Small Strongyle Arrested Development Not Found in Foals

A recent study suggests small strongyles might not behave the same way in foals as they do in adult horses.
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Researchers have rigorously studied small strongyles, or cyathostomins, and their impact on horses over the years. After all, these ubiquitous parasites affect virtually every grazing horse throughout the world. Scientists focused the bulk of that research, however, on adult horses rather than foals, and a recently published study suggests that small strongyles might not behave the same way in foals, potentially necessitating alternative treatment and management strategies.

“Data collected from 2014 to 2016 indicates that cyathostomin infection in foals progresses in a substantially different manner than adult horses,” said Martin Nielsen, DVM, PhD, Dipl. ACVM, associate professor and Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, in Lexington. He co-authored the study on the topic with Eugene T. Lyons, PhD, a professor at the Gluck Center.

Researchers have a good understanding of small strongyles’ lifecycle in horses, regardless of age:

  1. Adult parasites found in the lumen (middle) of the intestine lay eggs that pass in the feces and contaminate the environment;
  2. Larvae hatch from these eggs and develop through first-, second-, then third- stage larvae (L3s);
  3. Grazing horses ingest L3s, which travel through the gastrointestinal tract to the large intestine;
  4. The L3s penetrate the large intestine wall and become encysted; and
  5. Encysted L3s  molt into fourth-stage larvae (L4s) and re-emerge from the intestine’s wall before developing into adults and laying more eggs. 

Despite potentially being infected with tens of thousands of larvae, foals and horses rarely develop clinical disease associated with infection. The main exception is a condition referred to as larval cyathostominosis, which is reported to occur most frequently in horses 1 to 4 years of age. It occurs when large numbers of L4s emerge en masse causing damage to the wall of the intestine. Signs of disease include profuse watery, sometimes bloody, diarrhea; dehydration; and ventral edema (fluid swelling under the abdomen). While larval cyathostominosis is rare, its mortality rate can reach 50%

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Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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