Photo taken from The Big Book of Viruses(27)

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VIROLOGY

The measles virus is a spherical, nonsegmented, single-stranded RNA virus in the Morbillivirus family, closely related to the rinderpest and canine distemper viruses.  It contains six structural proteins, three that are complexed to the RNA and three that are associated with the viral membrane envelope.  Two of the membrane envelope proteins are important in its pathogenesis.  The F (fusion) protein is responsible for fusion of virus and host cell membranes, viral penetration and hemolysis.  The H (hemagglutinin) protein is responsible for adsorption of the virus to cells.  The virions enter the cell by binding to the human cellular receptor CD46, fusing with the membrane, and releasing the nucleocapsid into the cytoplasm.  The viral replication takes approximately 24 hours, then the infectious viral particles bud off from the cell membrane.(3)   Back to Top The entire genome of the strain of measles virus has been sequenced and is approximately 16,000 nucleotides long.  The measles virus is rapidly inactivated by heat, light, an acidic pH, ether and trypsin.  Its survival time in the air, on objects or surfaces is less than two hours.  Pneumonia occurs in up to 6 percent of reported cases and accounts for 60 percent of deaths attributed to measles. Encephalitis (inflammation of the brain) may also occur. Other complications include middle ear infection and convulsions. Measles is more severe in infants and adults.(8) The morbilliviruses are distinct from other paramyxoviruses because they do not have neuraminidase activity, and they cause the formation of intranuclear inclusion bodies.  The envelope of the virus carries surface proteins that are composed of the viral transmembrane hemagglutinin (H) and fusion (F) glycoproteins.  On the intracellular side of the envelope is the matrix or membrane protein (M) which is theorized to interact with H and F and the nucleocapsid to affect virion maturation.(23)

Schematic Diagram of the Measles Virus Photo Taken from Fields Virology(4)

Antigenic Determinants   The Morbillivirus variability has been described via the N, M, H and F genes by the analysis of the nucleotide sequence of complementary DNA.  The N proteins of wild-type viruses exhibit antigenic heterogeneity in the C-terminal region.  The F genes of the wild-type strains are almost invariant, with only three coding changes recognizable, while the H gene nucleotide is very variable, and base changes often cause changes in amino acids.  Specifically, the nucleotide change that predicts a new glycosylation site is present in the H variants.  There are few differences among the current vaccine strains. The susceptibility of the F0 protein to cleavage by a host protease is a significant determinant of virus host range and virulence.  Antibodies to the F protein prevent the spread of the infection, but the HN protein antibodies do not, even though they neutralize the released virus.  (7)