Measles virus

 Structure of Measles Virus

•  Measles virus is a typical paramyxovirus (spherical enveloped particles that contain a non segmented negative strand RNA genome with a linear arrangement of genes)
•  Measles virus have two glycoproteins  spikes that are important in pathogenesis:
•  F (fusion) protein, which is responsible for fusion of virus and host cell membranes, viral penetration, and haemolysis, and the
•  H (haemagglutinin) protein, which is responsible for binding of virus to cells
•  Measles virus has only one serotype i.e. Life long immunity occurs in individuals who have had the disease.
•  Hemagglutinin is the antigen against which neutralizing antibody is formed.
•  Infants are protected during the first six months of life ( they get maternal antibody as it passes the placenta)

Replication cycle of Measles Virus

•     Adsoprtion to the cell surface: via Hemagglutinin. Cellular receptor of measles virus is CD46 molecule.
•     Penetrates the cell surface and uncoats
•     Virion RNA polymerase transcribes the negative-strand genome to mRNA
•     Specific viral proteins are formed
•     Assembly to helical nucleocapsid
•     Release of virus by budding

Transmission and Epidemiology of Measles

•    Worldwide distribution, outbreaks in 2-3 years
•     Measles virus is extremely infectious, most children contract the clinical disease on exposure
•     Transmitted via respiratory droplets produced by sneeze or cough during prodromal period or direct contact with nasal or throat secretions from an infected person,  which continues up to few days after the rash appears.
•     Less commonly, it is spread by airborne aerosolised droplet nuclei or by indirect contact with freshly contaminated articles.
•     More serious outcomes in malnurished children, people with deficient cell-mediated immunity.

Pathogenesis

•     Measles virus invades the cells lining the upper respiratory tracts i.e respiratory epithelium of the nasopharynx and spreads to the regional lymph nodes
•     After 2-3 days of replication in these sites, a primary viraemia widens the infection to the reticuloendothelial system where further replication takes place.
•     Secondary viraemia occurs and the virus enters skin, conjunctivae, respiratory tract and other organs, including the spleen, thymus, lung, liver, and kidney and further replication occurs.
•     Appearances of rash (cytotoxic T cells attacks measles virus-infected vascular endothelial cells in the skin).
•     Formation of Multinucleated giant cells.

Laboratory diagnosis 

•    Demonstration of clinical form i.e koplik’s spots.
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• Microscopy – demonstration of multinucleated giant cells measuring 100nmin diameter obtained from nasopharyngeal secretion and stained with Giemsa stain

Virus isolation

•     The measles virus can be isolated from a variety of sources, e.g. throat or conjunctival washings, sputum, urinary sediment cells and lymphocytes.
•     The primary human kidney cell line can be used for the isolation of the virus.
•     A continuous cell line like a Vero cell line can be used.
•     The cytopathic effect can be seen in between 2 -15 days and consists of either a broad syncytium or a stellate form with inclusion bodies.
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• Antigen detection– Measles virus antigen detection is done by direct and indirect immunofluorescence from NPS specimens.

Antibody detection

•     Detection of antibody titers which rises by 4 fold between the acute and the convalescent phase or detection of measles-specific IgM.
•     Antibody detection is done by HAI, CF, neutralization and ELISA tests.
•     In the case of SSPE, the presence of measles specific antibodies in the CSF is the most reliable means of laboratory diagnosis.

Molecular diagnosis – It accounts for the identification of measles virus RNA from a clinical specimen by PCR.  

Treatment

•     There is no prescribed medication for measles, however acetaminophen to relieve fever and muscle aches, vitamin A supplements are given to patients and are suggested to drink plenty of water.
•     Administration of human anti-measles gammaglobulin is recommended.

Prevention and control 

•     The measles vaccine is most commonly administered as part of a combination of live attenuated vaccines that include measles, mumps, rubella or measles, mumps, rubella and varicella (MMR or MMRV).
•     The MMR vaccine is a three-in-one vaccination that protects individuals from measles, mumps, and rubella.
•     Children 12 months of age or older should have 2 doses, separated by at least 28 days.
•     Two doses of MMR (measles, mumps & rubella) vaccine is nearly 100% effective at preventing measles.
•     Practicing hygiene and cleanliness such as washing hands with soap and water or sanitizer, covering mouth and nose with a tissue when coughing or sneezing and avoiding close contact, such as kissing, hugging, or sharing eating utensils or cups, with people who are sick.