Top 5 Infectious Diseases

Keep your horse safe on the road and in the barn, part 1 of 2
by R. Paul Schwab, D.V.M.

This month Dr. Sauter and I were asked, “What are the top five infectious diseases to worry about when bringing horses 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?

Dr. Sauter and I 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 in a specific region. Below are our choices. Next month, part two of this article will discuss prevention and control of these syndromes as well as general disease prevention and health tips for going to events.

Equine Influenza
In Equine Infectious Diseases, author Debra Sellon states, “Influenza is the most frequently diagnosed and economically important cause of viral respiratory disease of the horse.” As 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 after 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 aside from 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 are common, as are 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 onto the virus in their nerve ganglia for years, ending up in a lifelong carrier state. This makes control more difficult. Transmission from horse to horse is via direct contact and indirectly from fomites (any agent capable of absorbing and transmitting a disease organism) such as people, tack, and contaminated surfaces. Aborted fetuses, placentas, and fluids are particularly high in the virus.

Strangles
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, which swell up and “strangle” the horse. This bacterium is extremely contagious and is spread easily 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 harbor the bacteria in their guttural pouches and become shedders for life. Clinical signs for Strangles are commonly high fever (104°F) and copious amounts of cloudy white nasal discharge, along with the abovementioned abscesses.

West Nile Virus
I debated whether or not to add this to the list. West Nile virus (WNV) is not very prevalent in Washington, though there have been a number of cases in Eastern Washington, as well as Oregon, Idaho and California. For those traveling with their horses through these and other areas, especially during the summer and fall, there is a higher risk. The 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 WNV will die.

Everything Else/Honorable Mentions
We couldn’t quite decide on the last disease to put on this list. There are several that could fit here, depending on where you live. Eastern and Western Equine Encephalitis are of concern in the Southeast; 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 stressed by weaning or traveling. And, of course, Equine Infectious Anemia is the viral disease that we test for with a Coggins 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. Check back with us next month for tips on disease prevention at shows and events.