Relative lack of clinical disease among household contacts of tuberculosis patients compared to leprosy households.

The incidence of clinical tuberculosis and clinical leprosy among household members of tuberculosis and leprosy patients in Sri Lanka was studied. The study period was approximately 20 years (January 1981 to December 2001) and the total number of patients and contacts were 325 and 968 for tuberculosis and 726 and 3066 for leprosy, respectively. While none of the tuberculosis patient households had more than 1 patient nor any contacts who developed clinical disease during the observation period, 20% (148/726) of the leprosy patients had more than 1 patient in the family and 0.9% (13/1403) of their contacts who were followed-up developed clinical leprosy during the observation period. Although the tuberculosis patient household contacts did not develop clinical disease, in 79% (88/112) of contacts who were tested by Western blot analysis, there was serologic evidence of Mycobacterium tuberculosis infection. These data show that in populations of comparable socio-economic, environmental and geographic locations, tuberculosis and leprosy show very different transmission patterns. In general, in tuberculosis household contacts, in spite of exposure, infection did not proceed to clinical disease. In contrast, a significant number of leprosy household contacts developed clinical leprosy. These findings have implications in the design and implementation of control programmes for these two diseases.