Interspecies Health Threats: Is Your Horse at Risk?

What protocols do you have in place to protect horses from diseases that are spread from wild animals and insects to your horse?

Photo: iStock

What’s your level of “wary” around the barn? Are you of the mindset that what you don’t see won’t necessarily hurt you or your horse, or are you a bit more barn-detective-paranoid? What was that rustling in the brush on the far side of the fence? Are the mosquitoes worse this year than last? What kind of vermin poop is that beside the feed bin? What the heck ... did you just see a raccoon in broad daylight?!

“Biosecurity protocols were largely designed to help mitigate pathogen transmission between horses and from humans to horses, but what protocols do you have in place to protect horses from diseases that are spread from wild animals and insects to your horse?” asks Scott Weese, DVM, DVSc, Dipl. ACVIM, professor in the Department of Pathology at the University of Guelph’s Ontario Veterinary College.

Sure, you use insect repellents and fly sheets, try to minimize turnout at dawn and dusk, and seal your barn from pests in the evening, but is there more you can do to protect your horses from critters? 

In this article we will review some of the most common diseases transmitted to horses from other animals and discuss ways to minimize the chances of your horses becoming ill. 

The Watcher in the Woods

Three days ago your horse was grazing peacefully in his paddock beneath a dense canopy of fragrant conifers that provides both natural shade and a home for squirrels, birds, and raccoons. Today, your horse is stumbling, seems uncoordinated, appears “checked out,” and is drooling profusely. 

Infected wildlife, such as raccoons, can transmit rabies to horses by bite or saliva.

Photo: iStock

What could it be? Well, the first ruleout would likely be rabies, as it remains an important (and deadly) viral disease. Infected wildlife can transmit rabies to horses either through a bite or via saliva coming in contact with a horse’s wound or mucous membranes. Unlike dogs, most horses suffer from the “dumb” form of rabies rather than the “furious” form that Stephen King has forever burned into millions of Americans’ memories with the thriller Cujo

“All mammals are susceptible to the rabies virus, but since the elimination of canine rabies from the United States back in the 1970s, the number of domestic animal cases has declined steadily,” says Jesse D. Blanton, MPH, epidemiologist at the Centers for Disease Control and Prevention, in Atlanta, Georgia. “Nonetheless, rabies remains an important disease in wildlife, including foxes, skunks, raccoons, and bats.” 

In a 2014 Journal of the American Veterinary Medical Association article, Blanton and colleagues reported that in 2013, veterinarians diagnosed 31 cases of rabies in horses and mules—significantly fewer than the 47 cases reported in 2012. By the end of 2014 that number had dropped again to 25.

“Over 5,300 cases of rabies in wildlife were reported in 2013, and many of those occurred in regions of the United States where horse populations are concentrated, predominantly the Northeast,” Blanton says. “Vaccination of domestic animals is a critical and cost-effective method of preventing rabies in domestic horses, especially because it is essentially impossible to prevent wildlife like bats, raccoons, and skunks from getting into your pastures at all times.”

The Winged Assassin

Your usually steady steed is weaving around his field and can’t seem to keep his feet underneath him. You’ve also witnessed him pressing his head into the fence rails. You’ve seen a few dead birds on your property lately, but figured they’d just hit a window. There have been so few reported West Nile virus cases in horses over the past few years—only 181 in 2015, compared to more than 15,000 cases in 2002—you didn’t consider West Nile virus a real threat anymore. When was the last time you vaccinated your horses against it?

Vaccination and insect control can help prevent potentially deadly mosquito-borne diseases.

Photo: iStock

Many mammals are susceptible to West Nile virus (WNV), but horses seem particularly so. As described in detail in our WNV special feature, vaccination is extremely effective in preventing this mosquito-borne disease—yes, this winged assassin is a tiny buzzing arthropod. 

“Like rabies, it is very important to remain vigilant with vaccination against preventable diseases,” says Angela M. -Pelzel-McCluskey, DVM, equine epidemiologist from the Surveillance, Preparedness, and Response Services of the USDA’s Animal and Plant Health Inspection Service. “Even if those diseases are not particularly common in your area, not vaccinating has unimaginable -repercussions,” including death. 

Veterinarians also advocate standard mosquito-control practices, such as fly masks and sprays and strategic turnout schedules (as night comes mosquitoes pose the greatest risk to horses), but we all know it is extremely difficult to fight every bite, which leads to our next topic.

