Experimental Transmission of Rocio Encephalitis Virus by Aedes Scapularis (Diptera: Culicidae) from the Epidemic Zone in Brazil1

Rocio encephalitis is an epidemic flaviviral infection of man first described in São Paulo, Brazil, in 1975. Rocio virus has been isolated from arthropods collected in nature only once, and the vector relationships of the virus in Brazil are essentially unknown. We report here on the first vector competence studies undertaken with mosquito strains from the epidemic zone. Aedes scapularis F1 progeny from females collected during May, November, December, January, and February at Pariquera-Açu, Brazil, were susceptible to per os infection with Rocio virus. The ID50 [ca. 103.5 Vero cell plaque-forming units (PFU)] is well within the range of a competent vector. Infection and transmission rates of 100% were achieved when Ae. scapularis fed on a chick circulating 107.5 PFU/ml of virus; however, infection rates generally were lower. The susceptibility of Ae. scapularis to oral infection with Rocio virus, its ability to transmit virus experimentally, and its abundance and close association with man in the epidemic zone make this species a prime suspect as a vector during the 1975 and 1976 epidemics in São Paulo State.