Results from a recent study performed on American and Danish horses identified an association between selective treatment and occurrence of the most pathogenic (capable of producing disease) parasite, Strongylus vulgaris, said Martin Nielsen, DVM, PhD, EVPC, assistant professor in the Department of Veterinary Science at the Gluck Equine Research Center.
In the study, presence of S. vulgaris was significantly associated with the time elapsed since a horse's most recent deworming. Frequent anthelmintic treatments tend to eliminate S. vulgaris completely.
The study involved 991 horses representing 53 different horse farms in Denmark and Central Kentucky. The data were subdivided based on whether farms used selective therapy as a treatment strategy and the time since the most recent deworming. The Danish results indicated a possible association between selective therapy use and S. vulgaris occurrence in both individual horses and at horse farms in Denmark.
"We found S. vulgaris on Danish farms representing both parasite control approaches, but the prevalence was significantly different," Nieslen said.
The overall S. vulgaris prevalence in Denmark was found to be approximately 12%. Farms basing parasite control on selective therapy had twice as much S. vulgaris as farms not basing anthelmintic treatments on fecal egg counts. Stud farms and training stables using selective therapy were particularly at risk of harboring S. vulgaris, which might be due to considerably higher traffic and the presence of young horses susceptible to parasite infection, Nielsen said.
However, when the most recent anthelmintic treatment had occurred less than six months ago, horses were significantly less likely to harbor S. vulgaris.
The parasite's six-month prepatent (incubation) period might explain this relation, he added. If treatment occurs within the prepatent period, the lifecycle can be interrupted effectively.
While the mean time since the most recent deworming was more than eight months on the Danish farms, the Kentucky farms were treating much more frequently, with the most recent deworming occurring about three months prior to the study. This likely explains why S. vulgaris was not encountered in any of the U.S. horses.
According to Nielsen, the possible reemergence of S. vulgaris in Danish horse establishments is most likely due to the current prescription-only restrictions of anthelmintic usage. This has lowered the treatment intensity dramatically and has led a majority of farms to adopt the selective therapy method. These regulations were introduced to encourage veterinary involvement in deworming programs and reduce further development of anthelmintic resistance. It appears to be an unforeseen consequence that we see S. vulgaris again.
"The good news is that this parasite is still fully sensitive to anthelmintic treatment," Nielsen said. "Anthelmintic resistance is a problem in other parasites infecting the horse: the cyathostomins (small strongyles) and the roundworm, Parascaris equorum."
According to Nielsen, the intensive treatment regimens commonly practiced on many American establishments on one hand appear to prevent S. vulgaris transmission, but on the other hand have also caused high levels of anthelmintic resistance in cyathostomins and P. equorum.
"We cannot completely avoid anthelmintic resistance unless we don't perform any treatment at all," he said.
Reduced treatment intensity, as represented by the selective treatment regimen, will still select for drug resistant parasites, but at a much lower rate.
However, the sparse treatment most commonly performed on Danish horse farms might pose a potential risk to equine health, Nielsen said.
"Overall, these results strongly indicate that the choice of anthelmintic treatment regimen represents a trade-off between anthelmintic resistance and S. vulgaris, which are both two undesired outcomes," he said.
Nielsen said parasite control should not be based upon just one potential parasitic risk but all parasites that might threaten equine health. Based on these study findings, Nielsen recommends applying a basic foundation consisting of one to two yearly anthelmintic treatments to all horses. This is likely to reduce the occurrence of S. vulgaris and can still be combined with selective therapy performed at other times of the year.
Shaila Sigsgaard is a contributing writer for the Bluegrass Equine Digest.
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Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.