The recommended management of parasitic nematodes in the intestinal tract of horses (including small strongyles or cyathostomins) is driven by fecal egg counts, which is an indication of adult parasite burden. However, “The most up-to-date data on dewormer resistance on larval populations are over 20 years old, leaving experts wondering where we stand in terms our ability to effectively control larval populations with larvicidal doses of fenbendazole and moxidectin,” said Martin Nielsen, DVM, PhD, DEVPC, DACVM, an associate professor at the University of Kentucky’s Gluck Equine Research Center.
According to Nielsen, there are currently only two larvicidal treatment options: moxidectin (Quest products) and a five-day regimen of fenbendazole given at twice the normal dose (often termed the PowerPac dose).
Why are cyathostomin larvae problematic when the adult small strongyle is considered fairly benign? “The accumulation of encysted cyathostomin larvae in the mucosa of the cecum and colon means synchronous emergence can result in a protein-losing enteropathy characterized by profuse, watery diarrhea,” said Nielsen.
There are larvicidal treatments—those aimed specifically at the larval stages— that can help lower the risk for parasitic disease in horses. However, Nielsen said, “We know that adult small strongyles have developed resistance to the abovementioned anthelmintics over the past 40 years.
“Considering the importance of controlling cyathostomin larvae populations, my colleagues and I conducted a study to determine the strongyle-killing efficacy of fenbendazole and moxidectin,” he continued. “This study was done without support from any pharmaceutical company.
“Larvicidal efficacy has not been evaluated in this country for the last 20+ years,” Nielsen emphasized. “Nobody had any information of whether it would still work or not. We were asking the simple question: With all the fenbendazole resistance that’s out there, can we expect the larvicidal treatments to still be effective? For that, we needed to document that the study population had fenbendazole resistance first.”
The study was undertaken by Nielsen with Craig Reinemeyer, DVM, PhD, and Julio C. Prado, DVM, both of East Tennessee Clinical Research. That team recruited young ponies harboring adult cyathostomins resistant to fenbendazole and dewormed them with the larvicidal doses of either fenbendazole, moxidectin or nothing.
Key findings from their study were:
• The five-day course with fenbendazole achieved only a 44.6% fecal egg count reduction (FECR) and had 56.4% activity against adult cyathostomins in the intestinal lumen.
• The five-day course of fenbendazole was 38.6% and 71.2% effective against encysted early third stage (EL3) and late third stage/fourth stage (LL3/L4) cyathostomin larvae, respectively.
• Moxidectin achieved a 99.9% FECR and removed 99.8% of adult small strongyles in the intestinal lumen.
• Moxidectin removed 63.6% and 85.2% EL3 and LL3/L4 mucosal (encysted) cyathostomins, respectively.
“The larvicidal efficacy of the five-day fenbendazole treatment regime was well below historical levels,” noted Nielsen. “Past studies indicate that the larvicidal treatment used to remove more than 95% of all larvae. In fact, this study provided clear evidence of resistance of cyathostomin larvae to fenbendazole.
Even though the larvicidal efficacy of moxidectin was not 100%, the reduction of LL3/L4 larvae was in agreement with historical efficacies, suggesting minimal resistance to moxidectin by cyathostomin larvae.”
It is important to note that this study also shows that resistance to a specific anthelmintic in adult parasites (evidenced by measuring the level of fecal egg count reduction) can be linked to resistance in the larval stages of the same parasite to the larvicidal regimen of the same anthelmintic.
“In other words, veterinarians should be aware that the widely used five-day fenbendazole regimen may not exert its intended effects anymore,” said Nielsen. “That said, bear in mind that both tested products did reduce encysted larval burdens, but neither reached 100%.”
There are no alternative chemical dewormers licensed for cyathostomin larvae and no new drugs on the horizon, and alternative strategies are being investigated.
“While this study illustrates that the five-day fenbendazole regimen is losing its value as a means for control of encysted small strongyle larvae, it is important to emphasize that it still has good efficacy against other important parasites, such as ascarids in foals,” stressed Nielsen. “Furthermore, fenbendazole remains a good choice for treating pinworm infections. All dewormers currently on the equine market still have a place as important means of parasite control. They are just not as broad spectrum as they used to be.”
Editor's note: This article was first published in the fall 2016 issue of EquiManagement magazine.