1. Instituto Evandro Chagas, Fundação Servicos de Saude Publica, Ministry of Health, C.P. 621, Belém, Pará 66,000, Brazil
2. Yale Arbovirus Research Unit, Department of Epidemiology and Public Health, Yale Arbovirus Research Unit, Department of Epidemiology and Public Health, Yale University School of Medicine, P.O. Box 3333, New Haven, Connecticut 06510