RESPIRATORY SYNCYTIAL VIRUS
Respiratory syncytial virus (RSV) is classified as a pneumovirus within the paramyxo-virus family. Its name is derived from its ability to produce cell fusion in tissue culture (syncytium formation). Unlike influenza or parainfluenza viruses, it possesses no hemag-glutinin or neuraminidase. The RNA genome is nonsegmented, negative sense, and single stranded and codes for at least 10 different proteins. Among these are two matrix (M) proteins in the viral envelope. One forms the inner lining of the viral envelope; the func-tion of the other is uncertain.
The antigens on the surface spikes of the viral envelope include the G glycoprotein, which mediates virus attachment to host cell receptors, and the fusion (F) glycoprotein, which induces fusion of the viral envelope with the host cell surface to facilitate entry. F glycoprotein is also responsible for fusion of infected cells in cell cultures, leading to the appearance of multinucleated giant cells (syncytium formation). Antibodies directed at the F glycoprotein are more efficient than anti-G glycoprotein antibodies in neutraliz-ing the virus in vitro.
At least two antigenic subgroups (A and B) of RSV are known to exist. This dimor-phism is due primarily to differences in the G glycoprotein. The epidemiologic and bio-logical significance of these variants is not yet certain; however, epidemiologic studies have suggested that group A infections tend to be more severe. RSV is the single most important etiologic agent in respiratory diseases of infancy, and it is the major cause of bronchiolitis and pneumonia among infants under 1 year of age.