Horse Illustrated Magazine

We all know it’s important to vaccinate our horses, but do we really know a lot about the diseases we are trying to guard against? Understanding these diseases can go a long way toward helping you make educated decisions about your horse, as well as help keep the equine and human communities healthy. In addition to the core vaccinations every horse needs, here are some  common equine diseases you should ask your vet about vaccinating against.

Strangles
Overview: Strangles, also known as distemper, is caused by a bacterium called Streptococcus equi. It is a highly contagious disease that most commonly affects young horses (weanlings and yearlings), but all ages can be infected. Some horses, called “shedders,” may continue to pass on the virus well after an active infection subsides. The organism is transmitted through contact with infected horses or shedders. Transmission can also happen through indirect contact with water buckets, troughs, hoses, feeders, pastures, stalls, grooming equipment, sponges, clothes, human hands and clothing, and even through insects that have come into contact with nasal discharge or pus.

Symptoms: Strangles affects the respiratory tract. Although usually not fatal, strangles can cause permanent damage to the lungs. Coughing, fever, weepy eyes and nose, loss of appetite and a fever of 102 to 106 degrees Fahrenheit are all signs of strangles. Swollen lymph nodes under the jaw are hallmarks of the condition, resulting in abscesses that eventually burst and release pus.

Treatment: Isolation and therapeutic doses of antibiotics, such as procaine penicillin. Everything the horse has come into contact with must be disinfected.

Further Information: “If a horse has had strangles, he’s not going to get it again unless he’s immuno-compromised,” says Julia Wilson, DVM, Dipl. ACVIM, member of the American Association of Equine Practitioners’ (AAEP) Infectious Disease Committee. Once strangles is on a premise, incoming horses may become infected because bacteria can survive for weeks, possibly even longer. “Strangles vaccination should definitely be discussed with your veterinarian. If strangles is a common problem in your area or your horse hasn’t had it and is exposed to many new horses, vaccination may be recommended,” says Wilson. “However, vaccination during a strangles outbreak should be approached carefully, as there is significantly increased risk of adverse reactions in [vaccinating] exposed horses.”

Strangles vaccination is not included in the AAEP’s list of core vaccines (those that all horses should receive); instead, it is listed as a risk-based vaccination.

Equine Influenza
Overview: Equine influenza is a very common and contagious virus that affects the upper and lower respiratory tracts. Transmission is mainly airborne, but indirect transmission can also occur through contaminated buckets or equipment.

“If your horse doesn’t come into contact with many other horses, there is less need for vaccination for influenza,” says Wilson. Influenza is listed as a risk-based vaccine by the AAEP.

Symptoms: Cough, fever, runny nose, loss of appetite and muscle stiffness.
u Treatment: As with humans, the flu must run its course, which takes time for full recovery. Rest is the best medicine.

Further Information: Infection of an entire herd can be avoided by quarantining all new horses for 14 days. Vaccination usually provides coverage for up to six months.

Potomac Horse Fever
Overview: Potomac horse fever (PHF) is a disease caused by the organism Neorickettsia risticii. It is prevalent near waterways throughout the United States from late spring to early fall, with most cases occurring in the summer. The organism uses water snails as its first intermediate host, which then infect aquatic insects (caddisflies, mayflies, damselflies, dragonflies and stoneflies) as its secondary host. These insects hatch in great numbers in the humid months of summer and are attracted to horse facilities by artificial lighting. PHF infection occurs when insect carcasses get into stalls and hay supplies where they are ingested by horses.

Symptoms: Mild colic, fever, diarrhea, and abortion in pregnant mares. Severe cases can lead to laminitis.

Treatment: PHF can be treated with the antibiotic oxytetracycline, fluids and NSAIDs. If PHF is caught early enough, horses usually respond quickly. Laminitis is usually severe and difficult to treat.

Further information: There is strong evidence to suggest that turning lights off at facilities goes a long way toward preventing PHF. “Motion detector lights are a good alternative to floodlights,” says Wilson.

Vaccination Schedule
Choosing the right time for vaccination is important in order to stimulate the horse’s immunity so his body is able to reject a disease-causing organism.

“Horse owners must think strategically. For instance, maybe you’ve just been riding at home but now you want to show,” says Wilson. “If your horse has never received the influenza vaccine, he might need to have a series of two before he is exposed to other horses in the community.

“There are nuances of timing for all vaccines. For example, if you vaccinate against Potomac horse fever in March, and the disease is most prevalent in July, you might not have enough protection to see a reduction in disease severity,” explains Wilson.

“It’s very important to involve your veterinarian in your vaccination decision-making because there are a number of factors to consider, including the horse’s age and lifestyle—whether the animal is breeding, competing or meeting other horses,” says Lunn. “Geography will also significantly affect how you might design a vaccination schedule. This is particularly important with the risk-based vaccines, and local veterinarians will be the best ones to speak to because they know the area and have experience with [the risk-based vaccination diseases]. I’m not a fan of people giving vaccinations themselves. You really do need that professional advice.”

Other Risk-Based Vaccinations
The following diseases also have vaccines available and are considered risk-based by the AAEP.

• Anthrax
• Botulism
• Equine Viral Arteritis
• Equine Herpesvirus
• Rotoviral Diarrhea  

For more information on these diseases, visit www.aaep.org/risk_based.htm

Read about the AAEP’s core vaccinations >>

SHARON BIGGS is a frequent contributor to Horse Illustrated and a dressage instructor.

This article originally appeared in the April 2011 issue of Horse Illustrated. Click here to subscribe.