Measles Virus: Properties, Pathogenesis, and Immunology

• Measles is caused by a helical, enveloped, single-stranded RNA virus.
• The Measles virus belongs to the Paramyxoviridae family and Morbillivirus genus.
• Measles is also known as Rubeola.
• Rubeola is one of the five classic childhood exanthems, along with Rubella, Roseola, Fifth disease, and Chickenpox.

Measles Keywords

• Cough
• Conjunctivitis
• Coryza
• Photophobia
• Koplik spots
• Rash
• Fever
• Subacute sclerosing encephalitis
• Post-measles encephalitis

Measles Virus Properties

• It has a negative-sense, single-stranded RNA genome.
• Nucleocapsid is helical.
• The virus has a pleomorphic envelope.
• It contains RNA-dependent RNA polymerase.
• Envelope spikes include hemagglutinin (HA) activity and fusion (F) activity (self-using and hemolytic activity).
• Measles has a single serotype.

Immunology and Pathogenesis

• Transmitted through respiratory droplets (coughing, sneezing) during the prodromal phase and a few days after the rash appears.
• Measles occurs worldwide, with outbreaks every 2-3 years.
• Outbreaks occur when the number of susceptible children reaches a high level.
• Many cases are in unvaccinated preschool-aged children in urban areas.
• The WHO estimates 30 million cases of measles each year globally.
• In 2000, the CDC declared measles eliminated from the U.S., but cases acquired abroad still occur, especially among immunocompromised, malnourished, and those with vitamin A deficiency.

Pathogenesis Details

• Local replication occurs in the respiratory tract's epithelial cells.
• The Measles virus infects monocytes and lymphocytes.
• The virus spreads through the lymphatic system and cell-associated viremia.
• Dissemination leads to infection of the conjunctiva, respiratory tract, urinary tract, small blood vessels, lymphatic system, and central nervous system.
• The maculopapular rash is caused by immune cells targeting infected endothelial cells lining small blood vessels.
• Recovery follows the rash, leading to lifelong immunity.
• Death can occur due to pneumonia, diarrhea, or encephalitis.

Encephalitis

• Measles can cause encephalitis in three ways: direct infection of neurons, post-infectious encephalitis (immune-mediated), and subacute sclerosing panencephalitis (SSPE).
• SSPE is caused by a defective measles virus variant that replicates poorly and causes neurological symptoms years after the acute disease.
• T-cell deficient children may have atypical presentations like GM cell pneumonia with opportunistic infections.

Clinical Symptoms

• The incubation period is 10 to 40 days.
• Prodromal phase symptoms include high fever, coryza, conjunctivitis, cough, and photophobia.
• Koplik spots (bright red lesions with a white central dot on the buccal mucosa) are virtually diagnostic.
• A maculopapular rash appears on the face and spreads down the body, including palms and soles.
• The rash turns brownish several days later.
• Measles can transiently suppress cell-mediated immunity against pathogens like Mycobacterium tuberculosis.

Measles Complications

• Encephalitis occurs in 1 per 1,000 cases with a 10% mortality rate.
• Permanent sequelae (deafness, mental retardation) occur in 40% of encephalitis cases.
• Primary measles pneumonia and secondary bacterial pneumonia are possible.
• Otitis media is common.
• Subacute sclerosing panencephalitis (SSPE) is a rare, fatal disease occurring years after the initial infection.
• Measles during pregnancy increases the risk of stillbirth, but not congenital abnormalities.
• Atypical measles occurs in those who received the killed vaccine and were later infected, characterized by an atypical rash and no Koplik spots.

Diagnosis and Treatment

• Clinical manifestations are usually characteristic enough for diagnosis.
• Confirmation can be made by RT-PCR on respiratory samples, urine, blood, or brain tissue.
• A fourfold rise in antibody titer can confirm diagnosis in difficult cases.
• Multinucleated giant cells with cytoplasmic inclusion bodies are characteristic pathological effects in respiratory samples.
• No specific antiviral treatment exists for measles.

Prevention

• Live attenuated measles vaccine is given to children after 12 months of age.
• It is administered as part of the MMR vaccine (measles, mumps, rubella) and with the varicella vaccine.
• A booster dose is recommended.
• The vaccine contains live virus and should not be given to immunocompromised persons or pregnant women.