Two viruses from Trinidad, Tr 4675 and Tr 5264, have been compared by means of hemagglutination inhibition tests, complement-fixation tests and neutralization tests with other previously described arthropod-borne (arbor) animal viruses. It has been found that the Tr 4675 and Tr 5264 are antigenically indistinguishable by HI and CF tests.
Tr 4675 has been found to belong to Casals' Group A of arbor viruses and to be most closely related to Semliki Forest virus. However, both by HI and CF tests, Tr 4675 is sufficiently distinguishable from Semliki Forest virus to justify its being considered as a new agent. It has been named Mayaro virus. Despite the fact that both animal and human immune sera react with the homologous (Mayaro) HA and CF antigens in significantly higher titer than with Semliki antigens, the degree of overlap as shown by intracerebral neutralization tests is none the less considerable. Convalescent sera from recently infected human beings have sufficient antibodies to be readily demonstrable in standard Semliki neutralization tests.