The paramyxoviruses include the most important agents of respiratory infections of infants and young children (respiratory syncytial virus [RSV] and the parainfluenza viruses) as well as the causative agents of two of the most common contagious diseases of childhood (mumps and measles). The World Health Organization estimates that acute respiratory infections and pneumonia are responsible every year worldwide for the deaths of 4 million children younger than 5 years. Paramyxoviruses are the major respiratory pathogens in this age group.
All members of the Paramyxoviridae family initiate infection via the respiratory tract. Whereas replication of the respiratory pathogens is limited to the respiratory epithelia, measles and mumps become disseminated throughout the body and produce generalized disease.
Rubella virus, although classified as a togavirus because of its chemical and physical properties (see Chapter 29), can be considered with the paramyxoviruses on an epidemiologic basis.
PROPERTIES OF PARAMYXOVIRUSES
Major properties of paramyxoviruses are listed in Table 40-1.
TABLE 40-1Important Properties of Paramyxoviruses ||Download (.pdf) TABLE 40-1 Important Properties of Paramyxoviruses
|Virion: Spherical, pleomorphic, 150 nm or more in diameter (helical nucleocapsid, 13 or 18 nm) |
|Composition: RNA (1%), protein (73%), lipid (20%), carbohydrate (6%) |
|Genome: Single-stranded RNA, linear, nonsegmented, negative sense, ~15 kb |
|Proteins: 6–8 structural proteins |
|Envelope: Contains viral glycoprotein (G, H, or HN) (which sometimes carries hemagglutinin or neuraminidase activity) and fusion (F) glycoprotein; very fragile |
|Replication: Cytoplasm; particles bud from plasma membrane |
Particles are labile yet highly infectious
Structure and Composition
The morphology of Paramyxoviridae is pleomorphic, with particles 150 nm or more in diameter, occasionally ranging up to 700 nm. A typical particle is shown in Figure 40-1. The envelope of paramyxoviruses seems to be fragile, making virus particles labile to storage conditions and prone to distortion in electron micrographs.
Ultrastructure of parainfluenza virus type 1. The virion is partially disrupted, showing the nucleocapsid. Surface projections are visible along the edge of the particle. (Courtesy of FA Murphy and EL Palmer.)
The viral genome is linear, negative-sense, single-stranded, nonsegmented RNA, about 15 kb in size (Figure 40-2). Because the genome is not segmented, this negates any opportunity for frequent genetic reassortment, resulting in antigenic stability.
Genetic maps of representative members of the genera of the family Paramyxoviridae. Gene sizes (boxes) are drawn approximately to scale. (Copyright GD Parks and RA Lamb, 2006.)
Most paramyxoviruses contain six structural proteins. Three proteins are complexed with the viral RNA—the nucleocapsid (N) protein that forms the helical nucleocapsid (13 or 18 nm in diameter) ...