Paramyxoviruses are roughly spherical-shaped viruses and usually vary in size from 100 to 300 nm. Sometimes, long filaments and giant forms of the virus measuring up to 800 nm are also found. These viruses consist of a negative-sense single-stranded RNA genome enclosed in a helical nucleocap-sid surrounded by a pleomorphic envelope. Paramyxoviruses resemble orthomyxoviruses in morphology but are larger, sur-face spikes are different, and their genomes are not segmented (see Table 62-1).
The family Paramyxoviridae consists of three important genera (Morbillivirus, Paramyxovirus, and Pneumovirus), which contain important human pathogens responsible for causing most of acute respiratory infections and contagious diseases of children and infants.
· The genus Morbillivirus includes the measles virus.
· The genus Paramyxovirus includes parainfluenza and mumps virus.
· The genus Pneumovirus includes respiratory syncytial virus (RSV), which is responsible for majority of acute respiratory infections in infants and children.
The nucleocapsid is associated with the nucleoprotein (NP), polymerase phosphoprotein (P), and larger (L) protein. The NP maintains genomic structure, the P facilitates RNA synthesis, and the L protein is an RNA polymerase. The nucleocapsid surrounded by envelope has the matrix (M) protein at its base. The virion envelope contains two glycoproteins, a fusion (F) protein and a viral attachment protein called hemagglutinin neuraminidase (HN), hemagglutinin (H), or G protein. The F protein facilitates fusion of the viral and host cell mem-branes, and HN promotes adsorption of the virus to the host cell surface. The F protein is activated by proteolytic cleav-age, resulting in the production of F1 and F2 glycopeptides held together by disulfide bond to express membrane-fusing activities.
The binding of the HN, H, or G protein on the virion enve-lope to these cell surface glycoproteins containing sialic acid is the first step in the replication of paramyxoviruses. This is followed by the fusion of the viral envelope with the plasma membrane of the cell, promoted by the F protein of the virion envelope. In addition, paramyxovirus also induces cell-to-cell fusion, resulting in production of multinucleated giant cells. Paramyxoviruses associated with human diseases are summa-rized in Table 62-1.