Agent Factors of Rubella
Rubella is caused by an RNA virus of the Togavirus family. Only one antigenic type of the virus seems to exist. The virus has been recovered from the naso–pharynx, throat, blood, CSF and urine. It can be propagated in cell culture.
Source of infection
Clinical or sub–clinical cases of rubella have been shown to be sources. A large number of rubella infections are, in fact, sub–clinical. This represents one of the major differences between measles and rubella. There is no known carrier state for post–natally acquired rubella. Infants born with congenital rubella may shed the virus for many months.
Period of communicability
Rubella is much less communicable than measles, probably because of the absence of coughing in rubella. It is difficult to state the exact period of infectivity. It probably extends from a week before the symptoms begin to appear, to about a week after the rash appears. Infectivity is greatest when the rash begins to erupt.
Host Factors of Rubella
It is mainly a disease of childhood, occurring particularly in the age group three to 10 years. Persons older than 15 years now account for over 70% of the cases in developed countries – this is similar to the changing epidemiological pattern with measles, following widespread immunization campaigns against the disease.
One attack results in life–long immunity, second attacks are rare. Infants of immune mothers are protected for four to six months. It is estimated that about 10 to 40% of the population could reach adulthood, without experiencing rubella infection in the absence of immunization. Thus, many women of childbearing age may remain rubella–susceptible. Studies in India indicate that approximately 40% of women of child–bearing age may remain rubella susceptible.
Transmission of Rubella
The virus is transmitted directly from person to person from one week before the onset of rash to one week after it has faded. The transmission occurs by droplets from the nose and throat, and droplet nuclei (aerosols). The portal of entry is through the respiratory tract. The virus is maintained in human populations by chain transmission. It can cross the placenta (verticle transmission) and infect the fetus in utero, leading to congenital rubella in the newborn.