MSD Manual

Please confirm that you are a health care professional

honeypot link
Professional Version

Strongyloides westeri-Associated Disease in Horses


Martin K. Nielsen

, DVM, PhD, DACVM, DEVPC, Maxwell H. Gluck Equine Research Center, Department of Veterinary Science, College of Agriculture, Food and Environment, University of Kentucky

Reviewed/Revised Dec 2019 | Modified Nov 2022
Topic Resources

Strongyloides westeri is an intestinal threadworm commonly infecting young foals, and it has been associated with small intestinal enteritis, resulting in diarrhea. The pathogenicity of the parasite remains unclear, however. The parasite is unique among equine helminths in having three possible routes of transmission: lactogenic, oral, and percutaneous. Furthermore, it can complete the entire life cycle in the environment without entering a host. The lactogenic route is considered the most important in foals, but skin reactions and hyperactive behavior have been observed in response to percutaneous invasion of larvae. Ivermectin and oxibendazole are effective against intestinal stages in foals, but it remains unclear whether deworming mares close to foaling has any effect on reducing lactogenic transmission.

Strongyloides westeri commonly occurs in foals around the world. Few large-scale prevalence studies exist, but Strongyloides eggs can typically be found in the feces of 5%–50% of managed foals that are < 2 months of age. Lactogenic transmission is believed to be the most significant route of infection in young foals. Mares harbor larvae in a hypobiotic arrested stage in their tissues, which become reactivated at foaling and migrate to the mammary gland. Larvae can be isolated in the milk at 5 days postpartum, but the highest concentration occurs 10–14 days postpartum. Larvae have been identified in milk until ~45 days postpartum. Foals have been reported as egg-count positive as early as 5 days old.

Two other routes of infection exist for this parasite—oral ingestion and percutaneous invasion—but the significance of these routes is unknown. Parasites obtained via either of these routes are believed to undergo a tissue migration taking them through the lungs, whereas lactogenically acquired parasites do not migrate extra-intestinally.

Two additional features separate S westeri from other intestinal helminths of horses: the nematode is a facultative parasite, because it can complete and maintain its life-cycle without entering a host, and only females are parasitic.

Clinical signs of Strongyloides westeri-associated Disease in Horses

Strongyloides westeri was once speculated to be the cause of “foal heat diarrhea,” because this is often observed around 10 days postpartum, when a proportion of foals can be fecal egg-count positive. However, other intestinal pathogens are likely to be implicated in disease at this age.

Nonetheless, the parasite has been associated with small intestinal enteritis with diarrhea, although no distinct clinical syndrome has been described. Epidemiologic studies suggest that disease can occur if Strongyloides egg counts exceed 2,000 eggs/g, and experimental inoculations with the parasite have led to small intestinal enteritis.

The percutaneous route of infection can cause dermatitis on distal extremities, and a hyperactive, “frenzied” behavior has been described in yearlings exposed to an infective environment.

Diagnosis of Strongyloides westeri-associated Disease in Horses

Fecal flotation or fecal egg counts allow identification of the Strongyloides eggs. They are ovoid-like strongyle eggs but shorter and more thin-walled and often contain larvae. Because they are thin-walled, the eggs tend to collapse in flotation media with specific gravities of above 1.25, so ~1.20 is recommended.

Treatment of Strongyloides westeri-associated Disease in Horses

The intestinal stages of the parasite are sensitive to ivermectin (0.2 mg/kg) and oxibendazole (15 mg/kg). There are no reports of anthelmintic resistance in this parasite.

Prevention of Strongyloides westeri-associated Disease in Horses

Deworming mares around foaling is widely used by farm managers and horse owners in an attempt to prevent lactogenic transmission, but the effectiveness of this procedure is unknown. No data are available on the efficacy of available anthelmintics against migrating tissue stages within the mare, and there is no information on the most effective dose, timing, and duration of such treatments.

Zoonotic Risk of Strongyloides westeri-associated Disease in Horses


Key Points

  • Strongyloides westeri commonly occurs in young foals but is extremely rarely reported to cause disease.

  • Tentative diagnosis is by clinical presentation and detection of eggs in the feces.

  • Ivermectin and oxibendazole are effective treatment options.

  • The effect of deworming mares at the time of foaling is unknown.

For More Information

quiz link

Test your knowledge

Take a Quiz!