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Vesicular stomatitis

by Elizabeth Mumford, DVM, and Josie Traub-Dargatz, DVM, MS

Depending on what region of the United States you live in, vesicular stomatitis (VS) may or may not be in your vocabulary of equine medical terms. If you live east of the Mississippi, you may never have heard of the disease unless you have recently shipped a horse to or from the western states. If you live in areas of the western USA, however, you have likely become familiar with VS as a result of the outbreaks that occurred in 1995, 1997, and 1998.

Vesicular stomatitis is an infectious viral disease that causes sporadic outbreaks in horses and cattle in the USA, especially in the Southwest and Rocky Mountains. It occurs every year in livestock in areas of Mexico, Central America, and South America. The disease has also been reported in swine, and one llama was determined to be VS-positive in the 1995 outbreak. There are at least two VS virus serotypes, Indiana and New Jersey, that cause outbreaks of disease in livestock in the western USA.

Clinical course of the disease:
In horses, infection with VS viruses commonly causes vesicles (blisters) or ulcers in the mouth or nose, which may be seen as drooling, swelling and/or crusting of the muzzle, and unwillingness to drink or eat. Signs of the disease last for 7 to 10 days. Vesicles or ulcers may also be seen on the coronary band(s), mammary glands, prepuce, or vulva. Horses with VS will often lose body weight, but regain the weight readily after the oral lesions heal. Clinical signs have been reported to occur 3 to 14 days after exposure to an affected animal. Any livestock with vesicles or other signs of VS should be promptly examined by a veterinarian.

Some owners report that infected horses are simply depressed or "off" not subjecting horses to undue stress all contribute to maintaining horses in good general health. Your veterinarian can help you develop a health maintenance plan to keep your horses healthy.

Changing or disinfecting clothes and boots and washing your hands after working with infected animals.

Disinfecting areas and equipment (including feeders and waterers) used by affected animals before introducing non-infected animals.

Some reports suggest that insects may play a role in transmitting VS viruses among horses. Although insect transmission has not been proven in the field, minimizing insect contact with livestock may help prevent the spread of VS viruses. Management practices aimed at reducing insect contact with horses include stabling horses during periods of increased insect activity, using insect repellents on horses, controlling insects in areas where horses are housed (using insecticides, screens, bug-zappers, insect traps, etc.), and moving animals from pastures containing waterways during the summer months.

People who work closely with infected animals have been reported to develop signs of VS including aches, pains, and fever. In rare cases, people infected with VS viruses may develop oral vesicles. If any of these signs develop, contact your physician promptly for diagnosis and treatment. Wearing disposable gloves and washing hands thoroughly with soap and warm water after handling animals with signs of VS can reduce the risk of becoming infected.

A safe, effective vaccine against VS is not currently available in the USA. A vaccine was developed and used during the VS outbreak in 1995, but its effectiveness at preventing clinical VS was not proven in the field. Several research groups are currently working on developing new vaccines for general use.

Regulation of the disease:
Outbreaks of VS have caused significant economic hardships for horse owners due to the mandatory restrictions imposed on livestock travel at the local, state, national, and international levels. The regulations and restrictions result, in part, from the relative lack of research-based information about the disease.

The disease is highly regulated primarily because the clinical signs of VS are indistinguishable from the clinical signs of foot and mouth disease (FMD). Foot and mouth disease affects cattle and swine, and has been completely eradicated from the USA and from many other countries around the world. Because FMD is a devastating disease that is strictly regulated worldwide, most countries do not want to risk the economic consequences of introducing any disease that clinically resembles it.

Therefore, in order to maintain good trade relations with the European Union, Russia, Eastern Europe, China, Japan, Canada, and others, the USA intensely monitors and attempts to control VS nationally. Considering the active leadership role of the USA in the North American Free Trade Agreement (NAFTA) and World Trade Organization (WTO), any restrictions in trade within North America or worldwide resulting from a failure to adequately regulate VS would have devastating effects on the equine and cattle industries.

Within the USA, many unaffected states impose livestock entry restrictions during VS outbreaks. The inability to ship horses into these states (or the cost of testing each horse to confirm that it is free of VS before shipment) can cause horse owners who regularly send animals to these states for breeding, showing, racing, or sale to lose substantial revenue.

Vesicular stomatitis is currently classified by the Office Internationale des Epizooties (OIE; the international authority for monitoring diseases of domestic animals) as a "List A" disease, along with diseases such as FMD and bovine spongiform encephalopathy (mad cow disease). According to the rules of the OIE, the United States Department of Agriculture: Animal and Plant Health Inspection Services (USDA:APHIS) must report all confirmed cases of VS within 24 hours.

Individual veterinarians identifying horses with clinical signs of VS are required to notify their State Veterinarian or USDA:APHIS:Veterinary Services immediately. Suspect horses are then inspected by a veterinarian from USDA:APHIS:Veterinary Services. At this time, a blood sample and an oral swab are collected from each suspect animal, and the premises is immediately quarantined pending laboratory confirmation of the disease (generally 48 hours). Premises that have confirmed VS-positive horses then remain quarantined for 30 days after clinical signs have been cleared from all livestock on the premises.

In 1995, premises in 41 states were investigated for suspect VS in livestock. Horses and/or cattle on 367 premises in New Mexico, Colorado, Utah, Arizona, Wyoming, and Texas were determined to be VS-positive. No animals were determined to be VS-positive in 1996, but in 1997 another outbreak of VS occurred, resulting in premises investigations in 40 states. During 1997, horses and cattle on 380 premises in Arizona, Colorado, New Mexico, and Utah were determined to be VS-positive. As of August 21, 1998, horses and cattle on 57 premises in Arizona, Colorado, New Mexico, and Texas were determined to be VS-positive.

Current studies: Currently, researchers from Colorado State University College of Veterinary Medicine and Biomedical Sciences (CSU-CVMBS) in conjunction with the Colorado Department of Agriculture, the USDA:APHIS:Veterinary Services, the USDA:Agriculture Research Services (ARS), and many other public and private agencies throughout North and Central America are conducting studies on VS. Some of these studies are examining VS viruses in the laboratory, and others are studying the disease on premises in areas where outbreaks have occurred. The cooperation of individual livestock owners throughout these regions has been invaluable to the success of these studies. Questions being investigated include: 1. Which insects can harbor and/or transmit VS viruses? 2. Where do VS viruses come from at the onset of an outbreak? 3. Why does VS sometimes occur in animals in the same locations outbreak after outbreak, and sometimes spread into new locations? 4. Why do some animals get sick and some do not? Is infection with VS viruses occurring outside the areas where it is typically seen, but in a different form?

Funding for VS research is being provided by the CSU-CVMBS College Research Council from Colorado Racing Funds, the USDA:APHIS:Veterinary Services, the USDA:APHIS:National Veterinary Services Laboratory, the USDA:Foreign Agriculture Services, and the USDA:ARS.

For further information about vesicular stomatitis, please contact the USDA: APHIS:Emergency Programs Staff at 4700 River Rd, Unit 41, Riverdale, MD 20737-1231 or phone 301-734-8073. Updates on current VS status nationally and for individual states are available through the USDA:APHIS web site or from the USDA:APHIS: Centers for Epidemiology and Animal Health 24-hour Voice Response Service at 1-800-545-8732.

From the American Association of Equine Practitioners

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