The photosensitivity of Semliki Forest and other viruses.

Summary Semliki Forest virus was inactivated by exposure to daylight or artificial light. The active wavelengths were in the region 3300 to 4700 A. Inactivation was reduced by calf serum and completely prevented by cysteine or anaerobic conditions. Light destroyed the infectivity of the virus-associated RNA at the same rate as that of whole virus; it did not affect the reaction of the virus in immunodiffusion tests nor the infectivity of previously extracted RNA. Inactivation was probably the result of photo-oxidation of the viral nucleic acid which was sensitized by some naturally occurring pigment. Although riboflavin and vitamin A were able to increase the photosensitivity of Semliki Forest virus, the evidence presented indicates that neither of these substances was the natural photosensitizer. Sindbis, Murray Valley encephalitis, influenza and rabbitpox viruses also lost infectivity when exposed to light, but poliovirus was photoresistant.