Definition
A vesicular viral disease capable of infecting cattle, horses, swine, people and many kinds of wildlife.
Occurrence
Vesicular stomatitis (VS) is endemic in various regions of North, Central, and South America. Widespread epidemics usually occur about every 10 years. The virus is endemic in sections of the southeastern U.S. but has not spread to swine. Most outbreaks of VS affect horses and cattle, less often swine. Infection also occurs in deer, bobcats, raccoons, feral swine and rodents.
Historical information
Vesicular stomatitis was first recognized in horses during the Civil War. In 1943, infection occurred in swine stemming from the use of VS-contaminated hog cholera serum. The endemic nature of VS in limited geographic areas and its epidemic nature in other areas remain unexplained by epidemiologists. The close resemblance of VS to foot-and-mouth disease and other exotic vesicular diseases continues to compel interest in the disease by animal health officials.
Etiology
Vesicular stomatitis is caused by a rhabdovirus. There are two serotypes of VS, Indiana and New Jersey (NJ). They do not cross immunize. Swine are susceptible to both serotypes. The Indiana serotype has three subtypes while the NJ serotype causes most major epidemics. VS virus is not considered to be highly contagious among domestic animals.
Epidemiology
The complete epidemiology of VS is not known. There may be unknown reservoirs and vectors of the virus, including insects. The disease remains endemic in certain river basins and coastal areas. In temperate zones, the appearance of VS usually coincides with the appearance of warm weather and ceases with the first frosts. This suggests virus is transmitted by insects, especially to horses and cattle, which frequently are infected while on pasture. Insect transmission is not essential; contact transmission has been observed. VS virus has been isolated from both biting and non-biting insects. Biting insects probably become infected by feeding on lesion areas although the viremic period is short. Non-biting insects may transmit virus mechanically. VS virus persists in feral swine and may provide virus for vectors.
The NJ strain of VS virus has been isolated from sandflies (Lutzomyia shannoni) and trans-ovarial transmission of virus has been demonstrated in sandflies. Movement of animals incubating or carrying virus may play a role in epidemics that occur far from endemically infected areas. Fomites from outbreaks in other species often have played a role in spread of VS to swine.
Pathogenesis
Virus appears to require introduction into epithelium through insect bites or abrasions of the skin. However, transmission may occur through the respiratory tract. Lesions appear initially as small, blanched papules. Lesions first develop in cells of the stratum spinosum and vesicles form. Epithelial cells above the stratum basale separate in about 30% of the cases. Loss of fluid through the stratum corneum results in inspissation (thickening by dehydration) of many lesions. The devitalized epithelium over vesicles soon erodes to leave shallow craters that fill with serum and erythrocytes. Since the basal layer of epidermis usually remains intact, most lesions soon heal. Gross and microscopic lesions of the vesicular diseases of swine are similar to those of other species and their morphology is not an adequate basis for differential diagnosis.
Clinical signs and lesions
Clinical signs are similar in all the vesicular viral diseases of most domestic species but there are a few differences peculiar to species and in the location of most lesions. Infection with VS virus stimulates a sharp rise in temperature that usually regresses after onset.
In swine, vesicles on the feet are common. Lameness is often the first sign that is noticed. Vesicles that form and coalesce around the coronary band may lead to loss of hooves, foot deformity and persistent lameness. Vesicles frequently occur on the snout and may extend into the nares. If lesions develop on the lips or in the mouth, there may be excessive salivation, slobbering and chomping. The vulva and scrotum occasionally have lesions. In nursing sows, vesicles may occur on the teats. Vesicles often develop at points of abrasion.
Vesicles usually rupture within 24 hours and the overlying epithelium detaches to leave shallow erosions with reddened or hemorrhagic surfaces. Lesions usually heal within two weeks unless secondarily infected with bacteria.
Pregnant sows may abort as a consequence of fever but none of the vesicular viruses have been reported to affect fetuses. Overall incidence is variable and mortality is low.
Diagnosis
Because signs and lesions of the vesicular viral diseases are similar, a variety of laboratory techniques have been developed for differential diagnosis. In the United States, the Federal Animal Disease Diagnostic Laboratory (FADDL) at Plum Island, NY usually is used for diagnosis. If signs or lesions consistent with VS are observed, state animal disease control offices should be contacted and will help make necessary arrangements for sample testing.
Epithelial and vesicular fluid samples can be used for complement fixation or enzyme-linked immunosorbent assay (ELISA) tests to identify viral antigen. Heparinized and clotted blood should be collected for virus isolation and identification, and for serologic testing respectively. A full set of tissues should be taken for histologic evaluation. Foot-and-mouth disease is of primary concern in differential diagnosis. If horses are affected, it is useful to know that horses are susceptible to vesicular stomatitis but not other vesicular viral diseases.
Control
Many laws that assist in control of VS are measures applicable to the control of all exotic diseases of livestock. They apply to all major categories of domestic animals. They control the importation of animal-derived food products, semen, embryos and biologics. In order to reduce the likelihood of spread of any of the vesicular viruses through garbage feeding, there are regulations that specify how garbage must be cooked before feeding it to swine. Other regulations are designed to prevent the discharge of garbage in ports.
Vaccines have not been used to control VS in swine although killed vaccines may be approved for use in cattle during an epidemic. Segregation of swine away from cattle and horses is an important, practical method of preventing spread of VS to swine. Control of flies, mosquitoes and other insects may be of value.
Emergency measures for control of outbreaks of vesicular diseases in swine require quarantine, identification of the disease, and slaughter of all infected and exposed swine. This is followed by a thorough cleaning and disinfection of the premises. For more information, see the table, Vesiculating Viral Diseases.