Strongyloides westeri

The nematode Strongyloides westeri occurs in horses around the world, including Canada.  It is most common in regions with a warm, humid climate and in foals and other young horses.  The adult female parasites live in the small intestine.  The life cycle is direct and typical of the genus Strongyloides.  Larvated eggs are produced parthenogenetically by the parasitic females are passed in the faeces.  The first-stage (rhabditiform) larva that hatches from each egg can then follow either a homogonic cycle (without the development of free-living adult males and females) or a heterogonic cycle (with such development).  Third-stage (filariform) larvae are infective for horses and infection is by ingestion or skin penetration.  Parasitic development involves tracheal (for some ingested larvae) or semi-tracheal or somatic (for some ingested and for percutaneous larvae) migrations.  In lactating mares some infective larvae that have undergone somatic migration enter the milk. These larvae can be important sources of the parasite for suckling foals for the first couple of months of life.  Infection by this route can be stopped by treating the mare with ivermectin (or probably moxidectin) at foaling.

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.

Phylum: Nematoda
Class: Rhabditea
Order: Rhabditida
Family: Strongyloididae

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.

Strongyloides westeri occurs in horses and other equids throughout the world, especially in foals and in warm and moist areas. It is reported rarely in western Canada but is probably more common in central and eastern regions of the country.

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

In general, S. westeri has the highest prevalence and intensity in young, suckling foals, and it is in this age group of hosts that most of the parasitic adults are found. The free-living stages of S. westeri prefer warmth and moisture, and are easily killed by adverse environmental conditions.

Many horses infected with S. stercoralis show no adverse effects. Clinical signs are most likely in young foals. Diarrhea is the commonest sign, and Strongyloides may have a role in "foal heat diarrhea" that occurs approximately 10 to 14 days after foaling. Skin lesions associated with entry of infective larvae are rare in horses.

Detection of the typical larvated eggs using a fecal flotation technique is the best method for diagnosis. The eggs of S. westeri are easily distinguishable from those of other enteric parasites of horses, although in feces that are not fresh they may be confused with strongyle and cyathostome eggs that have larvated.

Ivermectin (VARIOUS) and oxibendazole (ANTHELCIDE) are approved in Canada for treatment of Strongyloides westeri in horses. 

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.

Strongyloides westeri is not known to be zoonotic.