Paramyxo Viruses: Properties, Classification and Immunity

In this article we will discuss about Paramyxo Viruses, they belong to the family paramyxo-viridae and resemble Orthomyxo viruses in morphology and haem-agglutinate red blood cells but differ in structure, replication and other properties.

Contents:

1. Subject Matter of Paromyxo Viruses
2. Properties of Paramyxo Viruses
3. Classification of Paramyxo Viruses
4. Cultivation of Paramyxo Viruses
5. Replication of Paramyxo Viruses
6. Pathogenesis and Clinical Features of Paramyxo Viruses
7. Immunity
8. Laboratory Diagnosis of Paramyxo Viruses

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1.  Subject Matter of Paromyxo Viruses:

The envelope surface of paramyxo viruses is of lipoprotein membrane and is covered by projections of 12-14 nm x 2-4 nm. These projections are of two types: HN (haemagglutinin and neuraminidase) and F (fusion of protein) and glycoproteins. Here HN projection of paramyx­oviruses have both haemagglutinin and neuraminidase functions, but in myxo viruses separate projections carry each function.

2. Properties of Paramyxo Viruses:

They are spherical enveloped particles (100-300 nm diameter, giant form 800 nm diameter). They contain single stranded RNA genome and haemagglutinin in envelope and are sensitive to lipid solvent.

3. Classification of Paramyxo Viruses:

The family Paramyxo-viridae consists of three genera and infects man and animals:

i. Paramyxo Virus:

Para influenza, Mumps, Newcastle disease (NCD) virus, Simian virus 5 (SV5) contain both haemagglutinin and neuraminadase.

ii. Morbilli Virus:

Measles virus, Canine distemper virus, Rinderpest virus, possess haemagglutinin and no neuraminadase.

iii. Pneumovirus:

Respiratory syncytial virus (RSV), Pneumonia virus of mice (PVM) and Turkey rhino-tracheitis virus have neither haemagglutinin nor neuraminadase.

Differentiation of the Genera:

The nucleocapsid of all paramyxo viruses is 18 nm in diameter except pneumo virus (14_;nm in diam.). The spikes (peplomers) of Pneumo virus are longer than those of other paramyxo viruses.

4. Cultivation of Paramyxo Viruses:

Primary isolation is done in human and monkey kidney cell and in amniotic cavity of chick embryo. Cytopathic effects (CPE) are formation of large syncytia containing eosinophilic cytoplasmic inclusion bodies; viruses are identified by haemadsorption of fowl or guinea pig red cells.

5. Replication of Paramyxo Viruses:

Paramyxo viruses attach via the haemagglutinin glycoprotein (HN or H protein) to the sialic acid containing receptors on the host cells. The virus penetrates the cell by means of fusion of viral and cell membrane which is mediated by Fo glycoprotein.

Fo glycoprotein becomes activated when it is cleaved into two subunits F₁ and F₂. If Fo protein fails to cleave, the virus will attach but will not fuse with the host cell membrane and the viral genome cannot penetrate the cell.

The viral RNA polymerase transcribes m RNA in the cell cytoplasm; then viral components are synthesised in the cytoplasm. The surface HN and F proteins are incorporated in the membrane forming envelope. The enveloped virus buds out of the cell into the exterior. Activated fusion protein (Fo) causes fusion of adjacent cell membrane of host cells produc­ing syncytium (multinucleated giant cells).

6. Pathogenesis and Clinical Features of Paramyxo Viruses:

Incubation period of is 2-6 days. These viruses are widespread, cause common cold with sore throat in all ages, severe respiratory tract disease in infant and children (laryngo tracheo bronchitis—especially by types 1 and 2) and group (6-9%).

The group is characterised by a harsh brassy cough in children due to respiratory obstruction as a result of swelling of larynx and related structures. The infection may further spread down causing pneumonia and bron­chiolitis if it is due to type 3.

The transmission is by droplet aerosols or by direct person-to-person contact. Infection may produce both antibody and cell mediated immunity which may cause termination of illness with complete recovery in 10-14 days.

7. Immunity from Paramyxo Viruses:

Reinfection may occur in children; clinical picture of common cold may be modified by preexisting antibodies.

8. Laboratory Diagnosis of Paramyxo Viruses:

1. Isolation:

Throat swabs and mouth washings can be inoculated in monkey kidney tissue culture, CPF is minimal in tissue culture. Virus in infected tissue culture cells is identified by haemadsorption with guinea pigs red cells. Typing is done by inhibition of haemadsorption by type specific antisera before adding the guinea pig red cells.

2. Direct demonstration:

Viral antigen can be demonstrated in exfoliated respiratory cells with mixture of monoclonal antiviral antibodies of 4 types of virus by immuno­fluorescence or ELISA.

3. Serology:

Rising antibody titre can be detected by ELISA or CFT.