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Respiratory Syncytial Virus

ETIOLOGY RSV is an enveloped RNA virus with a single-stranded negative-sense genome that replicates entirely in the cytoplasm of infected cells and matures by budding from the apical surface of the cell membrane. Because this virus has a nonsegmented genome, it cannot undergo antigenic shift by reassortment like the influenza viruses do. The virus belongs to the family Paramyxoviridae, along with parainfluenza and measles viruses, and is in the subfamily Pneumovirinae, which also contains the human metapneumovirus (see Chapter 261). It is the only member of the genus Pneumovirus that infects humans. There are 2 antigenic subgroups of RSV, distinguished based primarily on variation in 1 of the 2 surface proteins, the G glycoprotein that is responsible for attachment. This antigenic variation caused by point mutations from infidelity of the virus RNA polymerase may to some degree contribute to the frequency with which RSV reinfects children and adults. RSV replicates in a wide variety of cell line monolayer cultures in vitro, and in HeLa or HEp-2 cells produces characteristic syncytial cytopathology, from which the virus derives its name. Interestingly, it is now known that the virus does not cause large syncytia in polarized epithelial cells in vitro, and it is not clear whether syncytium formation occurs to any significant degree in vivo.