EPIDEMIOLOGY OF CHILDHOOD TB
The source of transmission of TB to a child is usually an adult (often a family member) with sputum smear-positive PTB. Cases of TB in children usually represent between 10% and 20% of all TB cases. The frequency of childhood TB in a given population depends on the following: the number of infectious cases, the closeness of contact with an infectious case, the age of children when exposed to TB, and the age structure of the population. Children rarely have sputum smear-positive TB and so it is unlikely they are a powerful source of transmission. TB in children is mainly due to failure of TB control in adults. Failure of TB control in adults means failure to cure infectious cases (patients with sputum smear-positive PTB). The highest priority in TB control is to cure the infectious cases. However, it is still important to cure children with TB! Good treatment of TB in childhood will result in the following:
a) improved well-being through decreased morbidity and mortality;
b) improved credibility and reputation of the NTP;and c) less chance for children to have TB reactivation with cavitation in later life.
I PRACTICAL POINT I
A good TB control programme is the best way to prevent TB in children.
In many countries, newborns receive BCG immunization, and yet childhood PTB still occurs.This shows that BCG is not fully effective in protecting against PTB. BCG seems to give better protection against disseminated disease, such as miliary TB or TB meningitis, than it does against PTB. The effectiveness of BCG against PTB is variable between regions, and the reasons for this are not completely understood. One problem is likely to be the timing of the vaccination. In developing countries where TB is common, children will often be exposed to TB early in life and so immunization needs to be given as early as possible, i.e. soon after birth. However, the immune system of a newborn may be too immature to be able to produce an effective immune reaction to the BCG. BCG has been more effective when given to school-aged children. However, in communities where TB is common, this would be too late to protect against most disease. Other factors that reduce the effectiveness of BCG immunization are malnutrition and severe infections such as HIV or measles.
Risk of infection depends on extent of exposure to infectious droplet nuclei. An infant whose mother has sputum smear-positive PTB, for instance, has a high chance of becoming infected. Being in very close contact with the mother, he or she is likely to inhale a larger number of infectious droplets from the air than other household contacts. The greater the exposure to infection, the greater the likelihood of disease.
The chance of developing disease is greatest shortly after infection, and steadily decreases as time goes by. Infants and young children under 5 years of age have less-developed immune systems than school-aged children. They are therefore at particular risk (up to 20%) of developing disease following infection. Many will present with disease within one year following infection, most within 2 years. For infants particularly, the time-span between infection and disease may be quite short and the presentation of PTB is as an acute rather than chronic pneumonia.Almost always in those cases, the contact is the mother. The majority of HIVnegative children infected with M. tuberculosis do not develop TB disease in childhood. In these healthy, asymptomatic, but TB-infected children, the only evidence of infection may be a positive tuberculin skin test.
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