Learn the latest on keeping internal and external parasites at bay

No horse owner wants to see worms in their horses’ manure piles, or bugs jumping out of their manes. But parasite control is an inevitable part of horse husbandry, and the creepy-crawlies aren’t going anywhere. Knowing which parasites pose the highest risk of causing clinical problems, as well as how to keep their numbers at bay most effectively without promoting drug resistance, gives us the best chance at keeping our equids healthy for decades to come.

In this article we’ll examine the major parasites affecting horses both internally and externally.

Protecting Against Internal Parasites in Horses

Deworming horses has been a staple of equine preventive care for the past 50 years. At the time veterinarians began routinely recommending deworming, the large strongyle bloodworm, Strongylus vulgaris, was (and, though rare, still is) the most dangerous equine internal parasite known. Small strongyle eggs would reappear about two months after administering anthelmintics, leading to the prevalent prevention strategy of deworming horses every two months, “rotating” the drug classes, which was thought to prevent drug resistance over time. While this approach was successful in curbing large strongyle numbers substantially—to the point they are now considered a rare problem—it led to drug resistance among today’s most important parasites: small strongyles (cyathostomins) and roundworms (Parascaris spp).

Despite the known issues with resistance, the efficacy labels on all available anthelminthic (deworming) products are based on studies from decades ago, before widespread resistance emerged, and are no longer accurate, says Martin Nielsen, DVM, PhD, DVSc, Dipl. ACVM, Dipl. EVPC, Schlaikjer professor of equine infectious disease at the University of Kentucky’s Gluck Equine Research Center, in Lexington, and a top researcher studying equine internal parasites.

According to the American Association of Equine Practitioners (AAEP) Internal Parasite Control Guidelines, the goal of internal parasite control in horses is to “limit parasite infections so animals remain healthy and clinical illness does not develop.” Eradicating internal parasites is impossible; trying would result in rapidly increasing drug resistance even further. To limit parasitic disease, control programs should focus on reducing small strongyle egg shedding and maintaining the effective drugs we have available, Nielsen says.

Small strongyles

Luckily for 21^st century horse owners, small strongyles are considered mild pathogens and only cause clinical disease when numbers are extremely high. Small strongyles are found everywhere, and every grazing horse is infected. However, individuals vary widely in the numbers of eggs they shed in their manure.

Practitioners and labs quantify strongyle egg numbers as eggs per gram of feces via a fecal egg count (FEC). Adult horses tend to fall into one of three categories: low shedders (0-200 eggs per gram), moderate shedders (200-500 eggs per gram), and high shedders (more than 500 eggs per gram). Horses tend to remain in the same category for most of their lives, Nielsen says. Low shedders need minimal deworming targeting small strongyles, whereas moderate and high shedders should receive additional treatment to reduce the egg numbers going back onto the pasture.

Veterinarians and horse owners should time this strategic use to correlate with parasite transmission cycles and individual horse needs. Currently, the macrocyclic lactones, ivermectin and moxidectin, are the only drugs with substantial efficacy against small strongyles, though there are increasing reports of resistance to these drugs, too. Scientists report widespread resistance to the benzimidazole class of drugs (fenbendazole, oxibendazole) and the pyrimidine class (pyrantel drugs).

Roundworms

Roundworms, also known as ascarids (members of the Parascaris species), most commonly cause clinical issues in foals and young horses through age 2. In foals they can cause failure to thrive and a pot-bellied appearance. Large groups of worms can cause small intestine impactions, especially if a paralytic drug (pyrantel) kills a high burden rapidly. Due to the potentially severe clinical manifestations of roundworms, infected horses should be treated. Currently, researchers report high levels of resistance to ivermectin and moxidectin. Pyrantel and benzimidazoles are equally effective options, but Nielsen recently showed there are resistance reports to these as well.

Pinworms

Pinworms, or Oxyuris equi, lay eggs around the horse’s anus and perineum. The primary clinical sign of infection is intense tail rubbing and/or hindquarter itching. You can’t always find eggs on fecal egg counts but can perform a “Scotch tape test” around the horse’s perineal area to pick up eggs and identify them. Researchers report pinworms are commonly resistant to ivermectin and moxidectin but can be treated with benzimidazoles.

