Introduction
This topic covers the paramyxoviruses, enveloped negative-sense RNA viruses that spread by the respiratory route: parainfluenza viruses (HPIV 1-4), mumps virus (epidemic parotitis) and measles virus. It also covers respiratory syncytial virus (RSV), traditionally taught with this family but now classified in the separate family Pneumoviridae. For each virus the note gives the structure, pathogenesis, clinical picture, laboratory diagnosis and prevention.
General properties of the family
- The members are: parainfluenza virus, mumps virus, Morbillivirus (measles virus) and Pneumovirus (respiratory syncytial virus). Transmission is respiratory.
- Genome: a single molecule of linear, non-segmented, negative-sense ssRNA. The virion carries its own RNA-dependent RNA polymerase; replication takes place in the cytoplasm.
- Structure: pleomorphic enveloped virions (about 150-300 nm or more) with a helical nucleocapsid and a matrix (M) protein lining the envelope.
- Two envelope glycoproteins:
- An attachment protein: HN (haemagglutinin-neuraminidase) in mumps and parainfluenza viruses, H (haemagglutinin without neuraminidase) in measles virus, G in RSV (no haemagglutinin or neuraminidase).
- The fusion protein F (specialized spikes that initiate cell-to-cell fusion): activated by host-protease cleavage of F0 into F1 and F2, it fuses the envelope with the plasma membrane at neutral pH. It also fuses infected cells with neighbouring cells, producing syncytia (multinucleated giant cells), the typical cytopathic effect.
- Antigenic stability: no segmented genome, so no reassortment and no antigenic shift, unlike influenza. Measles and mumps each have one serotype; infection or vaccination gives lifelong immunity, and live vaccines are highly effective.