Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Vesicular Stomatitis, VS

Synonyms: Sore Mouth Disease, Indiana Fever


Vesicular Stomatitis (VS) is a viral disease that causes painful blistering of the gums, tongue and occasionally the udder, sheath or coronet band. Horses usually have a mild fever, salivation and may go off feed and act depressed for several days.

VS is thought to be transmitted by biting insects and occurs during insect season. VS is also thought to be spread by direct contact between saliva of infected animals and other horses. However, the precise means of spread is not entirely known.

Up to now, VS has occurred exclusively in the Americas. More frequent and severe outbreaks have occurred since the late 1990's. The disease is more common in southern Mexico and Central and South America. It is likely that a warmer climate in the last 15 years has favored the presence of the disease in North America, and is at least partly responsible for more frequent outbreaks here.

Because of its resemblance to Foot & Mouth Disease (another viral disease that has had a major impact on the livestock industry), VS is closely regulated by the USDA. For this reason, VS is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

VS is a zoonotic disease, meaning that it can be transferred from an infected horse to humans. It usually causes flu or cold-like symptoms in humans. If you have been exposed to a horse with (or suspected of having) VS you should contact your doctor.

Diagnosis is by oral examination by a veterinarian, along with blood tests and tests of swabs of the area for virus.

This disease is usually self-limiting, meaning that horses usually recover on their own with little or no treatment. This disease usually lasts about a week.

Treatment revolves around monitoring to be sure horses are eating and drinking until the problem resolves on its own. NSAIDS (phenylbutazone or similar) may be given to help with pain and inflammation. Some vets recommend oral rinsing but usually this is not necessary and it increases human exposure. Feeding moistened or soft feeds may help horses during the painful early phase.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

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The prognosis for horses with VS is good to excellent. Most recover fully in about 10-14 days.

Pastured horses have a much higher rate of disease than stabled horses. It is unclear why this is but may relate to exposure to the infecting insects or to grazing itself.

The infection rate is high in a group of horses but many horses do not show signs of disease. This relates to the immunity of some horses versus others. Usually 20%-30% of horses in a group will show signs of disease.

Horses less than one year of age have a lower incidence of infection than older horses.

VS is generally a summer-time problem. Colder weather in fall and early winter usually ends an outbreak.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • What precautions can I take to limit spread to other animals and to people?
  • What do I need to do to treat affected horses?
  • How long will it take for test results to be available?
  • How long will my facility likely be quarantined?

The most important factor in reducing VS incidence and spread is reducing horse exposure to biting flies. Practice good fly control, both during the day and evening. Stable horses as much as possible to reduce chance of infection.

Disinfect tack, equipment and premises that have come into contact with affected horses.

VS can be transmitted from horses to people. Wear gloves when handling animals affected by the disease or showing compatible signs. Wash your hands and arms with disinfectant soap following contact with affected animals.

further reading & resources

Author: Doug Thal DVM Dipl. ABVP