Epidemiology of equine strongyles

Seasonal changes in strongyle egg counts

Field observations of strongyle egg counts in mares and other adult horses throughout the Northern Hemisphere  show a distinct seasonal pattern. They begin to rise in March, peak anywhere from July to October and decline over winter to a low level in February and March before beginning to rise again. Differential larval counts have shown that more than 90% of these strongyle eggs come from small strongyles with only a small percentage (4-10%) coming from large strongyles. This is because under natural grazing conditions horses are infected with tens of thousands and sometimes even hundreds of thousands of adult small strongyles and only several hundred or at most a few thousand  h138a.gif (5735 bytes)
adult large strongyles.  Necropsies of horses with high strongyle fecal egg counts in March, April and May show large numbers of cyathostomes in the cecum and colon. Clearly these infections could not have come from ingested larvae that had over-wintered on pasture because, in fact, so few L3s survive this way. The rise in strongyle egg counts in grazing horses in spring comes primarily from adult cyathostomes that survived winter as hypobiotic EL3s in the intestinal mucosa and resumed their development to adults in late winter and early spring. As environmental temperatures rise these eggs hatch and develop rapidly to infective L3s providing a source for constant reinfection of grazing horses and, more importantly, infections of  newly weaned foals in spring and early summer.


Several studies in the southeast and midwest have shown a pattern of strongyle egg counts and pasture L3 levels, similar to that in the image to the right. An initial wave of  strongyle  egg output in horse feces occurs in May and this is presumably due to the emergence of arrested larvae from the mucosa in early spring followed by their development to egg-laying adults. Strongyle eggs on pasture develop rapidly to infective L3s  and their ingestion by grazing horses in June result in patent infections 6-8 weeks later as shown by the second smaller peak in egg counts. Pasture  levels of L3s fall off rapidly in July and August because hot summer temperatures are quickly lethal to all preparasitic stages. Eggs passed  h139a.gif (8287 bytes)
onto pasture in the second wave of egg counts in August are readily translated into rising levels of L3s. Since rainfall is more abundant and temperatures are lower than they were in July and August these L3s survive longer. Infections in grazing horses from this second wave of pasture L3s, will be seen as arrested EL3s in the mucosa over the subsequent winter. However, in certain parts of the southern United States where winters are exceptionally mild (e.g. Florida) high levels of pasture L3s may persist over winter and present problems to grazing horses during this time. In these areas, grazing horses may be at risk during spring, fall and winter. In the hot summer months, minimal transmission of small strongyles occurs because pasture L3s die off rapidly in the hot temperatures and, it is also postulated by some that small strongyles may undergo hypobiosis during the hot summers prevalent in the southeast.




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