The Stranger in the Night

A beautiful summer day has come to an end. As the sun sinks lower in the pink- and orange-hued summer sky, you dash out to get a quick ride in before dark. Your mare, who usually nickers a warm greeting, is standing morosely in her stall, head between her knees. It’s a little dark, and as you head in for a closer look, you notice she hasn’t touched her hay all day.

West Nile virus is only one of several mosquito-borne encephalitides that affect horses. Other examples include Eastern, Western, Venezuelan, and St. Louis encephalitis viruses. Each results in clinical signs similar to West Nile virus: fever, depression, gait abnormalities, and neurologic deficits. Many infected horses also have signs (difficulty swallowing, gait abnormalities, recumbency, fever) that resemble those caused by rabies. So always use caution and consult your veterinarian before handling a horse with clinical signs consistent with neurologic disease. 

Also like WNV, these encephalitides are largely preventable with vaccination. According to the American Association of Equine Practitioners, the mortality rates can range from 50-90%, and affected horses that survive often have lingering neurologic and/or vision problems.

The Barn Intruder

Yet another wobbly, uncoordinated, weak horse stars in this scene. Unlike the previous cases, this horse has a head tilt, difficulty swallowing, and is circling around instead of trying to walk straight. The signs look somewhat like those of the encephalitides, but your veterinarian suspects a different disease: EPM.

Opossums can excrete parasites in their feces that cause neurologic disease in horses.

Photo: iStock

Equine protozoal myeloencephalitis (EPM) is caused by microscopic parasites that are excreted by opossums and later ingested by horses when grazing or from contaminated feed,” says Weese. The parasite attacks the brain and spinal cord, which can lead to serious central nervous system damage and even death if not detected and treated.

The only recommended way to avoid EPM at this time is to prevent opossum access to your feed and hay and, thus, prevent your horse from ingesting opossum poop, both of which are easier said than done. Stabled horses fed from sealed feed bins are less likely to ingest opossum feces; however, horses turned out on pasture, which is widely recommended to avoid stable vices, allow natural foraging behavior, and social interaction, are particularly susceptible. 

The Bloodsucker

Unlike the more rapid disease onset frequently seen with the viral encephalitides and EPM, the horse trapped in this story suffers more subtly. At first you can’t put your finger on what is wrong: A slight lameness that shifts from one limb to another, a lack of energy, some subtle behavior changes, and perhaps muscle wasting.

Ixodes tick species, such as the blacklegged or deer tick, transmit the bacterium responsible for Lyme disease to horses. Lyme disease is difficult to diagnose, partly because the signs of disease are hard to pinpoint and partly because the blood test can only determine if the horse has been exposed to the bacterium Borrelia burgdorferi and not whether he is actively infected. Horses residing in areas known to harbor Ixodes ticks, such as the Mid-Atlantic and northeastern states, the Midwest, and California, might benefit from vaccination. (Note: There is no licensed Lyme vaccine for horses, but some veterinarians use a canine vaccine off-label, and researchers are testing it in horses: Check horses daily for ticks, especially around the ears, in the mane and tail, and in the hairs at the back of the fetlock, and remove ticks carefully. 

“Several canine-approved tick repellents are used in horses, but dose, application sites, and efficacy are unknown,” says Tom Divers, DVM, Dipl. ACVIM, ACVECC, Steffen Professor of Veterinary Medicine and section chief of large animal medicine at Cornell University’s College of Veterinary Medicine, in Ithaca, New York. “An equine-approved permethrin and piperonyl butoxide product for tick prevention is available, but published clinical studies (on its application and usefulness) in horses could not be found.”

Another way to reduce your property’s tick crop is by keeping pastures mowed and brush-free.  

Unlike other pathogens that politely follow the immune system’s common rule of, “once you’ve had it, you can’t get it again,” B. burgdorferi natural infection does not induce strong immunity, making reinfection a reality. Moreover, ticks can cause more diseases than just Lyme (such as piroplasmosis and anaplasmosis). 

The Microscopic Menace

While mucking stalls out at the end of the barn, you hear a splattering sound that concerns you. A search for its source leads you to your mare’s stall. She is groaning in discomfort and suffering from a terrible bout of diarrhea. Worse, she seems close to adopting the sawhorse stance synonymous with every horse owner’s worst nightmare: laminitis.

