ABSTRACT
In 1975, in the south coastal region (Ribeira Valley) of the state of Sao Paulo in Brazil, a sizable outbreak of encephalitis occurred [1,2], and a avivirus isolated from a fatal case was named Rocio virus (ROCV) after the name of the town where the victim lived. This episode marked the rst encephalitis outbreak by a avivirus in South America. Historically, in South America, St. Louis encephalitis virus (SLEV), another avivirus, had existed for years; and yet for a reason still unclear and unlike the strains in North America that caused frequent outbreaks of encephalitis, SLEV strains in South America did not cause such an outbreak until 2005. Thus, the outbreaks of encephalitis by ROCV in Brazil beginning in 1975 were not only the source of serious medical and public health concern but a source of scientic interest among virologists with respect to the mechanisms of the emergence of a new encephalitogenic avivirus, including the possibility of natural recombination.
