Integrated Parasite Control: How to Strike a Balance
Deworming protocols have changed radically over the past decade, thanks to research into how to best manage parasite control within herds and individual horses. One of the pioneers of this research is Martin Nielsen, DVM, PhD, Dipl. EVPC, Dipl. ACVM, professor at the University of Kentucky’s Gluck Equine Research Center. He described current parasite control recommendations and emerging technologies in this field during a presentation at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah.
For a parasite control strategy to be effective, the owner and practitioner must balance treatment and surveillance. “Monitoring of dewormer efficacy is required for herd health,” Nielsen began. “Egg counts are here to stay!”
He first listed individual worm types and described some significant points about each for horse owners and veterinarians to consider:
- Ascarids (roundworms) This year’s foals are infecting next year’s foals, said Nielsen, because ascarid eggs are environmentally resistant (i.e., capable of withstanding fairly warm summer temperatures and cold extremes in winter). Of the deworming drugs used, ivermectin and moxidectin are ineffective for controlling these worms; pyrantel salts might or might not work; and benzimidazoles are still doing a pretty good job, he said.
- Small strongyles (cyathostomins) There are 50 different small strongyle species, the larvae of which encyst in the walls of horses’ large intestines. Ivermectin and moxidectin do not control these worms as effectively as they did in the past, and Nielsen said we are now seeing shorter egg reappearance intervals after deworming. On a majority of farms, neither pyrantel salts nor benzimidazoles are effective against small strongyles.
- Tapeworms These parasites are present in 20-80% of horses, depending on geographic location, and can cause spasmodic colic in the cecum (the part of the gut between the small and large intestines) and impaction of the ileum (the last segment of the small intestine before the cecum). Veterinarians have not reported resistance of tapeworms to praziquantel and pyrantel salts because there are no methods to measure resistance, Nielsen said.
- Large strongyles These are now very rare because of all dewormer classes’ efficacy against them.
Nielsen notes that there is no single drug to treat all parasite categories. Therefore, owners and veterinarians must perform routine fecal egg counts to know which parasites are not being well-addressed by parasite drugs used on their farm.
Veterinarians and horse owners frequently ask whether any of the current larvicidal approaches are effective in controlling small strongyles in the face of widespread parasite resistance in herds they manage. Are there ways they should be adjusting their approach, or is it a lost cause?
In one recent study, Nielsen said researchers in Tennessee evaluated the two available larvicidal treatment regimens—moxidectin and the five-day double dose of fenbendazole—in a population of horses harboring small strongyles that were resistant to a single dose of fenbendazole. The group found that the five-day fenbendazole regimen was 33.4% effective for treating encysted small strongyles in the intestinal lining, as compared to 61.7% with moxidectin. While moxidectin efficacy was within the expected range, the fenbendazole regimen’s efficacy was significantly below historic levels. This trend is likely to be found elsewhere, Nielsen noted.
The best way to battle this trend is to know where your horses stand with serial testing. “To accurately determine fecal egg count reduction testing (FECRT), it is necessary to do both a pretreatment and follow-up egg count 10-14 days after treatment,” Nielsen said. Many people are only looking at pretreatment egg counts, yet post-treatment egg counts are important for evaluating drug efficacy, he explained.
Nielsen also pointed out that performing egg counts for a clinical diagnosis on a sick horse is useless: “The mere presence of parasite eggs and/or larvae in the feces does not equate with parasitic disease,” he said. A negative fecal egg count does not necessarily mean the horse is worm-free, just that the adult worms are not laying eggs at that time. And, just because a horse has a positive fecal egg count doesn’t mean that a parasite infection is the reason for an illness.
He finished with a few take-home messages:
- Parasite drug resistance is not an individual diagnosis. Many individuals within a herd must be tested to confirm resistance.
- Twenty percent of horses excrete 80% of the total egg output in a herd. Egg counts are very useful for identifying high strongyle egg shedders within a herd of adult horses.
- Obtain egg counts in foals at weaning age. “They can have ascarids, strongyles, or both,” Nielsen said. “In a majority of situations there is no single anthelmintic that would effectively treat both, so testing is required in order to select the correct dewormer.”
- Research results suggests that the cutoff point in fecal egg counts for treatment should be somewhere between 100 to 500 eggs per gram (epg).
- We cannot eliminate worm burdens entirely: “An immune system that coevolves with parasites is best, as the worms stimulate and improve the immune system,” Nielsen said.
One recently applied technique for monitoring ascarid burdens in foals is abdominal ultrasound. Nielsen’s team completed a study in which they used this technique with good results. Nielsen also described a technology called the Parasight system, which can quantify egg counts in a smartphone photo of a fluorescent-stained fecal sample. This will be available in the near future, he said.
Because internal parasites are here to stay, an integrated parasite control program is critical to providing science-based preventive health care. This relies on vigilant monitoring through once or twice yearly fecal egg count reduction testing balanced with deworming programs designed to treat the specific needs of each individual horse and farm.
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