After the introduction of safe, effective, easy-to-administer chemical dewormers in the 1960s, the equine industry enjoyed the luxury of being able to control equine internal parasites with a simple push of a plunger.

Parasite-related conditions such as colic, diarrhea, poor hair coat, ill-thrift, poor performance, etc. were essentially eradicated. Rotational deworming programs—involving the frequent administration of chemical dewormers with various products—rapidly became the mainstay. Almost hand-in-hand with the use of those dewormers, however, came the age of resistance: Populations of internal parasites that could not be killed by those coveted chemical dewormers.

“Populations of roundworms and small strongyles resistant to chemical dewormers, also called anthelmintics, have been identified in all parts of the world,” explained Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, assistant professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington. “This once again puts horses at risk for the development of parasite-related diseases.”

There are only three classes of chemical dewormers (benzimidazoles, tetrahydropyrimidines, and macrocyclic lactones), and parasites have developed either established or developing resistance to all three. Combined with the fact that there are no new chemical dewormers in the pipeline for horses (a process that usually takes a minimum of five years), horses with once-treatable parasite-related health issues are now in the pre-1960s position of having no (or very few at the least) treatment options.

“New, nonchemical deworming options are necessary to continue to control equine internal parasites and optimize horse health, welfare, and quality of life,” said Nielsen.

Dedicated to this endeavor, Nielsen and colleagues have devised several unique approaches for controlling equine parasites, including one nonpharmaceutical strategy to kill small strongyles.

Combination Deworming

This involves the co-administration of two chemical deworming products from different drug classes at the same time.

“This concept is based on research conducted in sheep,” Nielsen explained. “The theory is that if a horse has parasites that are already resistant to pyrantel and benzimidazoles, for example, using the two drugs together would result in improved efficacy of the dewormers.”

Nielsen and colleagues are currently putting this theory to the test. They are a few months into the one-year study and will be prepared to make clinical recommendations only once all the data have been gathered and analyzed, he said.

Fecal Egg Counts

As described by the American Association of Equine Practitioner’s Parasite Control Guidelines, surveillance-based deworming is strongly advocated.

“This involves using fecal egg counts to identify the type of internal parasite that is shedding eggs into the horse’s gastrointestinal tract, counting the number of roundworm and small strongyle eggs in one gram of feces, and recommending the appropriate timing and type of dewormer application,” Nielsen said.

Traditionally, fecal egg counts (FECs) have been performed at veterinary clinics or via mail-in services. But technology in this field is advancing. Nielsen and colleagues recently developed the Parasight System—a smartphone-based fecal egg diagnostic and intestinal parasite management tool.

“The Parasight System, which will be available next year, (is designed to allow) veterinarians to perform rapid, quantitative, on-site, and in-house FECs … in five minutes,” Nielsen said. He also noted that the system is designed to minimize the chances of misidentifying the parasite eggs.

Let the Germs Get the Worms

“This is one of our most exciting projects,” Nielsen said. “The premise is that we can use a crystal protein, Cry5B, produced by the bacterium Bacillus thuringiensis. Cry5B kills strongyle larvae in a laboratory setting, and we now need to determine the safety and efficacy of Cry5B to deworm live horses.”

Such studies are underway and are anticipated to take about five years and cost about $500,000. To help fund this research, Nielsen and colleagues are hosting a crowdfunding initiative. Details are available at

Ultrasonographic Roundworm Identification

We know that heavy roundworm burdens can put foals at risk for impactions. We also know that FECs, although beneficial for many horses, are not able to identify foals with heavy burdens 100% of the time. But there might be a better way to identify these foals on the horizon: ultrasounds.

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Recently, several Kentucky veterinarians underwent training for detecting adult roundworm infestations in young horses using ultrasound (see an example above). Nielsen said this technique is ideal for farms that already ultrasound foals aged 3 to 6 months to monitor for Rhodococcus equi abscesses and pneumonia.

Take-Home Message

In light of anthelmintics’ growing lack of efficacy and a dearth of future chemical dewormers, researchers are working diligently on projects to help us better evaluate and manage parasites in the future.