Deworming practices as a contributing factor
A major shift in the methods of equine parasite control has been underway during the past few years. Ray Kaplan, DVM, PhD,
a parasitologist at the University of Georgia's College of Veterinary Medicine, along with other parasitologists and researchers,
published a study in 2004 that looked at the prevalence and resistance of various equine parasites.1 Kaplan and Martin Nielsen, DVM, PhD, then at the Department of Large Animal Sciences at the University of Copenhagen, followed
with an evidence-based approach to deworming horses that essentially recommended increased fecal testing and decreased use
of anthelmintics based on that testing.2 Most equine veterinarians and horse owners have adopted some form of this program during the past few years. It's interesting
to note that we are now seeing the results of this selective therapy deworming, and certain problems, such as an increase
in summer sores, may be one of them.
When horses were dewormed every eight weeks, which was the previously recommended schedule, horses were never without anthelmintics
in their systems for very long, and deworming products commonly overlapped in coverage. Currently, horses that have low to
nonexistent parasite egg counts via fecal testing may go for extended periods (four months or longer) without a dose of dewormer.
Certain parasites have a higher chance of causing problems during these low-treatment periods. In the hot southern climates,
these low-treatment periods correspond to the summer months, and ticks and fly larvae may be developing an advantage.
Nielsen, currently at the University of Kentucky's Gluck Center, also acknowledges that certain parasites (e.g., Strongylus vulgaris) may be circumventing the selective therapy deworming programs and staging a comeback. In an article soon to be published
in Veterinary Parasitology, Nielsen looked at the prevalence of S. vulgaris at a number of horse farms in Denmark. He compared farms using selective treatment (based on regular fecal egg counts from
all horses) with farms that treated strategically (i.e., without egg counts). Farms using selective therapy had a horse and farm prevalence of more than twice that of farms not
using that approach. Nielsen concludes that "a strict interpretation of the selective therapy regimen can be associated with
an increased prevalence of S. vulgaris."
Both Kaplan and Nielsen suggest that modifications of their equine parasite-control program could be considered. Kaplan explains,
"The overriding important concept is that there is no one 'correct' parasite-control program. What's best for one farm won't
be the same for another. But we do know that the traditional [every eight weeks] approach is a very poor strategy for controlling
cyathostomes and often will fail to provide optimal control.
"However, there are side effects to this approach, such as the near eradication of some species of parasites," Kaplan continues.
"What we're doing now is taking what evidence we have to make sensible recommendations, but these will continue to evolve
as we learn more."
Additional use of ivermectin and moxidectin during the hot summer months to provide protection from ticks and fly larvae and
to keep summer sores from increasing may be one of the first modifications that practitioners should consider.
Dr. Kenneth Marcella is an equine practitioner in Canton, Ga.
1. Kaplan RM, Klei TR, Lyons ET, et al. Prevalence of anthelmintic resistant cyathostomes on horse farms. J Am Vet Med Assoc 2004;225(6):903-910.
2. Kaplan RM, Nielsen MK. An evidence-based approach to equine parasite control: it ain't the 60s anymore. Equine Vet Educ 2010;22(6):306-316.