Parasite resistance to at least one drug class is likely to be present in every horse operation across the world, a leading equine parasitologist says.
Even worse, an overwhelming majority will feature multi-drug resistance, says Martin Nielsen, an associate professor at the University of Kentucky’s Gluck Equine Research Center.
“With only three classes to choose between, we are running out of treatment options,” he writes in the latest issue of Equine Disease Quarterly.
“A pertinent question to ask is how to tackle this emerging crisis and what to expect in the future.
“The first step is to acknowledge the extent of the problem.
“Despite recommendations given during the past couple of decades, a majority of individuals in the industry continue to use old-fashioned parasite control programs based on frequent treatments given year-round without any consideration of treatment efficacy, parasites present, and climatic conditions.”
Nielsen says if no diagnostic testing is done, resistance to deworming drugs, known as anthelmintics, will not be identified.
He says no new anthelmintics with newer modes of action have been introduced since the early 1980s, and levels of anthelmintic resistance are ever-increasing in cyathostomin and Parascaris species.
“For the long term, we need new anthelmintic drug classes with new modes of action.
“It is important to learn from the past however, and realize that no drug class is going to remain effective indefinitely, and that reverting back to treatment regimens of the past would be a complete mistake.”
He says his colleague, classicial parasitologist Dr Gene Lyons, has clearly illustrated that once resistance appears in a given parasite, it is there to stay.
“The pharmaceutical industry is not anticipating developing any equine products in the foreseeable future.”
Recent pharmaceutical trends are aimed at combination deworming products – that is, formulations where two or more dewormers targeting the same parasites are combined into the same product.
Research in the sheep industry has also highlighted the importance of reducing treatment intensity to avoid development of multi-drug resistance.
Nielsen says one of his recent projects highlights the importance of having a high starting level of effectiveness if these drugs are given in combination. If its efficacy is markedly less than the desired 95 percent or above, resistance may develop quickly.
His laboratory is also testing a bacterial dewormer. Strains of naturally occurring Bacillus thuringiensis produce crystal proteins capable of killing worm parasites. If successful, he says, this could become a deworming product in the future.
“Perhaps the most important element in future parasite control programs is utilization of good diagnostic tools.
“Fecal egg counts will remain a cornerstone of control programs, but they have limitations in not providing information about larval stages and specific types (species) of parasites present.
Nielsen traverses several developments in the area of diagnostics, including development of an automated smartphone-based fecal egg-counting system, which allows easier, quicker, and more precise fecal egg counts to be determined.
“Taking these diagnostic approaches collectively, the goal is to enable veterinarians and their equine clients to make more informed decisions about parasite control.”
The road to effective and sustainable parasite control is evidence-based, he says, with veterinarians playing a central role.
Lyons, also from the University of Kentucky’s Gluck Center, reviews the history and current status of parasite treatments in the same issue.
He says internal parasites of horses have been recognized for centuries, but methods for their control lacked a scientific basis until the early 1900s.
“For example, in the 1600s one recommendation was to incise the horse’s palate with the intent that the ingested blood would kill any internal parasites.”
Beginning in the 1940s and extending to the 1980s, new classes of antiparasitic compounds were developed around every 10 years.
Currently in the United States, only benzimidazoles (fenbendazole and oxibendazole), tetrahydropyrimidines (pyrantel pamoate and pyrantel tartrate), and macrocyclic lactones (ivermectin and moxidectin alone or combined with praziquantel) are commercially available for parasite control in horses.
The major endoparasites of horses include bots, large strongyles, small strongyles or cyathostomes, ascarids, and tapeworms. Large strongyles (Strongylus spp.) are one of the most significant equine parasites. The larval stages can cause disease due to migration in blood vessels and abdominal organs.
Drug resistance is not evident in the case of large strongyles.
Cyathostome larvae do not migrate parenterally like Strongylus spp., but encyst in the the large intestine. Intestinal disease can arise when large numbers of larvae emerge from the lining of the large intestine, a condition called larval cyathostomiasis.
Resistance to fenbendazole, oxibendazole and pyrantel pamoate is now common among cyathostomes. Also, both ivermectin and moxidectin have become less effective against immature cyathostomes in the lumen of the large intestine; thus the life cycle is shortened.
Heavy infections with adult ascarids (Parascaris spp.) can cause intestinal blockage and rupture because of their bulk. These, too, have become resistant to ivermectin, moxidectin, and pyrantel pamoate, he says.
The final group of equine endoparasites, tapeworms (Anoplocephala spp.), can also result in serious intestinal problems, but do not show drug resistance.
Parasite treatment schedules have been based on the life cycle of the parasites since the early 1900s. In the mid-1960s, it was suggested that horses should be dewormed for strongyles every six to eight weeks.
This frequent deworming was thought not to provide time needed for Strongylus species to mature, was thought to help decrease potential cyathostome egg deposition on pastures, and not to allow time for ascarids to mature.
High strongyle fecal egg counts indicate contamination of pastures and increased potential for ingestion of infective larvae by grazing horses.
“Thus, profiling the number of eggs per gram (EPG) of feces has been used in updating deworming schedules.
“Since Strongylus spp. are now rarely encountered, a deworming schedule can be more flexible,” he writes.
Unfortunately, there is no direct relationship between EPG values and cyathostome numbers.
Lyons suggested the following deworming program:
- Establish a strongyle EPG profile for individual horses rather than deworming all horses; a study of 1114 Thoroughbred mares showed that one fecal sampling per horse was sufficient for establishing a strongyle EPG profile.
- For strongyles, use ivermectin or moxidectin alone or in combination with praziquantel twice a year — consider treating in the spring and fall. While benzimidazoles and pyrantel may be ineffective on cyathostomes, they may be effective in treating other parasite species.
- Treat foals every eight weeks for ascarid infection until they become yearlings; oxibendazole is currently considered the drug of choice followed by fenbendazole.
- Control Strongyloides with ivermectin or oxibendazole and tapeworms with praziquantel or pyrantel pamoate/tartrate.
Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.
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