Is Your Horse at Risk for Infectious Disease?

Like colds and the flu in humans, horses are at risk for contracting a number of common diseases such as mild respiratory ailments (colds) and flu, along with others that can have mild to devastating consequences.


Like colds and the flu in humans, horses are at risk for contracting a number of common diseases such as mild respiratory ailments (colds) and flu, along with others that can have mild to devastating consequences. Fortunately, most of these diseases can be prevented or the symptoms reduced through preventive management. Here are the diseases you should watch out for.

Respiratory Infections

By far, the most likely infections a horse might get are respiratory diseases. "They are still the number one problem as far as contagious diseases go," says Robert Hertzog, DVM. "It's an ongoing, year-round problem."

Of these, influenza and Streptococcus equi (strangles) are frequent assailants. Horses most at risk are those exposed to transient populations, those stressed by shipping or heavy training schedules, youngsters, and seniors.

Although not usually life-threatening, influenza makes a horse uncomfortable and vulnerable to other diseases. Clinical signs include fever, lethargy, cough, nasal discharge, muscle aches, and inappetence. Treatment includes a minimum of three to four weeks of rest to avoid development of long-term respiratory problems (for every day of fever, stall-rest the horse for one week), antibiotics if secondary bacterial infection is suspected, drugs to control fever and aid in airway clearance, palatable feed, and housing in a low-dust environment.

Preventive Measures

Vaccination remains the best defense against many infectious diseases. However, risks are not equal. Where the horse lives, what he's used for (lawn ornament versus show), and his exposure to new populations of horses affect the decisions on what he should be vaccinated against. In other words, a vaccination program should be tailored to the individual.

Rob Keene, DVM, suggests that owners ask:

  • How is the disease spread? Some diseases, such as influenza, rhinopneumonitis, and strangles, require direct horse-to-horse contact. If your horse does not travel or see new populations of horses, then he is at low risk; but if you're campaigning a horse or your horse is at a facility where other horses come and go, then his risk to exposure increases. Diseases such as tetanus, equine protozoal myeloencephalitis, West Nile virus (WNV), Eastern equine encephalomyelitis (EEE), and Western equine encephalomyelitis (WEE) can afflict a horse without him ever leaving the property or being exposed to another horse; in areas of the country where the disease is present, all horses are at risk.
  • Is the disease life-threatening or does it cause permanent damage? Strangles can be fatal, as can tetanus, rabies, and the encephalomyelitides. Foregoing vaccinating against serious and fatal diseases is gambling with your horse's health and his life.
  • Is the disease difficult or expensive to treat? "If you have a disease that is 100% treatable, inexpensively, and doesn't result in any long-term consequences for the horse, there may be a vaccine that you want to eliminate from the protocol," Keene says. Other diseases, like EPM, are expensive to treat.

Because not all diseases are present in every part of the country, along with individual factors that will influence exposure, owners should work with their local veterinarians to devise appropriate vaccination programs. That said, Keene recommends a preventive program that at least includes vaccinating against tetanus, WEE, EEE, WNV, and strangles. Depending on the area of the country, some veterinarians might add the rabies vaccine to the protocol.

Marcia King

Josie L. Traub-Dargatz, DVM, MS, Dipl. ACVIM, explains that influenza is spread by horses shedding the virus by aerosol or by fomites (any objects capable of mechanically transporting an infectious agent). Keep in mind that an infected horse can in some cases continue to shed the virus and be contagious for more than a week after the fever has broken. She notes that exposure needs to be fairly immediate, as the virus is short-lived in the environment. Once the horse is over the disease, he no longer sheds the organism.

It's a different story for strangles. This bacterium can live in the environment for weeks under ideal conditions and can be shed for months, even a year, by a small number of recovered horses. Says Traub-Dargatz, "Horses can look normal, but still shed this bacterium. It's been hypothesized that's how this disease moves around."

Clinical signs include cough, purulent (pus-like) nasal discharge, fever, decreased appetite, and abscesses in associated lymph glands, reports Stacy B. Smith, DVM. In more serious cases, internal abscesses can occur in other areas of the body.

"Diagnosis is confirmed by bacterial culture (or polymerase chain reaction, or PCR, examination) of nasal swabs, transtracheal washes, and swabs of abscesses," says Smith. "Treatment consists of penicillin and non-steroidal anti-inflammatory drugs (NSAIDs) to control fever; occasionally a tracheotomy is necessary if severe swelling of submandibular lymph nodes causes respiratory distress. All halters, leads, waterers, feed buckets, etc., need to be disinfected to prevent contamination and spread of the disease."

