Seroepidemiologic studies in Oaxaca, Mexico. II. Survey for arbovirus antibody.

A serologic survey was conducted in south-western Mexico to obtain information on human experience with arbovirus infections. Sera were collected from two semitropical areas along the Pacific coast of Oaxaca State, two mountain areas above 1,700 meters and the interior valley at 1,500 meters. Of the 610 sera tested for group A antibody, 4.9 per cent were positive in the hemagglutination-inhibition (HI) test to Venezuelan (VE), 11 per cent to Eastern, and none to Western encephalitis viruses. In neutralization tests the antibody was shown to be probably due to VE virus infections. When sera were screened for group B antibodies in the HI test, 32 per cent were positive with St. Louis encephalitis (SLE), 19 per cent with Ilhéus, and 4 per cent with yellow fever viruses. The pattern of reactions suggested that SLE or an antigenically related virus was responsible for the antibody detected. An unusually high rate was found in a mountain area at 2,000 meters: 41 per cent of 113 persons tested were seropositive to SLE. Of 493 sera screened by complement-fixation test, 6 per cent were positive to Nepuyo, 4 per cent to Patois, and 3 per cent to Tlacotalpan viruses.