Bots

Bot fly larvae, Gasterophilus species—are not usually associated with clinical disease. Using ivermectin or moxidectin in late fall or early winter can decrease transmission the following season, and scientists have shown ivermectin to be a bit more effective than moxidectin.

Tapeworms

Tapeworms are very common throughout the United States, and heavy infections have been associated with ileocecal impactions and spasmodic colic. Routine fecal egg count tests do not reliably reveal tapeworm infections, so veterinarians typically recommend empirical treatment at least once a year, usually in the late fall to curb pasture contamination going into the cold season. Scientists say praziquantel and “double-dose” pyrantel pamoate are the only options for treating tapeworms; praziquantel is sold in combination with ivermectin or moxidectin. They report emerging evidence of reduced efficacy with these drugs, as well.

Developing a Deworming Program for Your Barn

Nielsen is the lead author of the AAEP Internal Parasite Control Guidelines, of which an updated version will soon be released. “The principle of AAEP guidelines is to take a two-tiered approach,” he says. “We identify a baseline of treatments that should be considered for all horses—usually one or two treatments a year—and then we add additional treatments to individual horses based on diagnostic findings.”

Fecal egg counts are our most reliable diagnostic tool to determine strongyle egg-shedding levels, identify the presence of roundworms, and test treatment efficacy, Nielsen adds. Once veterinarians have identified the parasites on a given farm using FEC, “the most important thing to focus on is to make sure that you are deworming with products that actually work as intended,” he says. “There are no dewormer products that can be expected to effectively work against all parasite categories, so it is important to find out which products work against which parasites in each equine operation.”

Several variables affect the type and severity of parasite infections present in a herd. “In younger horses, we have higher strongyle egg counts, which may warrant more treatments,” says Nielsen. “And the foals and weanlings may have ascarids, which require different drug classes.

“Young and geriatric horses may be more likely to be high strongyle egg shedders, which means that they may require more treatments over the course of a season,” he adds. “In addition, there is no apparent immunity to tapeworm infections, so horses of all ages can harbor these.”

Climate also affects deworming programs. “The climate determines the parasite transmission season,” says Nielsen. “Some regions have very long transmission seasons, and others have very short and condensed transmission seasons. Treatments aimed at reducing strongyle fecal egg counts should be administered during these seasons, and not in the off-season—winter in a northern climate, for example.” Both arid and frigid conditions are unfavorable to parasite transmission, so those are appropriate climates to avoid deworming.

After using FECs to determine the egg-shedding status of each horse on a farm and identifying if strongyles, roundworms, or both are present, Nielsen says veterinarians can use fecal egg count reduction tests (FECRT) on the higher shedders to test the efficacy of the dewormer used. He says it’s a good idea to do a FECRT annually on each property to make sure the products being used are effective on that premises.

In summary, when addressing adult horses, practitioners and owners should focus on controlling small strongyles with one or two treatments per year for every horse, regardless of shedding status. Generally, one treatment with ivermectin and one with moxidectin—which has some efficacy against encysted small strongyles—can be somewhat effective if parasite resistance hasn’t developed in the population being treated. Include praziquantel for tapeworms at least once a year, Nielsen says. Treatments should be administered during peak transmission seasons—usually spring and fall. Any additional treatments should target just the high shedders of strongyle eggs.

Young horses must be treated a bit differently. Fecal egg counts are still useful to identify which parasites are present, says Nielsen, as roundworms are common in this age group, but strongyles are also often found. Horse owners should first deworm foals at 2-3 months of age with a benzimidazole, such as oxibendazole or fenbendazole, then again at weaning with a benzimidazole, followed by ordering a fecal egg count to assess strongyle shedding. Additional treatments should happen at 9 and 12 months, targeting strongyles. Treat horses through age 2 automatically as high shedders and deworm three to four times per year, depending on the grazing season.

Other common recommendations to decrease parasite transmission include pasture rotation, dragging pastures during the dry season, and keeping stocking density low. Nielsen says these strategies depend on climate and season, with little research available to back up specific guidelines. Also note that “natural” deworming supplements such as diatomaceous earth are not effective for treating internal parasites and should be avoided.

Nielsen warned that the drugs we have will not change for the foreseeable future. “We have to make use of what we have,” he says. “Each of the four classes we have available have issues with drug resistance in at least one equine parasite, but at the same time, each of these classes still maintains efficacy against some parasites.

