Clinical Significance

Measles is an acute viral illness caused by a morbillivirus of the paramyxovirus family and is one of the most easily transmitted diseases. Transmission is primarily by large droplet spread or direct contact with nasal or throat secretions from an infected person.
After infection, measles virus invades the respiratory epithelium of the nasopharynx and spreads to the regional lymph nodes. After two to three days of replication in these sites, primary viremia widens the infection to the reticu-loendothelial system. Following further replication, secondary viremia occurs five to seven days after infection and lasts four to seven days. During this viremia, infection and further virus replication may occur in skin, conjunctivae, respiratory tract and other organs, including spleen, thymus, lung, liver, and kidney. Viremia peaks 11-14 days after infection, and then declines rapidly over a few days .
Prior to vaccine availability, measles was mostly a disease of childhood, but measles vaccination programs (part of measles, mumps, rubella, varicella [MMRV] vaccination) have had a marked effect on the incidence of the disease and the complications associated with it. After prolonged periods of high vaccine coverage in developed countries, measles transmission now occurs mainly in people that have never been vaccinated and in older children who did not seroconvert following vaccination. Measles outbreaks can still occur in countries with high immunization coverage. Such outbreaks demonstrate an immunity gap in the population involved.
Clinically, the diagnosis of measles is supported if Koplik's spots are detected and if the rash progresses from the head to the trunk and out to the extremities. The non-specific nature of the prodromal signs and the existence of mild cases, however, make clinical signs unreliable as the sole diagnostic criteria of measles disease. As disease prevalence falls, many medical practitioners are inexperienced in recognizing measles, increasing the need for laboratory serological method of distinguishing measles from other clinically similar diseases .
Both IgM and IgG antibodies are synthesized during the primary immune response and can be detected in the serum within a few days of rash onset. IgM antibody levels peak after about seven to ten days and then decline rapidly, being rarely detectable after six to eight weeks. IgM is generally not detected in an immune individual following re-exposure to measles virus . Re-exposure to the measles virus induces a strong anamnestic immune response with a rapid boosting of IgG antibodies, which prevents clinical disease.