Vesicular stomatitis in horses treatment and prevention

Etiology
Vesicular stomatitis viruses are members of the family Rhabdoviridae and genus Vesiculovirus. 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 compone

nt 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. Epidemiology and Transmission
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, with some outbreaks overwintering and continuing into a subsequent year or years. Outbreaks occurred in the USA in 1995, 1997 to 1998, 2004 to 2006, 2009, 2010, 2012, 2014 to 2015, and 2019. The largest outbreak in the past decade occurred in 2015 and resulted in 823 affected premises in eight 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 biting insects. Black flies (Simulidae), sand flies (Lutzomyia), and biting midges (Culicoides spp) have been shown to be competent vectors, but other insects may act as mechanical vectors as well. 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, but no definitive reservoir or amplifying host of vesicular stomatitis viruses has been identified. Clinical Findings
The incubation period for vesicular stomatitis 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, ears, 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. 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 10–12 years after an outbreak, but reinfection can occur after a second exposure. Diagnosis
Clinical signs with serologic or antigen detection
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. Whereas cELISA and virus neutralization titers persist for many years after an outbreak, complement fixation titers are relatively short-lived and provide the best serologic evidence of recent infection. Antigen detection assays to confirm diagnosis include PCR tests and virus isolation. 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, swine vesicular disease, and vesicular exanthema of swine. Horses are not susceptible to foot-and-mouth disease. Both noninfectious and infectious causes of oral lesions must be considered. Treatment, Control, and Prevention
Supportive care
Vesicular stomatitis is self-limiting, with no specific treatment other than supportive care. Cachexia can be avoided by providing softened feeds. Cleansing lesions with mild antiseptics may help avoid secondary bacterial infections. Aged animals or those with underlying medical conditions may require administration of IV fluids if oral lesions result in a reluctance to drink. 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. 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 vectors carrying vesicular stomatitis virus. 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. In the USA, affected premises are placed under state quarantine for a period of at least 14 days from the onset of lesions in the last affected animal. 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. Lesion swab and serum samples from suspected animals are submitted for testing to approved 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 Animal and Plant Health Inspection Service of the USDA website. Zoonotic Risk
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. Personal protective equipment should be used when handling lesioned animals to avoid contact with the virus present in the lesions.