Overview of Vesicular Stomatitis
Vesicular stomatitis is a viral disease caused by two distinct serotypes of vesicular stomatitis virus—New Jersey and Indiana. Vesiculation, ulceration, and erosion of the oral and nasal mucosa and epithelial surface of the tongue, coronary bands, and teats are typically seen in clinical cases, along with crusting lesions of the muzzle, ventral abdomen, and sheath. Clinical disease has been seen in cattle, horses, and pigs and very rarely in sheep, goats, and llamas. Serologic evidence of exposure has been found in many species, including cervids, nonhuman primates, rodents, birds, dogs, antelope, and bats.
The viruses are members of the family Rhabdoviridae and genus Vesiculovirus. Vesicular stomatitis viruses are the prototypes of the Vesiculovirus genus. They are bullet shaped and generally 180 nm long and 75 nm wide. The genomic structure is a single strand of negative-sense RNA composed of five genes (N, P, M, G, and L, representing the nucleocapsid protein, phosphoprotein, matrix protein, glycoprotein, and the large protein, which is a component of the viral RNA polymerase). Although there are many members of the Vesiculovirus genus, the New Jersey and Indiana serotypes are of particular interest in the Western hemisphere. These two viruses are similar in size and morphology but generate distinct neutralizing antibodies in infected animals. They have both been isolated in recent outbreaks in the USA.
Vesicular stomatitis is seen sporadically in the USA. Outbreaks historically occurred in all regions of the country but since the 1980s have been limited to western states and occur seasonally, usually May through October. Outbreaks occurred in the USA in 1995, 1997, 1998, 2004, 2005, 2006, 2009, 2010, and 2012. The largest outbreak in the past decade occurred in 2005 and affected nine states. Vesicular stomatitis viruses are endemic in South America, Central America, and parts of Mexico but have not been seen naturally outside the Western hemisphere. The virus can be transmitted through direct contact with infected animals with clinical disease (those with lesions) or by blood-feeding insects. In the southwestern USA, black flies (Simulidae) are the most likely biologic insect vector. In endemic areas, sand flies (Lutzomyia) are proven biologic vectors. Other insects may act as mechanical vectors. Exposure to insects that carry the virus is often associated with nearby moving water sources such as creeks or rivers or irrigation of pastures. Experimental studies have shown that feeding of infected insects on mucosal surfaces and nonhaired areas of the body were more often associated with development of lesions at those sites than if insects fed on haired areas of the body. The prevalence of clinical cases in a herd is generally low (10%–20%), but seroprevalence within the herd may approach 100%. Viremia has not been detected in livestock species that exhibit clinical signs of vesicular stomatitis, although experimental studies have shown transmission of virus, presumably via lymphatics, between co-feeding black flies on cattle. Virus is routinely isolated from active lesions in affected animals, and these lesions serve as a source of virus spread by direct contact and contamination of shared feed and water stations. Many vertebrate species have serologic evidence of exposure and may serve as reservoirs of infection. No definitive reservoir or amplifying host of vesicular stomatitis viruses in the USA has been identified.
The incubation period is 2–8 days and is typically followed by a fever. By the time animals develop other signs and are examined, however, they are rarely febrile. Ptyalism is often the first sign of disease. Vesicles in the oral cavity are rarely seen in naturally occurring cases because of rupture soon after formation; therefore, ulcers are the most common lesion seen during initial examination. Ulcers and erosions of the oral mucosa, sloughing of the epithelium of the tongue, and lesions at the mucocutaneous junctions of the lips are commonly seen in both cattle and horses. Ulcers and erosions on the teats are not uncommon in cattle and may result in secondary cases of mastitis in dairy cows. Coronitis with erosions at the coronary band are seen in some cattle, horses, and pigs, with subsequent development of lameness. Crusting lesions of the muzzle, ventral abdomen, sheath, and udder of horses are typical during outbreaks in the western USA. Loss of appetite due to oral lesions, and lameness due to foot lesions, are normally of short duration, because the disease is generally self-limiting and resolves completely within 10–14 days. Virus-neutralizing antibodies to either serotype persist and have been documented in individual horses that had previous clinical disease for >8 yr after an outbreak, but reinfection can occur after a second exposure.
In most areas, including the USA, vesicular stomatitis is a reportable disease. Samples for diagnostic purposes are generally taken by a foreign animal disease diagnostician or other regulatory veterinarians and are tested by officially designated government laboratories. Diagnosis is based on the presence of typical signs and either antibody detection through serologic tests, viral detection through isolation, or detection of viral genetic material by molecular techniques. Samples for viral isolation may include vesicular fluid, epithelial tags from lesions, or swabs of lesions. Vesicular stomatitis viruses are easily propagated in cell culture. Three commonly used serologic tests are competitive ELISA, virus neutralization, and complement fixation. PCR tests may also be used to identify the virus. Of primary concern in diagnosis is differentiation of vesicular stomatitis from clinically indistinguishable but much more devastating viral diseases, including foot-and-mouth disease in ruminants and swine (see Foot-and-Mouth Disease), swine vesicular disease (see Swine Vesicular Disease), and vesicular exanthema of swine (see Vesicular Exanthema of Swine). Horses are not susceptible to foot-and-mouth disease. Both noninfectious and infectious causes of oral lesions must be considered.
No specific treatment is available or warranted. Cachexia can be avoided by providing softened feeds. Cleansing lesions with mild antiseptics may help avoid secondary bacterial infections. Management factors suggested to reduce risk of exposure to the virus include limiting time on pasture during insect season, providing shelters or barns during insect feeding times, and implementing other procedures that reduce animal contact with insects, such as application of insecticides. This should include application to the inner surface of the pinna, where black flies feed. If livestock need to be kept on pasture during outbreaks of vesicular stomatitis, then keeping them pastured away from moving surface water (such as streams, irrigation canals, or rivers) may reduce the risk of exposure to Vesiculovirus. Affected animals should be isolated, and movement of other animals from the affected premises restricted. Vesicular stomatitis is a reportable disease in most areas, including the USA, so state and federal animal health officials must be notified when it is suspected. Commercially produced vaccines are not available in the USA, but vaccines for livestock are available in some Latin American countries.
Veterinarians act as a part of the surveillance network as they examine animals involved in shows, exhibitions, races, and interstate or international movement in order to write a health certificate (ie, certificate of veterinary inspection). When practitioners observe suspect cases of vesicular stomatitis, they should report to both their state and federal animal health officials. Reporting will prompt a regulatory investigation. Mucosal swab and serum samples from suspected animals are submitted for testing to veterinary diagnostic laboratories. During outbreak years, data regarding laboratory-confirmed cases of vesicular stomatitis, along with the number of premises with cases, are posted on the Web site of the Animal and Plant Health Inspection Service of the USDA at http://www.aphis.usda.gov.
The vesicular stomatitis viruses are zoonotic and may cause self-limiting influenza-like disease (headache, fever, myalgia, and weakness) lasting 3–5 days in people working in close contact with the virus (eg, laboratory exposure, direct contact with lesions in infected animals). Rarely, people can develop vesicles on the buccal and pharyngeal mucosa, lips, and nose. More severe signs, including encephalitis, are rare.