Strongyloides westeri		Contents		Rev 10/14/2025
Equine intestinal threadworm		Classification		Biology and Ecology
		Morphology and Anatomy		Life Cycle
Return to Strongyloides Menu				Ecosystem Functions and Services
		Distribution		Management
Return to Strongyloididae Menu		Feeding		References
		Go to Nemaplex Main Menu		Go to Dictionary of Terminology

Classification:

Synonyms:

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Morphology and Anatomy:

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Distribution:

Nematode with parthenogenetic adult females that live within the mucosa of the intestinal epithelium of their hosts.  Distributed worldwide.

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Feeding:

Parasite of the small intestine of horses, donkeys and zebra. It has also been reported from pigs (Marchiondo, 2020)

Adult female nematodes are encountered principally in suckling and weaning foals. Adult Strongyloides westeri infections in foals appear to originate mainly from larvae passed in the milk of foals from 4 to 47 days postpartum. Most naturally infected foals begin passing eggs at 2 weeks of age and lose adult infections 20-25 weeks of age. The prepatent period is 10-14 days 

Foals 4-6 months old can develop strong immunity to threadworms after a few infections.

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Biology and Ecology:

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Life Cycle:

Like other Strongyloides species, S. westeri can cycle through a free-living generation feeding on soil bacteria. When the cycle involves the development of free-living sexual generations, it is termed a heterogonic cycle. The free-living stage, which may or may not occur. If the free-living stage occurs, the threadworm reproduces sexually (heterogonic cycle). In the parasitic stage the nematode reproduces asexually where the female adult lays eggs in the intestine, and the eggs develop by parthenogenesis (homogonic cycle).

Only parthenogenetic parasitic females exist in the host. The filariform parasitic female produces embryonated eggs by mitotic parthenogenesis that are passed in the feces, hatch and develop directly into infective larvae. Thery are termed homogonic rhabditiform larvae.

• Parasitic adult females of  S. westeri colonize the mucosal layer of the host small intestine. They produce eggs via mitotic parthenogenesis and deposit them in the mucosal epithelium.
• The eggs hatch into male and female post-parasitic first-stage larvae which migrate to the host intestinal lumen
• Male larvae pass into the environment with the feces and develop into free-living adults.
• Female post-parasitic larvae have three potential developmental routes:
  • (i) pass into the environment with the feces and execute four molting cycles to become free-living adults which reproduce sexually to produce female larvae which develop to the L3 stage where development is arrested;
  •  (ii) develop directly to the L3 stage and on to free-living male and female adults which reproduce;
  • (iii) remain in the host intestine and become autoinfective within the host.
• Soil-dwelling L3s locate and penetrate the skin of a host (causing dermatitis) or are ingested by the host.host, emerge from the developmentally-arrested state (activation), migrate to the lungs then through capillaries to the alveoli, are coughed up and swallowed by the host. Migration of the larvae through the lungs can cause severe hemorrhage and respiratory distress. Once they pass through the stomach into the intestine, the larvae develop into adult females  (Castelleto et al., 2024; Marchiondo, 2020)..

Ecophysiological Parameters:

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Ecosystem Functions and Services:

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Management:

Strongyloides westeri infection is usually associated with fecal contamination of soil or water.  In fections can be reduced by improved sanitation

Anthelmithics including ivermectin

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References:

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Copyright © 1999 by Howard Ferris.
Revised: October 14, 2025.