University of Vermont AAHS

Questions on Vesicular Stomatitis Answered

Elizabeth Greene, Ph.D.
Equine Extension Specialist
Unversity of Vermont
Member, AAHS Board of Directors

[reproduced from Caution: Horses, Vol. 3, No. 4, Winter 1998]

The recent false alarms involving Vesicular Stomatitis (VS) outbreaks on horse farms in Auburn, WA and Albany, OR have brought forth questions by backyard and professional horsemen alike regarding potential spread of this disease.  In order to minimize the possibility of VS outbreak on your farm, it is necessary to understand more about transmission, signs, symptoms, treatment, and prevention of VS.

What is Vesicular Stomatitis?

Vesicular Stomatitis is classified as a rhabdovirus since it has a bullet-like shape when examined under a microscope.  Signs and symptoms very closely resemble those of Hoof and Mouth disease, which was eradicated from the United States in the late 1920's.  Hoof and Mouth disease and VS may contribute to major negative economic impacts on the livestock industry.  Cattle tend to suffer the greatest economic losses from these diseases since they develop lesions on their teats.  This can result in decreased milk production, painful milking, depression, loss of appetite and potentially serious cases of mastitis.   The similarities in outward appearance between the two diseases resulted in vesicular stomatitis becoming a "reportable" disease.  This means that any veterinarian or livestock owner that suspects an animal may have vesicular stomatitis must immediately contact his/her state or federal authorities that deal with animal health.

How is VS transmitted?

Vesicular Stomatitis usually occurs between late spring and early fall. The seasonal time frame of outbreaks lends to the theory that insects serve as vectors.  Phlebotomine sandflies are responsible for the spread of one type of VS.  In addition, mechanical vectors such as shared feed tubs and water troughs, or exposure to saliva or fluids from lesions can increase transmission potential of VS.  People can become infected and pass the virus between horses.  The virus cannot, however, pass through intact skin; it is opportunistic, and thus may enter through a break in the skin or a wound.  This may be an important consideration if horses are being vaccinated in the vicinity of infected animals.

What signs will I see?

The incubation period for the disease can vary from two to eight days.  The horse may develop a fever at the time that blisters begin to form on the tongue, gums or hoof area.  Unfortunately, the blisters often go unnoticed until the horse goes off feed due to painful lesions left after the blisters have burst.  Then, by the time the lesions are discovered and the animal quarantined, the entire herd may have already been exposed.  The most common signs in horses include frothing or drooling from the mouth and blisters or lesions as a result of broken blisters.  Occasionally, a horse may show lameness or develop laminitis if he has ulcers in the hoof area.  Infrequently, this can result in the sloughing of his hoof.  Vesicular Stomatitis is rarely deadly, and horses without secondary bacterial infections will usually recover within two weeks.

Could it be anything else?

There are several other potential causes that might have symptoms resembling VS.
Hence, it is important to distinguish between VS infection and other potential mouth irritations including cheat grass irritation of the gums, Phenylbutazone toxicity, or blister beetle ingestion.  Usually, the blister beetle ingestion will also have other symptoms involving the gastrointestinal tract, or colic like symptoms.  Your veterinarian will either draw blood or take swabs of blisters and lesions for laboratory testing.

How do they test for it?

According to Dr. Jim Evermann at Washington Animal Disease Diagnostic Laboratory (WADDL) in Pullman, WA, there are two tests that are used most frequently by veterinarians.  The first involves isolating the virus from a swab taken from blisters or ulcers.  The virus isolation test takes about 48 hours for an answer.  For the second test, the veterinarian collects a blood sample in "red top" tubes in order to run a serum test when the blood coagulates. Two different serum tests check for antibodies that the infected horse will have in his bloodstream.  The Complement Fixation Test (CF) detects early antibodies (i.e. recent infection of VS).  If the horse was infected previously, and is not currently positive by the CF test, the serum will come back positive in the Serum Neutralization Test, but negative by the CF test.  This may help you determine if the horse is actively fighting VS.

Can I vaccinate against it?

There is a vaccination made specifically for horses, but the State Veterinarian controls distribution of the vaccine. Since the vaccination is comprised of killed virus, a vaccinated horse's serum will show positive for both serology tests.  Therefore, a vaccinated horse will test positive and may have the same limitations of an infected horse.  An owner must have proof of identification and excellent vaccination records to avoid travel limitations on his/her vaccinated horse.  Horses are rarely vaccinated, except in the case where an outbreak has occurred in the vicinity.

What if I suspect my horse has it?

If your horse shows signs of mouth ulcers or painful ulcers around the coronary band, be sure to contact your vet immediately for evaluation.

Can I decrease potential spreading of VS?

It is important to quarantine the horse immediately.  Minimize potential infection by having completely separate water sources, and by decreasing potential cross contamination via shared salt blocks, care or feeding utensils.  Also decrease the possibility of passage through insect vectors by removing insect breeding grounds (i.e. standing water or mud) and establishing a proactive insect control program.

NOTE: Each state has specific guidelines for transport of VS infected animals.  Livestock transportation or quarantine laws for each state can be examined at the following web site:

Additional information on this as well as other diseases can be found at the United States Department of Agriculture Animal and Plant Health Inspection Service web site: gopher://


Personal communication. Dr. Jim Evermann, Washington Animal Disease Diagnostic Laboratory, Pullman, WA 1998.

Rose, R. J. and D. R. Hodgson. Manual of Equine Practice. W. B. Saunders Company. Philadelphia, PA 1993.

Maas, J. UCCE Livestock Health Fact Sheet No. 6: Vesicular Stomatitis. University of California-Davis.

USDA-APHIS-VS-Emergency Programs.  Vesicular Stomatitis Fact Sheet. 1995.

AAEP Online: Precautions for Horses Diagnosed with Vesicular Stomatitis.

Oglesby, R. N. Vesicular Stomatitis in Horses. The Horseman's Advisor.

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