Investigation of an epidemic of unknown etiology in a community of quarry and forest workers on the River Guama, 120 miles east of Belém, Brazil, resulted in the isolation of six strains of virus, subsequently shown to be Mayaro, from the blood of six patients. The common complaints were moderate and continuous fever and severe frontal headache of two to five days' duration. There were no deaths.
Sera collected from 50 persons on April 15 were found to have no neutralizing antibodies against Semliki virus, which is antigenically closely related to Mayaro virus. Seven, or 18.9 per cent, of 37 sera collected from these same persons three weeks later showed protective antibodies by the same technique. Samples obtained at this time from 37 persons not bled earlier revealed that 7, or 18.9 per cent, also had demonstrable antibodies. Two later conversions were demonstrated in second samples collected June 26. Six of the serum conversions were in persons from whom the virus strains were isolated by mouse inoculation.
Previous neutralization tests with Semliki virus and the sera from 551 adults in 17 localities throughout the Amazon Valley had shown protective antibodies in 53, or 9.6 per cent, of the persons examined. It now seems probable that these antibodies represent infections with Mayaro virus.