Household contacts of leprosy patients are the group with the highest risk of developing the disease, and although many risk or prevention factors have been identified, they have not been employed in leprosymonitoring programs. This investigation aimed to establish the relative risks or the preventive effects of the presence of BCG vaccination, the Mitsuda test, and the ML-Flow assay. Household contacts (1,396) were monitored for a 5-year period. Twenty-eight contacts (2%) developed leprosy and had their clinical and operational classifications established. All immunological tests were performed, and intradermal BCG vaccination was given after the BCG scar count. Of the affected contacts, 75% developed the disease in the first year, and 71.4% were classified as having paucibacillary forms. Contacts of lepromatous leprosy patients presented a 3.8-fold-higher risk of developing leprosy. BCG vaccination and the Mitsuda test showed a protective effect against leprosy of 0.27 (at least one scar) and 0.16 (>7 mm), respectively, and the positive ML-Flow test indicated a relative risk approximately sixfold higher for occurrence of the disease. All unfavorable combinations of two and three assays generated significant risk values that ranged from 5.76 to 24.47, with the highest risk given by the combination of no BCG scar, negative Mitsuda test, and positive ML-Flow test. We suggest that the BCG vaccination may be given to stimulate Mitsuda test positivity, reducing the patient's risk of developing multibacillary forms. The high significance of these tests may have a great impact on programs to monitor contacts and should be used to improve early detection and treatment.

Leprosy transmission still occurs despite the availability of highly effective treatment. The next step towards successfully eliminating leprosy is interrupting the chain of transmission of the aetiological agent, Mycobacterium leprae. In this investigation, we provide evidence that household contacts (HHCs) of leprosy patients might not only have subclinical infections, but may also be actively involved in bacilli transmission. We studied 444 patients and 1,352 contacts using anti-phenolic glycolipid-I (PGL-I) serology and quantitative polymerase chain reaction (qPCR) to test for M. leprae DNA in nasal swabs. We classified the patients according to the clinical form of their disease and the contacts according to the characteristics of their index case. Overall, 63.3% and 34.2% of patients tested positive by ELISA and PCR, respectively. For HHCs, 13.3% had a positive ELISA test result and 4.7% had a positive PCR test result. The presence of circulating anti-PGL-I among healthy contacts (with or without a positive PCR test result from nasal swabs) was considered to indicate a subclinical infection. DNA detected in nasal swabs also indicates the presence of bacilli at the site of transmission and bacterial entrance. We suggest that the concomitant use of both assays may allow us to detect subclinical infection in HHCs and to identify possible bacilli carriers who may transmit and disseminate disease in endemic regions. Chemoprophylaxis of these contacts is suggested