Reported by Michael Manno, DVM, MS on behalf of the USPA Equine Welfare Committee

The U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service have identified and are currently tracking new cases of vesicular stomatitis virus (VS). Please be aware of a current health alert concerning both the health and movement of horses to, from, and through California and Texas, and plan accordingly.

Vesicular stomatitis is a contagious disease that afflicts horses, livestock, wildlife and even humans. The disease is caused by a virus, which although rarely life threatening, can have significant financial impact on the horse industry. In affected livestock, VS causes blister-like lesions to form in the mouth and on the dental pad, tongue, lips, nostrils, hooves and teats. These blisters swell and break, leaving raw tissue that is so painful that infected animals generally refuse to eat and drink and show signs of lameness. Severe weight loss usually follows.

The Southwestern and Western United States have experienced a number of vesicular stomatitis outbreaks, the most recent and largest VS outbreak occurred in 2015. Outbreaks usually occur during the warmer months, often along waterways. The time from exposure to the onset of clinical signs is 2-8 days.

An updated situation alert is released weekly by the USDA. Below is a link to the most current information.USDA VESTICULAR STOMATITIS SITUATION ALERTS


The following information is provided by the American Association of Equine Partners (AAEP).

When vesicular stomatitis occurs in horses, blister-like lesions usually develop on the tongue, mouth lining, nose or lips. In some cases, lesions can develop on the coronary bands, or on the udder or sheath. When VS is suspected, an exact diagnosis should be obtained by testing the blood for virus-specific antibodies or by testing swabs from the lesions to identify the presence of the virus. Testing is necessary to rule out the possibility that the lesions are caused by photosensitivity (sunburn), irritating feeds or weeds, or toxicity from non-steroidal anti-inflammatory medications like phenylbutazone.

A fever may develop initially as blisters form on the tongue, gums or coronary bands. One of the most obvious clinical signs is drooling or frothing at the mouth and potentially a reluctance to eat. This occurs following rupture of the blisters that create painful ulcers in the mouth. The surface of the tongue may slough. Excessive salivation is often mistaken as a result of a dental problem just as a horse that is not eating well may be suspected as having colic. Weight loss may be a secondary effect, as a horse with mouth ulcers finds it too painful to eat. If lesions form around the coronary band, inflammation within the foot may result in lameness or laminitis.

The disease generally runs its course within two weeks, although it may take as long as two months for the sores to entirely heal. Live virus can often be isolated from the lesions for up to a week after the lesions appear. During this time, the horse remains infective and the potential remains for the disease to spread to other animals.


While a horse is suffering from vesicular stomatitis, feeding soft feeds may reduce mouth discomfort. Anti-inflammatory medications may be used as supportive care help to minimize swelling and pain so a horse will continue to eat and drink. If the horse becomes dehydrated from not drinking enough water, your veterinarian may need to provide additional supportive care using intravenous fluids. Secondary bacterial infection of ulcerated areas is another concern. If fever, swelling, inflammation or pus develops around the sores, treatment with antibiotics may be required. Flushing the mouth regularly with dilute antiseptic solutions, such as chlorhexidine in water, may reduce secondary bacterial contamination of the sores and speed healing. However, there is little more an owner or veterinarian can do but wait for healing to occur and take appropriate precautions to minimize the risk of spread of the disease to other horses and livestock.


There are still some questions regarding how vesicular stomatitis is transmitted and why it only occurs sporadically in the U.S. The disease is distributed only in North, Central, and South America, with a greater incidence in warmer regions. Due to the seasonal occurrence of VS during summer through early fall, it is believed that insects such as biting flies and midges contribute to maintaining the lifecycle of the virus. Black flies, sand flies and midges are known to transmit the virus, but there may be other insect vectors that have not yet been identified. VS can also be passed from horse to horse by contact with saliva or fluid from ruptured blisters. Physical contact between animals, or contact with buckets, equipment, housing, trailers, feed, bedding, shared water troughs or other items used by an infected horse can provide a ready means of spread.


By observing the following guidelines you can help prevent the occurrence of VS:

  • Healthy horses are more disease resistant so provide good nutrition, regular exercise, deworming and routine vaccinations.
  • Isolate new horses for at least 21 days before introducing them into the herd or stable.
  • Observe your horse closely. Immediately isolate any horse that shows signs of infection and contact your veterinarian.
  • Implement an effective insect control program. Keep stabling areas clean and dry. Remove manure and eliminate potential breeding grounds (standing water, muddy areas) for insect vectors.
  • Use individual rather than communal feeders, waterers and equipment.
  • Clean and disinfect feed bunks, waterers, horse trailers and other equipment regularly.
  • Be sure that your farrier and other equine professionals who come into direct contact with your animals exercise due care so as not to spread the disease from one horse or facility to the next.
  • On farms where VS has been confirmed, isolate any animals with lesions away from others and handle healthy animals first, ill animals last. Handlers should then shower, change clothing and disinfect equipment to prevent exposing others.
  • Anyone handling infected horses should implement proper biosafety methods, including wearing latex gloves and washing hands after handling animals with lesions.

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