Many horses infected with S. westeri never show any clinical signs, but heavy infections in young foals and in older animals that have concomitant disease or are otherwise stressed can show gastro-intestinal symptoms, especially diarrhea and dehydration. Also, may affected foals may cease to thrive and grow. Rarely there may also be skin lesions caused by larval penetration. Because of the routes of infection with S. westeri, a sub-optimal (dirty) environment can increase the risk of clinical problems. The possible association between S. westeri infection and foal heat diarrhea has not been fully explored.
Strongyloides westeri is not known to be zoonotic.
Strongyloides westeri is a rhabditid nematode related to the other species of Strongyloides that infect cattle, sheep, pigs, dogs and a range of other mammals, as well as amphibians, reptiles and birds. All species of Strongyloides have a similar structure and life cycle. Most rhabditid nematodes are free-living, and under some circumstances parasitic Strongyloides species can have a free-living phase in their life cycles. Also related is the genus Rhabditis, which is free-living, but larvae of which occasionally cause cutaneous larva migrans in a variety of hosts, especially dogs.
With the Strongyloides species that infect mammals, the parasitic adults are all female. They are very small, up to approximately 10 mm in length, and cannot easily be seen without a microscope. The pharynx occupies almost half of the total length of the nematode.
The larvated eggs of S. westeri passed in the feces of an infected horse are oval with a delicate smooth shall and measure approximately 50 µm by 40 µm. Each egg contains a rhabditiform first-stage larva.
Host range and geographic distribution
Life cycle - direct
Adult female S. westeri are located deep in the mucosa of the small intestine. The larvated eggs pass in the feces. In the environment, a rhabditiform larvae develops within each egg, which then hatches. The rhabditiform larva from each egg can either moult twice, developing into infective third-stage (filariform) larvae – this is the homogonic cycle, or moult four times and develop into free-living adult males and females. These free-living adults can then produce rhabditiform larvae which moult twice, developing into filariform larvae – this is the heterogonic cycle. The factors determining which cycle is followed in a particular situation are not fully understood.
Infection of the horse follows skin penetration by, or ingestion of, filariform larvae. In the horse, some ingested larvae migrate in the vasculature through the liver and lungs, where they break out from the branches of the pulmonary artery into the airways. Subsequently the larvae are coughed up and swallowed and move to the small intestine, where they complete their development to adult females. This is essentially a tracheal migration. Other ingested larvae penetrate the buccal mucosa and follow a semi-tracheal migration, as do percutaneous larvae. Also, some ingested larvae follow a somatic migration. Subsequently in lactating mares these larvae are present in the milk are are a major source of S. westeri infection for suckling foals. These larvae can be present in the milk for up to approximately eight weeks of lactation. Following this trans-mammary transmission, foals can become patent by 10 to 14 days of age, and the infection can persist for several months.
Strongyloides stercoralis In people, and perhaps in dogs, is capable of auto-infection, where rhabditiform larvae become infective filariform larvae either in the intestinal lumen or in the peri-anal area. In people, particularly those who are immuno-suppressed, autoinfection can result in large and/or very persistent infections. It is not known whether S. westeri can auto-infect its hosts.
Life Cycle: Strongyloides westeri
Pathology and clinical signs
Treatment and control
Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.
Administration of ivermectin to the mare at foaling very significantly reduces trans-mammary transmission.