Researchers found no seasonal differences in strongylid egg shedding in a herd of naturally infected horses in Kentucky – a finding in stark contrast to previous research overseas.
Horses are host to a range of parasites. Knowledge of the seasonality of parasite egg shedding and transmission is important for designing parasite control programs. However, studies describing these patterns are sparse, and have largely been conducted only in Britain.
Ashley Steuer and her fellow researchers set out to evaluate strongylid egg shedding patterns and transmission dynamics of Strongylus vulgaris in naturally infected and untreated mares and foals through the 2018 calendar year in Kentucky.
Equine strongylid parasites are common around the world. The Strongylidae family in equids comprises two subfamilies: Cyathostominae (cyathostomins, or small strongyles) and Strongylinae (strongylins, or large strongyles).
Cyathostomins are commonly reported to show resistance to all currently available drug classes, whereas the group of Strongylinae, which includes the bloodworm S. vulgaris, has not been reported resistant to these products.
The researchers, reporting in the journal Parasites & Vectors, also looked at whether strongylid egg counts varied in mares around the time of foaling, and collected information about Strongyloides westeri and Parascaris species in the foals.
The study involved 18 mares, one stallion, and 14 foals from the University of Kentucky’s closed herd, which has never received deworming drugs and has been located on the same grazing pasture since 1979. The foals in the study all arrived during the year of the study.
Samples were collected for analysis biweekly from all mature horses and weekly or biweekly from the foals, depending on the behavior of the individual animals.
The researchers found a general lack of seasonality in strongylid egg shedding throughout the year among the mature horses.
Shedding of S. vulgaris eggs displayed a higher abundance in spring, but findings were variable and not statistically significant.
Concentrations of antibodies against S. vulgaris did not fluctuate significantly in the mature horses throughout the year, and the parasite was steadily detected in fecal samples.
There was evidence of passive transfer of antibodies to the foals through the mare’s colostrum, and foals assumed their own production of antibodies starting from about 20 weeks of age.
Overall, colts shed higher numbers of strongylid, ascarid, and S. westeri eggs than fillies. It was an interesting finding, they said. “This has not been previously demonstrated in this herd, and we are not aware of any such findings in other studies.”
Mare pregnancy, foaling, and lactation did not affect parasite fecal egg counts, the study team found.
Professor Martin Nielsen, of the Maxwell H. Gluck Research Center at the University of Kentucky, who took part in the study, said: “We learn so much from this unique herd of horses.
“First and foremost, they remind us that parasitism is a natural state, and that worms only extremely rarely cause disease or ill-thrift”.
Nielsen said several of the study findings were surprising, as seasonality in parasite egg shedding had been reported in other countries, and many people had speculated that pregnancy and foaling could affect parasite transmission.
“This study demonstrates the importance of investigating these things properly, and this research herd offers excellent opportunities for doing so.”
The authors, in their conclusion, said their findings will be useful for understanding parasite epidemiology in a broader context, and emphasize the value of collecting this type of information from a variety of climatic settings.
“Certainly, this study demonstrated substantial differences from findings in previous studies primarily in the United Kingdom.”
The study team comprised Steuer, with the School of Veterinary Medicine at Texas Tech University; Haley Anderson, with the College of Veterinary Medicine at Auburn University; Taylor Shepherd and Morgan Clark, with the College of Veterinary Medicine at Lincoln Memorial University; Jessica Scare, with the Department of Animal Science; Eastern Kentucky University; and Holli Gravatte and Nielsen, both with the Gluck Center.
Steuer, A.E., Anderson, H.P., Shepherd, T. et al. Parasite dynamics in untreated horses through one calendar year. Parasites Vectors 15, 50 (2022). https://doi.org/10.1186/s13071-022-05168-z