Microepidemics of tuberculosis in schools. How should we select contacts

BACKGROUND: Outbreaks of microepidemics of tuberculosis indicate the existence of Mycobacterium tuberculosis transmission. We describe a microepidemic in a school in which an index case with low infective capacity gave rise to a high percentage of tuberculosis infection and disease in the center. METHODS: Contact investigation was performed in 423 pupils of a school after a recently arrived teacher with few symptoms was diagnosed with tuberculosis. A cross-sectional study was carried out to calculate the prevalence of tuberculosis infection and disease in the school center. RESULTS: Statistically significant differences were found in the distribution of positive results in the first tuberculosis skin test for primary and secondary students, with more positive results in secondary school pupils (18.4%) than in primary school pupils (6.1%). The incidence of skin tuberculosis test converters was 5.7%. The prevalence of infection by educational level was 10% in primary education and 23.4% in secondary education. The risk of infection in secondary school pupils was more than twice that in primary school pupils (OR = 2.4; 95% CI, 1.5-3.5). There were 6 new cases of tuberculosis in pupils. CONCLUSION: The high percentage of conversions in the second tuberculosis skin test indicated the existence of transmission in the school. The results of this study support contact investigation in schools where there is exposure to a case of tuberculosis, whether highly infective or nort.