Epidemiology of Equine Strongyle Infections

Development of strongyle infections in foals

Foals will be infected with strongyle  larvae as soon as they begin to graze while on pastures with their dams. Initial infections in early spring result primarily from ingestion of L3s that have successfully overwintered on pasture. However,   overwintered pasture larvae that fail to be ingested by a host will die off rapidly in spring  as temperatures rise and their food reserves are quickly used up.   The primary source of infections in foals will therefore be L3s that have developed from eggs passed by their dams and other horses on the same pasture. These eggs will develop rapidly to infective L3s as temperatures rise in spring and summer, producing dangerous levels of  L3s on pastures during late spring, summer and fall.


Several studies have shown that strongyle eggs may appear in the feces of foals, on pasture, as early as 4 weeks of age. The majority of these eggs are small strongyles but fecal cultures show that as many as 25% may be eggs of Strongylus vulgaris and Strongylus edentatus. Clearly these eggs cannot be from adult worms  because the prepatent periods of these species are 6 months and 11 months respectively and also because prenatal infections  with equine strongyles do not occur.  In fact  Strongylus eggs may be found  in the feces of foals up to 12 weeks of age and it is believed that their presence  results from ingestion of maternal and other adult feces by foals to populate the large intestine with  microorganisms necessary for cellulose digestion. h140.gif (7358 bytes)
In the absence of an effective parasite control program, the strongyle egg counts of foals will increase steadily during the first 12 months of life and this is shown in the accompanying graph. Patent infections of Strongylus vulgaris are detected by the time foals are 7-8 months old and eggs from Strongylus edentatus  appear at 11-12 months of age consistent with the prepatent periods of these species. However, it should be remembered that during their first 6 months of life, foals are at risk from migrating larvae of Strongylus vulgaris. It is therefore important that foals are introduced to a parasite control program that includes larvicidal  anthelmintics by the time they are 2 months old. Depending on when they are born, the initial source of infective larvae for foals will be either L3s that have successfully over-wintered, or larvae that have developed from eggs passed in spring by their dams and other adult horses on the same pasture. Whatever the source of their strongyle infections it is clear that this epidemiological picture shows clear evidence that foals are at risk from strongyle infections early in life and certainly by the time they are 2-3 months old.



Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000