“The key here is testing,” he adds. “We can’t just keep sticking to our old regimens of blind rotational deworming using everything on the shelf. So, good stewardship means basing our program on parasite surveillance. And, most importantly, routine surveillance of treatment efficacy.”

Ectoparasites in Horses

Horses are also susceptible to parasites that live on the outside of their bodies. Most of the common equine ectoparasites fall in the arthropod category: ticks, lice, and mites (which cause mange). Stomach worms  (Habronema and Draschia species) can also manifest on horses’ skin in the form of summer sores.

Many of these parasites can be easily seen and positively identified without a lot of testing. The most common clinical sign is pruritis, or itchiness.

Susan White, DVM, MS, Dipl. ACVIM, distinguished professor emeritus at the University of Georgia College of Veterinary Medicine, in Athens, has focused on equine dermatology in her practice and lectures extensively on the topic. If a horse is showing clinical signs of ectoparasites, she recommends veterinarians perform a visual examination and skin scrape. With the visual exam they can identify ticks and, by parting the hair, sometimes lice. They can identify the type of lice under a microscope, while a skin scrape allows them to diagnose mites (as well as other causes of skin abnormalities, such as fungal disease).

Ticks

Aside from pruritis, ticks can cause large nodules to form and also cluster together on the skin. Deer ticks, or black-legged ticks, serve as vectors for diseases such as anaplasmosis (caused by the bacterium Anaplasma phagocytophilum) and Lyme (caused by Borrelia burgdorferi). A horse’s tick exposure largely depends on climate and vegetation; ticks often live in areas with a lot of brush and trees. White recommends keeping pasture areas mowed and limiting access to areas where ticks are more likely to reside.

Lice

Both equine-specific biting lice and sucking lice can affect horses. Besides pruritis, lice infestations can cause a dull, scruffy-looking hair coat and the appearance of poor condition. Lice favor cooler climates and rarely survive in hot conditions. Sucking lice feed on blood, and severe infestations can result in anemia. Biting lice feed on dry skin and secretions, commonly infesting the head, mane, shoulder, and tail. Lice spread from horse to horse via direct contact or shared items such as grooming tools and saddle pads.

Eradication of lice involves repeated treatment with either pyrethrin or pyrethroid sprays, lime sulfur dips, or the organophosphate coumaphos. Farmers use coumaphos primarily to treat cattle, but White says the formulations can be safely used on horses and can work for killing ticks as well. (Always consult your equine veterinarian before instituting any off-label treatment.)

Treatment for lice is usually performed at two-week intervals. With some heavy infestations horse owners might first have to clip hair in affected areas.

Mange

Several species of mites can cause mange in horses, with Chorioptes bovis causing the most common form, chorioptic mange. This mange most commonly affects the feathered legs of draft horses. It causes a pruritic dermatitis, followed by the development of papule lesions, hair loss, crusted skin, and moist dermatitis. Horse owners can treat chorioptic mange with lime sulfur at 12-day intervals; in addition, veterinarians believe the macrocyclic lactones ivermectin and moxidectin have some efficacy against most of the mange mites. “Treatment needs to be over several weeks,” says White. “The environment should be treated, and horses with feathers should have the hair clipped.”

Summer Sores

Summer sores, or habronemiasis, occur when Habronema or Draschia species (referred to as stomach worms—usually internal parasites with little clinical significance) larvae burrow into damaged skin or mucous membranes, causing a local inflammatory reaction that consists of an ulcerated lesion with yellow granules in the middle. Veterinarians often need a biopsy to confirm diagnosis, and treatment can be challenging—surgical debridement, corticosteroids, and ivermectin are all used. Unfortunately, once a horse develops summer sores, they usually recur each spring or summer. Fly control stands as the most important factor in preventing summer sores.

 “As is often the case, there is no ‘one shot’ solution to the problem of external parasites,” says White. “Environmental management, persistence, and attention to detail is needed.”

Take-Home Message

Horse owners need to mindfully manage internal and external parasites. Understanding their clinical significance and the best practices for controlling them are the most important pieces of the parasite puzzle.

Editor’s note: This article was originally published in the 2024 Preventive Care issue of The Horse: Your Guide to Equine Health Care.