Surprising findings about how parasites affect horses

A study from the University of Kentucky challenges long-held assumptions about the seasonality of parasite egg shedding among infected horses. The researchers also found that pregnancy, foaling and lactation did not affect fecal egg counts.

A mare and foal standing in a field
Pregnancy, foaling and lactation does not affect fecal egg counts, according to a University of Kentucky study.

The study was conducted using 20 study horses that are part of a closed herd maintained by the university on the same pasture since 1979. For research purposes, the herd has received no deworming treatments since its formation. In addition to the rate of strongylid egg shedding, the researchers focused on the presence of the parasite Strongylus vulgaris, one of several “large strongyle” species known to infect horses.

Every other week for a year, blood and manure samples were collected from all the horses. The researchers tested the blood for antibodies and used coprocultures, PCR analysis and ELISA tests to estimate the number of S. vulgaris parasites the horses carried. Blood samples from mares and foals were also analyzed to document passive transfer of antibodies to S. vulgaris from mare to foal.

The researchers found that the rate of egg shedding did not vary by season, findings that contradict similar studies in Europe. Blood concentrations of antibodies against S. vulgaris also remained consistent throughout the year.

“We expected some seasonality, because that had been described in the few other studies that have been done,” says Martin Nielsen, DVM, PhD. “But those studies were done in very different climates, and most were done a very long time ago. So, it is always healthy to investigate things rather than extrapolate and assume. And this herd offers the perfect opportunity to do so.”

Nielsen adds that while the consistency of egg shedding is surprising, it may ultimately have little bearing on recommendations for surveillance-based parasite control programs. “The seasonal deworming recommendations have more to do with when the eggs would develop into infective larvae on pasture. If it is winter and below 6 degrees Celsius (42.8 degrees Fahrenheit), there is no development, and therefore no point in reducing egg shedding through deworming. This is why we say that parasite control should occur during the active parasite transmission season.” He adds, though, that further study is needed to determine the potential effects of different climates and locations on shedding patterns.

As for the portion of the study focusing on mares and foals, the researchers found evidence of passive transfer of S. vulgaris antibodies from mares to their foals through colostrum. Foals began pro-ducing their own antibodies about 20 weeks after birth. There was no evidence that pregnancy, foaling or lactation affected a mare’s parasite  fecal egg counts.

Reference: “Parasite dynamics in untreated horses through one calendar year,” Parasites and Vectors, February 2022.

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