Note that treating strangles with antibiotics is controversial, since it is debated whether antibiotics just keep the disease condition at the status quo so that abscesses don't mature and burst and don't shrink and go away. Also, the form that the bacterium takes on in an abscess might make it difficult for the antibiotic to have an antimicrobial effect. Traub-Dargatz notes, "Some clinicians do not treat with penicillin if the abscess(es) are limited to the head region and are close to rupturing when the disease is recognized in a given horse."

Reduce the spread of strangles by isolating affected horses from healthy ones for several weeks after all clinical signs of the disease have resolved; quarantine new or traveling horses from resident populations (those that don't travel); screen bacteriologically or by PCR examination to determine any asymptomatic carriers, and keep at-risk horses current on strangles and influenza vaccinations. Adds Traub-Dargatz, "It may be appropriate to test that shedding of Streptoccocus equi has resolved prior to moving or introducing new horses."

Skin Irritation

Ringworm and rain rot are skin diseases transmitted by direct contact with an infected horse, often among horses which share grooming equipment, tack, and blankets. The diseases are enhanced by broken or damaged skin (from insect bites, trauma, poor grooming, etc).

Ringworm (dermatophytosis, also called dermatomycosis) is a fungal disease typified by circular patches of hair loss, reddening, scaling, crusting, and itching. Lesions commonly and initially affect areas where tack contacts the skin, then can spread to the head, chest, flanks, and croup. Spontaneous recovery can occur in five or six weeks, although lesions could be prolonged if secondary infection is present. Anti-fungal topical treatments can aid recovery. Infected stalls and woodwork should be treated by pressure-hosing or using bleach, and you should disinfect tack, grooming tools, and other objects that have come into contact with the infected horse.

Moisture and damaged skin are the two agents needed to produce rain rot or rain scald (dermatophilosis, streptothricosis). Clinical signs include crusty or scabby skin, matted hair, hair loss across the back and trunk, reddened skin beneath scabs, and pus. Treatment depends on severity; mild cases can resolve spontaneously in drier weather, while severe cases might require systemic antibiotics. Horses can be treated topically with 1% povidone iodine shampoo for 10 days to aid healing. In all cases, affected horses should be protected from rain, biting insects, and prickly vegetation.

"For skin diseases such as ringworm or rain rot, horses that are parasitized or receive poor nutrition seem to succumb first," Hertzog states. "Their immune systems are not the strongest."

Minimize risk by isolating affected animals, reducing the horse's exposure to moisture, maintaining healthy skin via fly control and good grooming, providing good nutrition and regular deworming, and avoiding shared equipment.

Little Bites Can Cause Big Problems

The mosquito-borne disease making big news is West Nile virus (WNV), which can cause serious and frequently fatal inflammation of the spinal cord and brain in up to a third of infected horses. Says William Saville, DVM, PhD, Dipl. ACVIM, "Based on data from 2001, the greatest risk of disease is in horses aged 25 years and older, probably due to immune suppression.

"Neurologic signs include weakness, depression, and loss of coordination (ataxia)," says Saville. "No specific cure exists. Treatment is supportive therapy."

Anti-inflammatories can be administered, but prevention is the best protection. "Utilize an integrated mosquito control program on your farm," Saville recommends. "Keep horses in during times of highest mosquito activity (dawn and dusk), use fans on them to keep mosquitoes off of them, and remove standing water on your property--buckets, barrels, wagons, tires--so mosquitoes don't have a place to breed."

Vaccinating against WNV is usually recommended; one challenge study done by Fort Dodge Animal Health showed that the vaccine was efficacious (up to 94%). Current vaccination protocol, says Smith, consists of an initial series of two vaccinations with follow-up boosters as recommended by your veterinarian.

"The horse is not considered protected for at least two weeks after the second shot," she says.

Touchy Tummies

Salmonellosis is one of the most common infectious causes of diarrhea in mature horses.

"Salmonella (bacteria) are passed from animal to animal or through environmental contamination," says Traub-Dargatz. "Horses can acquire the infection by ingesting the organism in feed or water or by contact with surfaces such as a caretaker's hand that is contaminated by infected fecal material. The bacterium may be shed in feces by affected and recovering horses for as little as a few days or up to a few months."

Clinical signs are severe watery or bloody diarrhea, weight loss, inappetence, fever, or lethargy. Treatment consists of supportive therapy to replace fluids and electrolytes and isolation for up to six weeks. Contaminated stalls should be disinfected.

Salmonellosis in horses occurs sporadically or in outbreaks, Traub-Dargatz says. Salmonellosis is a leading cause of hospital-acquired infections in North American veterinary teaching hospitals.

These are some of the most common ailments. Keep an eye out for them so that if and when they occur, you'll recognize and begin treating them early to minimize your horse's distress.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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