||The Mantoux test using 5 tuberculin units (TU) is the optimal test to identify TB infection. The use of different tuberculin strengths complicates interpretation of TST results and comparison between studies. The degree of induration is usually <10mm in children vaccinated with BCG soon after birth and >10mm in children with natural infection.
BCG-induced hypersensitivity diminishes with time and tuberculin responses frequently become negative after a variable time period depending on the BCG strain used. After natural infection, permanent reversion occurs in less than 0,5% of children. Temporary tuberculin inhibition may occur and is associated with wasting, viral illnesses and MMR vaccination, severe burns, immunosuppression, severe forms of tuberculosis etc.
Following prolonged household contact with a sputum smear-positive source case, 60-80% of children become infected. When the source case is smear-negative and culture-positive, 30-40% of children become infected. Probability of infection in children depends on the infectivity of, the proximity to and duration of contact with the source case. Some local cultural practices may have contributed to different epidemiological patterns of disease spread, e.g extensive socialization, isolation of children and women etc. Also, factors such as family size, population density and the median age of TB patients in a given setting, determines the number of infected persons per infectious source case. Younger TB patients are more likely than older TB patients to have young children in their household environment. Some studies have found that younger TB patients are more contagious than older ones. More than 60% of the children who became infected did so within 3 months of symptom onset in the adult source case.
Ample evidence exists that TB incidence is increasing in many parts of the world. Much of this increase is due to the human immunodeficiency virus (HIV) pandemic, which disproportionately affects adults of reproductive age, many of whom live in close proximity to young children.