If One Dewormer Works, Try Another

Deworming drugs designed to combat equine parasitic infection may no longer be working as they should.


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Deworming drugs designed to combat equine parasitic infection may no longer be working as they should.

Research has shown that the overuse of oral dewormers is fueling parasite resistance to tried-and-true drugs, some that have been in use for 40 years.

“If horse owners and veterinarians continue to use drugs that are no longer effective against certain parasite populations,” says Wendy E. Vaala, VMD, Dipl. ACVIM, “resistant worms will have an advantage and will slowly become the predominant population on a farm and in the horse.”

Dr. Vaala is senior equine technical service veterinarian with Intervet/Schering-Plough Animal Health.

“Drug resistance is the result of random mutations that occur naturally within a parasite population,” Vaala says. “Drugs do not cause resistance but rather select for those parasites that have developed the resistance mutation.

“Using more than one drug class during the year is encouraged since there is no one class of drug or single dewormer that can or should be used exclusively for a prolonged period of time.”

Researchers are examining the mutations that are occurring to learn more about specific mechanisms of resistance.

“Tests are being developed to help identify the actual worm burdens of individual horses,” Vaala says. “For example, we do not yet have a sensitive and reliable method for detecting tapeworm infections and encysted small strongyle infections.”

Questioning Rotation

Veterinarians may be hard-pressed, though, to convince horse owners that the traditional method of rotational deworming is not the best strategy any more.

“The term ‘rotation’ may be misused,” Vaala says. “Random rotation between drug classes without fecal egg count monitoring is not recommended. All deworming programs, including daily deworming, should be monitored to be certain the drugs prescribed are still working as expected.

“Some drugs are more effective against roundworms, others are more effective against bots, others more effective against encysted small strongyles, while others help control large strongyles,” she says. “So whether you call it rotation or targeted deworming, I believe more than one product should be used during a 12-month period to help control all the important parasites a horse encounters.

Targeted strategies can help keep worm loads manageable. And fecal egg count monitoring must be part of all deworming programs.

“The goal is to use only the necessary drugs in the horses that require them,” Vaala says. “Rather than refer to continuous vs. rotational, I prefer the terms ‘strategic’ or ‘targeted deworming,’ since that suggests there is a specific reason to use a particular drug in an individual horse at a certain time of year.”

Julia H. Wilson, DVM, Dipl. ACVIM, an associate professor at the University of Minnesota College of Veterinary Medicine, agrees that a strategy targeting specific species at logical points of time is most worthy.

“Simple rotational deworming is no longer advisable, particularly in areas with high stocking rates or where resistance has been documented,” she says.

“Continuous deworming has merit still in some regions against strongyles,” she says, “but pyrantel resistance has now been demonstrated on some horse farms. There are still benefits from continuous deworming, but it is certainly not a panacea for parasite control.

“Even with the Pfizer program using the daily dewormer,” Wilson says, “at least twice a year deworming with an avermectin is recommended, along with monitoring for resistance with fecal egg counts.”

Tom R. Lenz, DVM, MS, Dipl. ACT, says veterinarians need to offer owners up-to-date deworming strategies.

Dr. Lenz, senior director of equine veterinary services for Pfizer Animal Health, points out that 30 years ago, most parasite control was performed by veterinarians through nasogastric tubing.

“Since the mid-1980s, paste dewormers have pretty much taken veterinarians out of the parasite control business,” he says. “New thinking from parasitologists is that veterinarians have to get involved again for the health of horse.”

He also advises practitioners that owners need to know that using different brand-name dewormers on a rotating basis doesn’t mean they are using different classes of drugs.

“There is no doubt that there is resistance within different chemical classes,” he says. “Mindlessly rotating dewormers is not only a waste of money, but is not good for the health of the horse.”

Current thoughts, says Dennis D. French, DVM, Dipl. ABVP (equine practice), are to deworm only horses that are shedding greater than 500 fecal eggs per gram, based on the life cycles of the parasite and existing environmental conditions. He is a professor at the University of Illinois College of Veterinary Medicine.

By the Numbers

Minnesota’s Wilson says parasitologists are working to document the magnitude of resistance and are advocating for more targeted deworming based on need as determined by fecal egg counts. They also are advocating for the concept of refugia—not deworming horses with low loads of parasites to maintain drug-sensitive populations.

Millions of tubes of anthelmintic being administered to horses every year kill very few parasites, says Ray M. Kaplan, DVM, Ph.D., Dipl. EVPC, because there are few worms in the horse to kill, or because the drug is ineffective as a result of resistance.

“There is a widely held notion among horse owners that all horses are highly susceptible to worms and therefore should be treated the same,” he says.

About 20 to 30 percent of horses harbor about 80 percent of all the worms. Some horses carry extremely high worm burdens, even when treated frequently, while others with strong immunity are infected with few worms.

Kaplan, a parasitology professor at the University of Georgia College of Veterinary Medicine, administered a study in 2001-02 to determine the prevalence of resistance on horse farms in the southern U.S. The study included 1,274 horses on 44 large farms.

“The fecal egg count reduction test was performed on each farm using fenbendazole, oxibendazole, pyrantel pamoate and ivermectin,” he says. “Resistance testing was only done for the cyathostomes.

“The percent of farms found to harbor resistant worms were: 97.7 percent for fenbendazole, 0 percent for ivermectin, 53.5 percent for oxibendazole and 40.5 percent for pyrantel pamoate.

“The prevalences of resistance to fenbendazole, oxibendazole and pyrantel pamoate found in this study were far greater than in any previously published report,” Kaplan says. “Results from all five southern states were remarkably similar despite major differences in the types of farms and in physical geography.

“These results,” he says, “indicate that drug resistance in small strongyles is common, that the problem of anthelmintic resistance in cyathostomes is constantly worsening, and anthelmintic resistance may be more severe in the United States than elsewhere in the world.”

He says three major classes of anthelmintics are used to control nematode parasites in horses: benzimidazoles, tetrahydropyrimidines and avermectin/milbemycins, or macrocyclic lactones.

“Of these classes, resistance to BZ is the most prevalent and widespread, with reports of resistance from more than 21 countries,” he says. “Reports of resistance to pyrantel are less common. The true prevalence of resistance in most of the world is unknown.

“Many parasitologists have suspected that low-dose daily feeding of pyrantel may lead to resistance,” Kaplan says. “Because the United States and Canada are the only countries in which daily feeding of low-dose pyrantel tartrate is practiced, one must wonder whether this mode of administration is having a major impact on the selection for resistance to pyrantel.”

“The food animal side of veterinary research is actually where most of the ideas on battling resistance were developed,” Wilson says.

“Sheep and goats are experiencing a more serious drug-resistant parasite issue,” Vaala agrees. “Cattle are also affected. New concerns are that flea, tick and heartworm products may be facing similar issues in our dogs and cats. Certain parasites in humans are also becoming more difficult to manage.”

The veterinarians are interested in the development and marketing of new anthelmintics—entirely new drug classes, not just new products of existing classes.

“The great cost associated with the development of new drugs and the modest size of the equine anthelmintic market make it very likely that few new anthelmintic classes will be developed and marketed in the foreseeable future,” Kaplan says.

“Thus, even if a new drug class is introduced sometime in the near future, it is clear that drug resistance will continue to outpace the introduction of new anthelmintics.” 

This article first appeared in the June 2010 issue of Veterinary Practice News. Click here to become a subscriber.

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