1. Instituto Evandro Chagas, Fundaçāo Servicos de Saúde Pública, Ministerio da Saúde, CP 621, 66.000 Belém, Pará, Brazil
2. Yale Arbovirus Research Unit, Department of Epidemiology and Public Health, Yale University School of Medicine, P. O. Box 3333, New Haven, Connecticut 06510
3. U.S. Army Medical Research Unit, Belém, Pará, Brazil