“There are multiple causes of diarrhea in horses that can be spread from other animals, one of the most notable being the bacterium Salmonella,” says Weese. 

Salmonellosis affects many mammals, including horses, cattle, pigs, dogs, cats, wildlife, and even humans. Salmonella enterica serovars most commonly cause diarrhea, septicemia (bacterial invasion of the bloodstream), or both. Horses become infected when they ingest the bacterium in the environment from either other sick animals or apparently healthy animals that are “carriers.” Once ingested, the bacterium invades the gastrointestinal tract lining and causes profuse diarrhea. Horses that are carriers can shed bacteria after stressful scenarios, such as transport or parturition (foaling), posing a serious risk for other horses and foals. 

“Strict biosecurity protocols should be utilized immediately on any farm with a diarrheic horse,” advises Weese.  

Another cause of diarrhea in horses that shouldn’t be overlooked is Potomac horse fever (PHF). The animals responsible for sharing this disease with horses are snails and aquatic insects, such as mayflies and dragonflies, parasitized by Neorickettsia risticii-infected flukes. Horses get infected when they inadvertently ingest infected aquatic insects. In addition to diarrhea, classic signs of PHF often include a very high fever (104-105°F), colic, anorexia, depression/lethargy, and possibly laminitis.

Due to PHF’s potentially devastating consequences, many field veterinarians in high-risk areas, such as the northeastern United States, will almost automatically treat horses with a fever that live close to water during the summer months, says Peter R. Morresey, BVSc, MACVSc, Dipl. ACVIM, Dipl. ACT, of Rood & Riddle Equine Hospital, in Lexington, Kentucky. 

The Neighboring Livestock

Your beloved broodmare is in the third trimester of her pregnancy when she aborts suddenly and inexplicably. She’s otherwise healthy, so what could have gone wrong? As you examine her pasture for clues, you remember that the creek you’re stepping over is downstream from a cattle and farm. 

In addition to Salmonella spp, other pathogens can spread from cows and pigs to horses: Leptospira interogans and Lawsonia intracellularis, respectively.

Wildlife and livestock species such as pigs might serve as reservoirs for proliferative enteropathy.

Photo: iStock

Livestock and wildlife can excrete Leptospira spp capable of infecting horses. Many animals carrying the pathogen appear healthy, making this disease difficult to spot and prevent. Horses become infected by ingesting water (such as standing water in pastures) contaminated with the bacteria from infected animals’ urine. Infection can cause abortion, kidney disease, and recurrent equine uveitis (also called moon blindness, an important cause of vision loss). Veterinarians might administer antibiotics in some cases and/or the recently released vaccine to at-risk horses (see To further prevent this disease, don’t pasture your horses downstream from cattle.

L. intracellularis can cause equine proliferative enteropathy (EPE). A common disease of pigs, proliferative enteropathy has been diagnosed more frequently in horses for more than a decade now. Infection causes a thickening of the small intestine, resulting in ill thrift, weight loss, poor hair coats, and mild to moderate diarrhea in young horses. While researchers have not yet determined the exact means by which L. intracellularis is transmitted to horses, certain wildlife, livestock, and domestic animals might serve as reservoirs. Appropriate biosecurity and rodent/wildlife control measures are likely the best means of decreasing the likelihood of disease transmission to horses.

Horses are also at risk of contracting the fungal infection ringworm from cows housed on the property. Veterinarians most commonly diagnose ringworm in young or debilitated horses, rather than healthy adults residing in barns with appropriate biosecurity protocols (designated tack, brushes, etc., for each horse).

“I think the concept of ‘keep like with like’ is always good,” says Weese. “Whether that’s keeping broodmares together and away from yearlings, or all horses together but away from cattle, the same basic aspects apply.”

So while your horse isn’t going to catch mad cow disease from nosing the cute calf over the fence next door, it never hurts to be aware of the risks farm animals can pose if unhealthy. 

Take-Home Message

“The major message of these stories is to vaccinate horses annually with all core vaccines recommended by the American Association of Equine Practitioners, including rabies and West Nile virus,” says Pelzel-McCluskey. “In addition, discussing the risk-based vaccines, such as … Potomac horse fever, might be beneficial.”

The second line of defense against animal-borne illness is to protect horses from insects and from contact with wildlife, with the help of fences, doors, and feed bin lids. Wildlife removal services are options for some regions and situations. For more information on controlling nondomestic animal populations on your property, contact your local animal control agency or Extension office.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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