[Seroprevalence of infection by St. Louis encephalitis virus in the Province of Formosa].

The aim of this work was to determine the prevalence of antibodies against St. Louis encephalitis virus (SLE) in human sera provided by the Laboratory of Epidemiological Surveillance from Formosa District (Province of Formosa, Argentina) in 1995 and 1997. The tests used for this study were hemagglutination inhibition (HI) and neutralization (NT). The screening performed by NT test showed prevalences of 21% (60/284) and 32% (50/157) of antibodies in samples obtained in 1995 and 1997 respectively. In 1995, 14% of tested sera showed low titer of neutralizing antibodies (NT) (1/20 and 1/40) whereas in 1997, 19% of the sera presented titers of NT antibodies equal or greater than 1/80. It was observed that sera with low titers of NT antibodies (1/20-1/40) resulted negative in HI in the simultaneous titration of antibodies by NT and HI whereas other sera presented high titers for both tests. This relation between the low and the high titers of antibodies indicates the presence of past and recent infections and the continuous circulation of this virus. Moreover, the prevalence of NT antibodies in the surveyed population increased significantly in 2 years (p < 0.0075) confirming the endemicity of this agent in this area and showing the need to perform studies of non-confirmed viral etiology febrile diseases to determine its importance in human pathogenicity.