Arch Virol (1990) 114:229-235 Archives Vi rology © by Springer-Verlag1990 Ultrastructural study of Mayaro virus replication in BHK-21 cells Brief Report J. M. S. Mezeneio 1' *, W. de Souza 1, M. E. F. Fonseca 2, and M. A. Rebeilo 1 1 Instituto de Biofisica Carlos Chagas Filho and 2Instituto de Microbiologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil Accepted July 2, 1990 Summary. The replication of Mayaro virus in BHK-21 cells was studied by electron microscopy. The infected cells show an intense vacuolization and pro- liferation of membranous structures. At 5 h post-infection, precursor virus par- ticles were seen in the cytoplasm of infected cells. Later, mature virus particles were found outside the cells and budding from the plasma membrane. Enveloped virus particles were also observed inside the vesicles and budding across their membrane. The release of virus particles into the extracellular space by exo- cytosis was also observed. In a later stage of the infection, inclusion bodies were sometimes present in the cytoplasm of infected cells. We conclude that in BHK-21 cells, budding from the plasma membrane is the main process of Mayaro virus maturation, and in this kind of cell replication differs significantly from that observed in Aedes albopictus cells. The Mayaro virus is a member of the family Togaviridae, genus Alphavirus, antigenically closely related to Semliki Forest virus [-4]. This virus has been isolated in Brazil, from human and other mammalian species, especially at frontiers of colonization in the Amazon region [5]. Mayaro virus, as other Togavirus, is perpetuated in nature by its ability to infect and replicate in vertebrate and invertebrate cells [1]. The morphogenesis of the alphavirus in these cells has been analysed by electron microscopy [-2, 3, 7, 9]. The first ultrastructural study of the Mayaro virus was reported by Saturno in 1963 [10]. This author described an enveloped virus-like particle with 40 4-5 nm in * Present address: Departamento de Biologia Geral, Universidade Federal de Viqosa, Vigosa, Minas Gerais, Brasil.