Paramyxoviridae - Overview
The Paramyxoviridae have a helical nucleocapsid surrounded by an envelope comprised of host cell membrane spiked with viral glycoproteins responsible for haemagglutinin, neuraminidase and haemolytic activities. The genome of the Paramyxoviridae is single-stranded, negative-sense RNA which is used as a template for the production of messenger (positive-sense) RNA and further genomic material. Paramyxoviridae are sensitive to heat, dessication and most disinfectants, and so are not resistant in the environment. The Paramyxovididae family is divided to two sub-families, the Paramyxovirinae and the Pneumovirinae. It is within the Paramyxovirinae sub-family that morbilliviruses fall, along with respiroviruses, henipaviruses, rubulaviruses and avulaviruses. As well as canine distemper virus (CDV), the morbilliviruses include rinderpest, peste des petits ruminants, measles, phocine (seal) distemper and dolphin distemper.
The virus is constructed of single-stranded negative-sense unsegmented RNA. Reassortment and antigenic shift cannot occur with this virus.
Spike proteins include HN (Haemagglutinin and Neuraminidase) and F (Fusion glycoprotein), which allows the virus to fuse directly to the plasma membrane and release its RNA. F also causes syncitium to form, which aids diagnosis. The host antibody response to the F protein is the basis for vaccination.
Paramyxoviruses replicate in the epithelium of the upper respiratory tract as well as occasionally in the gut as these are sites of spike protein cleavage. Virulence varies by virus, see below.
Types and Subtypes
Antigenic conservation allows some cross protection by vaccination as conservation of major virus-specific F/HN antigens means vaccines protect against all isolates of the same virus. Minor morbillivirus-specific epitopes on F allows some cross protection between canine distemper, measles, and rinderpest.
Antigenic "fingerprinting" is possible for some viruses based on minor variable epitopes of HN, F and NP on specific isolates as detected by monoclonal antibodies. These are detected by immunostaining infected cells.
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