s milk.
- Larvae can be identified using the larvae culture and the Baermann technique.
Treatment
De-worming the Jenny with an appropriate drug such as ivermectin within 24 hours post-partum will reduce or prevent larvae migrating via the mammary glands. Foals should be de-wormed using an appropriate drug such as ivermectin at six weeks of age.
Control
Control measures are important with older animals, primarily to protect youngsters. Jennies in particular should be dewormed soon after giving birth in order to prevent the passage of larvae to nursing foals in the milk. As larvae can persist in the soil, clean pasture should be used for each new group of foals to minimise the parasitic burden.
Literature Search
Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).
Strongyloides species in donkeys publications
References
- Trawford, A. and Getachew, M. (2008) Parasites In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 6
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