Disease in United States

The first report of vesicular stomatitis (VS) in the United States was in 1916. However, anecdotal reports from the Civil War period leave little doubt that VS was occurring in horses during that time.⁵ In 1926 an extensive outbreak of VS occurred in New Jersey in which approximately 750 cattle on 33 farms were affected. The disease appeared in very few horses. The agent was isolated and determined to be distinct from the Indiana strain isolated in the previous year. This new strain of VS was termed “vesicular stomatitis New Jersey strain.”⁶ Over the next six decades, VS occurred sporadically throughout the United States. Only states in New England appear to have been spared incursions of VS.

In the 1990s, three outbreaks of VS occurred in the southwestern United States. On May 9, 1995, the first case of VS was confirmed in Las Cruces, New Mexico. A “case” was defined as an animal with positive virus isolation or serologic test results, in combination with clinical signs consistent with VS. Bridges et al.⁷ have provided an extensive review of this outbreak. Briefly, 1162 investigations were conducted in 42 states during the outbreak. VS was confirmed in six states, including Colorado (165 premises), New Mexico (186 premises), Utah (6 premises), Texas (1 premises), Arizona (1 premises), and Wyoming (8 premises). Overall, 78% of the positive premises housed horses that were positive for VS, 22% of positive premises housed cattle that were positive for VS, and there was one VS-positive llama. All cases where virus isolation was successful were caused by the VSV-NJ serotype.

McCluskey et al.⁸ have provided a detailed review of the 1997 outbreak. The index case for this outbreak in the United States was identified on May 27 in Yavapai County, Arizona, after a report of suspicious vesicular lesions in a horse by a private practitioner. During the 1997 outbreak, 689 investigations for suspect VS occurred in 40 states. There were 380 (55%) premises identified as housing animals positive for VS in four states: Arizona, Colorado, New Mexico, and Utah. Similar to the 1995 outbreak, clusters of cases occurred in the Albuquerque, New Mexico, and Grand Junction, Colorado, areas. However, unlike the 1995 outbreak, a large number of cases were identified in the counties east of the Continental Divide in Colorado, extending from Pueblo as far east as Brush, Colorado. Nationwide, horses comprised 704 of 802 (88%) of the examinations conducted for suspect VS, and 362 of 374 (97%) positive premises had horses positive for VS. Cattle comprised 78 of 802 (10%) of the examinations conducted, and 12 of 374 (3%) positive premises had positive cattle. No premises had both positive cattle and positive horses. Both VSV-NJ and VSV-IN were isolated from clinical cases.

In 1998 the index case of VS occurred in a horse in Tularosa, New Mexico; virus isolation was confirmed as VSV-IN on May 18. Overall, 130 of 232 (6%) investigations nationwide were positive for VS. A total of four states were affected, including Texas (1 positive premises), Colorado (102 positive premises), New Mexico (12 positive premises), and Arizona (15 positive premises). Premises where an equid was positive for VS represented 99% of all positive premises. Only 1 premises housed cattle positive for VS, and this occurred in only one cow.

In 2004 a total of 294 positive premises were identified in three states (Colorado, New Mexico, and Texas) and in 2005, 445 positive premises were identified in nine states (Arizona, Colorado, Idaho, Montana, Nebraska, New Mexico, Texas, Utah, and Wyoming). In both the 2004 and 2005 outbreaks the virus isolated was VSV-NJ. The outbreak in 2005 occurred over the widest geographic area since the 1980s.

VS is endemic on Ossabaw Island, Georgia, where cattle, raccoons, white-tailed deer, horses, and feral swine are seropositive to VSV-NJ.^9,10 Serologic data indicate that transmission occurs annually, is seasonal, and is associated with maritime forest habitat.¹¹ Clinical disease is rare and is only observed in feral swine.

VS is a disease of the Western Hemisphere. Although only one endemic focus of disease occurs in the United States, areas throughout South America, Central America, and Mexico are considered endemic for VS. There are reports of endemic VSV and other vesiculoviruses in Brazil,^12,13 Argentina,¹⁴ Columbia,¹⁵ and other South American countries.¹² Extensive research in Costa Rica has demonstrated the endemic nature of the virus and the disease.^16–19 Work conducted by the author in El Salvador also indicated that the virus and disease are endemic in that country. A review of VS in Mexico revealed that cases occurred in every year between 1981 and 1995, with both serotypes identified in most years. VS has a national distribution in Mexico, although most cases occur in the southern states of Chiapas, Veracruz, and Tabasco. Disease is considered endemic in the central area of Mexico, but at a lower level than in southern states; viral activity in the northern area of Mexico is sporadic.

Transmission

Vesicular stomatitis viruses are considered arboviruses because they use insect vectors as their primary means of transmission. Although experimental and epidemiologic evidence for the role of arthropods as biologic vectors in the transmission of VSV is convincing, the mechanisms of virus acquisition by arthropod hosts and potential amplification of disease and the identity of reservoir hosts remain an enigma. Propagation of VSV outbreaks may be enhanced by movement of infected horses and spread by direct contact between infected and uninfected mammalian hosts.

Risk Factors

A case-control study of horses, cattle, and sheep on 395 premises in Colorado, New Mexico, Utah, and Arizona identified management factors affecting the risk of animals developing VS in the southwestern United States.³⁶

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