Etiology and Epidemiology

Rubella is caused by the rubella virus, which is in the genus Rubivirus of the family Togaviridae. Rubella virus is 50 to 60 nanometers in diameter and contains a single-stranded RNA genome.

Rubella is a highly contagious disease transmitted by contact of susceptible individuals with the nose and throat secretions of infected persons, primarily by droplet spread. Infection also occurs by direct contact, by indirect contact through freshly soiled articles, and by airborne transmission. There is no reservoir for rubella other than human beings, which means that a continuous chain of susceptible contacts is necessary to sustain transmission. The period of communicability is from about 1 week before rash onset to at least 4 days after. There is no carrier state except for infants with congenital rubella, who may shed virus for many months after birth. Rubella's incubation period from time of exposure to onset of rash is 16 to 18 days, with a range of 14 to 23 days.

In populated areas with no or low vaccination coverage, rubella is primarily an endemic disease of children with periodic epidemics. However, a significant proportion of adults remain susceptible, and thus congenital rubella may result if a pregnant woman contracts rubella. In more remote isolated populations, rubella is not endemic and disease is dependent upon introduction of the virus from the outside, at which time an epidemic may occur, affecting all age groups born since the last epidemic. There is no evidence for a sex difference in incidence or severity of rubella, although more cases may be reported in women because of concern for congenital rubella. There is no evidence for a racial difference in incidence or severity.

The risk of congenital rubella is related to gestational age at the time of maternal infection. Fetal or placental infection has been shown to accompany 85 percent of maternal infections that occur during the first 8 weeks of pregnancy. Data from the last major epidemic in the United States, which occurred from 1964 to 1965 and resulted in 20,000 infants born with congenital rubella syndrome, showed that the risk of developmental defects was about 50 percent in infants whose mothers were infected during the first month of pregnancy, 22 percent during the second, 6 percent during the third, and about 1 percent during the fourth month of pregnancy. Other investigators have shown that infection in the first 8 weeks of pregnancy also leads to high rates of abortion or stillbirth.

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