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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.
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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.
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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.
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PROPERTIES OF PARAMYXOVIRUSES
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Major properties of paramyxoviruses are listed in Table 40-1.
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Structure and Composition
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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.
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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.
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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) ...