Immunolocalization of vesicular stomatitis virus in black flies (Simulium vittatum).

Vesicular stomatitis, a disease of cattle, horses, and swine, is caused by either vesicular stomatitis virus, New Jersey serotype (VSV-NJ), or vesicular stomatitis virus, Indiana serotype, which are related viruses in the genus Vesiculovirus, family Rhabdoviridae. Although recognized for at least 160 years, the epidemiology and pathogenesis of this disease remains undefined. Black flies have been suggested as a vector for VSV-NJ. In this study we infected three- to four-week-old female black flies with VSV-NJ via feeding of virus-spiked ox blood or intrathoracic inoculation, and demonstrated the location of virus by immunohistochemistry. These preliminary findings suggest that VSV-NJ initially infects the gut in the natural situation but that subsequent spread to the salivary gland may be blocked in older flies, decreasing their ability to transmit the virus. The pattern of staining was different in intrathoracic inoculated flies. In these flies, salivary gland involvement was more likely, and extensive staining of eye, brain, and hemolymph suggested a more generalized infection that apparently circumvented the gut. We conclude that intrathoracic inoculation may be an inappropriate method of infection for determining vector competence and that the age of the vector should be considered when conducting competency studies.