An Update on Multi-Drug-Resistant Parasites in Horses 
Dr. Martin Nielsen discussed the current state of parasite drug resistance in horses and teased the upcoming equine parasite control guideline updates.
Horses on the University of Kentucky Main Chance farm are a part of equine parasite research conducted by Dr. Martin Nielsen, faculty member in the Department of Veterinary Science at the Gluck Equine Research Center
Horses on the University of Kentucky Main Chance farm are a part of equine parasitology research conducted by Dr. Martin Nielsen, faculty member in the Department of Veterinary Science at the Gluck Equine Research Center. | Courtesy Dr. Martin Nielsen

For years Martin Nielsen, DVM, PhD, DACVM, DEVPC, DVSc, Schlaikjer Professor of Equine Infectious Diseases in the Gluck Equine Research Center at the University of Kentucky, has been warning equine practitioners and horse owners/managers that multi-drug resistance to the existing anthelmintics on the market is inevitable. He echoed that sentiment at the University of Kentucky Equine Vet CE event held in Lexington on Jan. 16, 2024. 

“As parasitologists we’ve been accused of crying wolf for a long time, but it’s here. It’s happening,” he said. 

During his talk, Nielsen provided veterinary attendees with an update on the current state of parasite drug resistance as well as the most current guidelines for diagnosing drug resistance in horses. 

Dealing With Outdated Drug Classes 

Historically, veterinarians have had—and continue to have—only four anthelmintic drug classes to choose from: benzimidazoles, pyrimidines, macrocyclic lactones, and quinolone pyrazines. Despite product label claims, “you can’t trust what’s on the package or insert. It’s outdated,” said Nielsen. He listed the parasite categories each drug class is registered to treat, followed by its true efficacy level: 

Benzimidazoles claim to be effective against: 

  • Strongyles – ineffective  
  • Ascarids – increasing resistance 
  • Pinworms 
  • Threadworms 

Pyrimidines claim to be effective against:  

  • Strongyles – ineffective 
  • Ascarids – increasing resistance 
  • Pinworms 
  • Threadworms 
  • Tapeworms – increasing resistance 

Macrocyclic lactones claim to be effective against:  

  • Strongyles – increasing resistance, which Nielsen said is a very recent and concerning development that hints at triple drug resistance.  
  • Ascarids – ineffective 
  • Pinworms – ineffective  
  • Threadworms 
  • Bots 

Nielsen noted the egg reappearance periods for ivermectin and moxidectin used to be quite impressive at 8-10 and 12-16 weeks, respectively, but it’s now averaging 4-5 weeks. “Don’t expect these drugs to work more than a month,” he said. “This doesn’t mean we need to deworm monthly to keep counts down; we have to just realize and accept that horses have parasites and for the most part they’re healthy.” 

Praziquantel and pyrantel pamoate, labeled to treat tapeworms, are showing evidence of reduced efficacy, as well. Nielsen described a recent field study showing treatment failure of praziquantel and pyrantel pamoate and pyrimidines—the only two drugs for tapeworms—in yearlings and broodmares on several Central Kentucky farms. 

“I consider the tapeworms to be the most pathogenic of all the parasites we deal with in this area,” he said. “That’s a big concern. If you oversee or are involved with the management of these types of farms, keep an eye out for [tapeworm resistance].” 

Anthelmintic Resistance Testing Recommendations 

Anthelmintic resistance testing is important to monitor which drug classes are still working. “Talk with your clients, farm managers, and owners to make sure they’re not still using something that hasn’t worked for a decade or longer,” Nielsen said. 

He contributed to a paper the World Association for the Advancement of Veterinary Parasitology (WAAVP) published last year with guidelines for diagnosing anthelmintic resistance using fecal egg count reduction tests in ruminants, horses, and swine. Nielsen is preparing equine-practitioner-specific recommendations based on that publication for the updated version of the AAEP’s parasite control guidelines to be released later this year. Some of those include: 

  • The number of eggs counted. If, using the modified McMaster technique, an egg count of 100 EPG with a multiplication factor of 25 means you’re only counting four eggs. “If you take that same horse but apply a different technique that counts more eggs, you have better resolution of your data,” Nielsen explained. “Think about which technique you use, and consider that you might be missing early signs of anthelmintic resistance if you’re not counting enough eggs.” 
  • The number of horses tested. Nielsen shared recommended group sizes for FECRT testing of strongyles and, for the first time, ascarids. Testing can still be done on smaller groups of horses, but it will result in wider confidence intervals and less statistical certainty, he said. 
  • Thresholds for defining reduced efficacy. To accurately classify a drug’s resistance status, the WAAVP now recommends calculating confidence intervals using upper and lower efficacy thresholds specific to each drug and parasite species. “We base our classification on how the confidence limits fall relative to these two thresholds,” Nielsen explained. “The interval between the thresholds is referred to as the grey zone, within which data are inconclusive and more testing is needed.” He added that user-friendly online interfaces are available for quick and easy calculation of these metrics. 

Dealing With Anthelmintic Resistance 

The new parasite control guidelines will provide recommendations for practitioners if they suspect or confirm anthelmintic resistance. Nielsen reviewed these briefly:  

  • Check storage conditions, expiry date, and dosage of the product administered to rule those issues out first. 
  • Repeat the treatment and determine new FECs at 14 days post-treatment. If efficacy still falls below expected levels, it’s strongly suggestive of anthelmintic resistance. Now you’ve double-checked and verified that finding, he said. 
  • Once you’ve confirmed resistance, choose a different anthelmintic class and repeat the exercise.  
  • In cases of documented resistance to all available drug classes, use the class with the highest observed efficacy.  

“For farms with documented multi-drug-resistant parasites, do this and keep monitoring it,” Nielsen said. “We’re not primarily concerned about small strongyles. We want to keep the large strongyles, the very pathogenic parasites, away, and we want to make sure we have good control of ascarids, because we still have drugs that work against those.” 

His take-home message, however, remained the same as his opening remarks: “We’re running out of options. We need new solutions.” 

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