The American Association of Equine Practitioners (AAEP) recommendations for deworming horses have changed substantially over the years, with the current emphasis on strategic targeting (treating certain horses) rather than the outdated rotation-based treatment of all horses at a facility.

“The primary objective of managing parasites in horses (as well as other equids) is to minimize parasite infections, ensuring the well-being of the animals and preventing the onset of clinical illness,” says Jonathan Yardley, DVM, associate professor of clinical equine field services at The Ohio State University’s College of Veterinary Medicine, in Columbus. Of the internal parasites found in adult (over 18 months of age) horses, the most damaging and potentially fatal for the horse are large strongyles (bloodworms), small strongyles (cyathostomes) and tapeworms. Other intestinal parasites include pinworms, threadworms, stomach worms and lung worms. In foals less than 1 year old, ascarids (roundworms) are the most problematic. “It’s worth noting that adult horses develop immunity to ascarids, making this less of a concern in older horses,” says Yardley.

The Life Cycle of Internal Parasites in Horses

The life cycle of most equine internal parasites includes egg, larval, and adult stages. In general, the horse eliminates parasite eggs in the manure, depositing them onto the ground. The larvae develop in eggs and might hatch out, depending on the species, which allows the horse to ingest the parasite’s infective stage. The larvae then develop into adults within the gastrointestinal tract of the horse, with some parasite species migrating throughout the horse’s various organ systems as developing larvae before completing their life cycles.

Previous Equine Deworming Practices

In the past, owners dewormed their horses every four to eight weeks to treat them for any possible intestinal parasites, but more recent research shows this practice leads to parasite resistance. Scientists predicted resistance to horse dewormers (anthelmintics) a long time ago, says Antoinette Marsh, PhD, associate professor and service head at Ohio State’s Veterinary Medical Center Diagnostic Parasitology Lab. “(Because) roundworms, small strongyles, and tapeworms reproduce quite rapidly and display high fecundity (the ability to produce a large number of offspring), the opportunity for genetic change significantly increases,” she says. “The presence of viable, reproducing worms remaining after any deworming gives surviving worms an advantage to reproduce as compared to nonresistant worms no longer around.”

The greater the deworming frequency, the faster resistance develops and, once this resistance occurs, it is not reversable, Marsh says. One parasite generation can pass drug resistance or susceptibility to the next, creating more resistant parasites over time if all the anthelmintic-susceptible parasites are killed off.

Updated Equine Deworming Practices

Current deworming practices involve only treating the horses that need it the most (young horses and those with a heavy parasite burden) to slow the rate of parasite resistance development by maintaining a population of nonresistant parasites in the horses that are not receiving any deworming treatment.

“By leaving some drug-susceptible worms behind to mate or reproduce with drug-resistant worms, the drug-resistant genes in the worms can be diluted out when examining the parasite population as a whole,” says Marsh. This nonresistant, untreated population is called refugia and is key to the success of current strategic deworming practices. The refugia will shed eggs containing the nonresistant genetic code into the environment and, over time, this will help dilute the drug-resistant parasite population.

Because tapeworms are difficult to identify in fecal examinations, horses should be dewormed for tapeworms biannually or annually with a product containing praziquantel, says Marsh.

Safe Deworming Practices for Adult Horses

“It’s important to note that the aim is not the complete elimination of all parasites within a specific individual,” says Yardley. “Pursuing total eradication is not only unattainable but also leads to the accelerated emergence of drug-resistant parasites.”

Because a heavy burden of internal parasites can cause weight loss, rough hair coats, poor performance, diarrhea, and colic in horses, the deworming plan should target maintaining the health of the horse and prevention of drug-resistant parasites. “Fecal egg counts (FECs) are useful to determine the magnitude of egg shedding of an individual horse and the level of pasture contamination,” says Marsh.

After a FEC veterinarians categorize horses as nonshedders (no eggs seen), low shedders (usually less than 150 eggs per gram (EPG), moderate shedders (usually 150-500 EPG), or high shedders (usually more than 500 EPG). The FEC allows veterinarians to more clearly identify horses with consistently higher parasite egg counts. These horses are most likely to show signs of parasitism, such as weight loss, dull coat, or pot belly, and pass large numbers of infective eggs into the environment.

Veterinarians select dewormers based on the FEC results, the horse’s age, and the type of parasite identified or suspected. Repeating the FEC 12-14 days after deworming provides information on the effectiveness of the dewormer used, says Marsh. “The (repeat) test is referred to as a fecal egg count reduction test (FECRT).  Essentially, do an FEC, deworm, repeat the FEC, and then use a formula to determine the treatment efficacy. When there is a 95% or greater reduction in parasite egg number in the feces, then that dewormer is still quite effective.”

Deworming Foals

While strategic targeting works well in adult horses, owners should take a different deworming approach for foals (under 1 year of age), says Yardley. “Younger horses, due to their developing immune systems, should be treated as high shedders from birth to 2 years of age,” he explains. “Typically, high shedders require deworming every four months with a crucial assessment of their status through a fecal egg count.” Foals aged zero through 6 months are especially vulnerable, having a higher population of roundworms in their intestines, he adds. “It’s noteworthy that these roundworms can migrate from the small intestine to the liver and then to the lungs of the foal, potentially leading to clinical signs resembling pneumonia. First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against ascarids. Second deworming is recommended just before weaning (approximately 4-6 months of age).” Using ivermectin in young foals can be fatal if they have a heavy burden of adult roundworms in their small intestine, says Yardley, so it is important to have an FEC in a young foal before using ivermectin.

Take-Home Message

Most horses do not need to be dewormed as frequently as researchers previously thought. The new equine deworming protocol allows veterinarians and horse owners to tailor the deworming program to each horse. Individualizing deworming strategies can minimize parasite contamination of the environment, improve the horse’s overall health, and limit parasite resistance.