By R. Paul Schwab, D.V.M.
This month we had some help from Karen in choosing a topic. “What are the top five infectious diseases in horses to worry about when going to a show or event?” Tough question.

Several factors need to be considered: 1) where is the horse going (state, country, distance, etc.) 2) What diseases are prevalent in that region 3) where are the animals at the event coming from, and 4) what vaccinations has your horse been given and how long ago? We came up with a long list to choose from:

Influenza Rhinopneumonitis Equine Infectious Anemia
Strangles Salmonella Potomac Horse Fever
Rabies West Nile Disease Vesicular Stomatitis
Ringworm Lawsonia Anaplasmosis
Pleuropneumonia Lyme Disease MERSA
Botulism Tetanus Corynebacterium pseudotuberculosis
Misc. Fungal Infections EPM Eastern and Western Encephalitis

To narrow down this long list we tried to think of the top five that horses are likely to come into contact with when in a show situation with other horses and/or disease-carrying insects in a specific region. Here are our choices. Part II of this article (next month) will discuss prevention and control of these syndromes as well as general disease prevention and health tips for going to events.

Equine Influenza

In Sellon’s Equine Infectious Diseases, it is stated “Influenza is the most frequently diagnosed and economically important cause of viral respiratory disease of the horse.” Like in people, Influenza or “flu” is seen throughout the world. The Influenza virus has a very short incubation period with clinical signs occurring within 48 hours post infection. Horses are exposed to the virus via nasal secretions from infected horses or horses still shedding the virus up to a week post clinical disease. The virus can be transmitted not only directly, but also through the air in confined areas (stalls and barns), and it can survive in the environment for 72 hours on moist surfaces. Clinical signs include fever (up to 106° F), clear nasal discharge followed by cloudy discharge within 48-72 hours, coughing, and lethargy. Affected horses usually don’t eat well and may have swollen lymph nodes under the jaw or neck. Other signs may include muscle soreness, secondary pneumonia, and limb swelling. There is no treatment besides supportive care, anti-inflammatory medications, and antibiotics to prevent or treat secondary bacterial infections.

Equine Herpesvirus

There are two types of Equine Herpesvirus of concern; EHV-1 and EHV-4. Both are capable of causing respiratory disease or Rhinopneumonitis(“Rhino”). EHV-4 is a common cause of viral respiratory disease in horses, especially in young animals. Affected animals will have a “biphasic” fever that goes up and down often for several days. Coughing and clear nasal discharge is common as well as secondary bacterial infections. EHV-1 can also cause abortions in pregnant mares and neurologic disease in horses. The neurologic form has gotten recent press because of several outbreaks between 2000 and 2006. Clinical signs of the neurologic form include hind-limb weakness or paralysis , bladder and/or tail paralysis, and recumbency. Unfortunately, horses can hold on to the virus in their nerve ganglia for years ending up in a life-long carrier state. This makes control more difficult. Transmission from horse to horse is via direct contact and indirectly from fomites such as people, tack, and contaminated surfaces. Aborted fetuses, placentas, and fluids are particularly high in virus.


No list would be complete without this one. Strangles is the common name for the infection caused by the bacteria Streptococcus equi. It’s known for the classic abscesses that it causes under the jaw that swell up and “strangle” the horse. This bacterium is extremely contagious and is spread easily through a barn through contact with nasal secretions whether directly or indirectly from water troughs, tack, or people. Infected horses can shed the bacteria for weeks after clinical disease, and in some cases horses harbor the bacteria in their guttural pouches becoming shedders for life. Clinical signs for Strangles are classic with high fever (104°F) and copious amounts of cloudy white nasal discharge being the most common along with the above described abscesses under the jaw and throat.

West Nile Disease

I debated whether or not to add this to the list. West Nile Virus is not very prevalent here in Washington (there have been a number of cases in Eastern Washington), but for those taking their horses out of the state, especially during the summer and fall, there is a higher risk. West Nile Disease is caused by a virus that is spread to horses (and people) through mosquitoes. The virus damages the neurologic tissue in the brain and/or spinal cord causing symptoms such as extreme lethargy, depression, difficulty eating, and difficulty walking. Thirty percent of unvaccinated horses that get infected with West Nile Virus will die.

Everything Else/Honorable Mention

We couldn’t quite decide the last disease to put on the list. There are several that could fit here depending on where you live. Eastern and Western Equine Encephalitis is of concern in the southeast and Potomac Horse Fever is of concern on the east coast. Lawsonia intercellularis is a bacterial disease that causes severe diarrhea in young horses especially those that are stressed by weaning or stress of traveling. And of course Equine Infectious Anemia is the viral disease that we test for with a Coggin’s Test for health certificates. It is not extremely common, but the equine community and federal and state governments have worked hard to keep it so.

It’s best to talk to local event officials or veterinarians to help decide which diseases are of more concern for a particular area. Part II will focus on disease prevention at shows and events.