Molecular evidence for the aerial route of infection of Mycobacterium leprae and the role of asymptomatic carriers in the persistence of leprosy.

BACKGROUND:  Leprosy persists as public health problem. The chain of transmission and mechanism of infection are not completely understood. Here we investigated the route of infection and of disease onset, from airways exposure, colonization, and bloodstream dissemination.

METHODS:  M. leprae DNA was detected through qPCR in nasal vestibule, nasal turbinate mucosa, and peripheral blood samples, plus anti-PGL-I serology and skin tests, from the same individual, of 113 leprosy patients and 104 household contacts of patients (HHCs). Bivariate statistics and multiple correspondence analysis were employed.

RESULTS:  DNA positivity among patients: 66.4% (75/113) in nasal swabs, 71.7% (81/113) in nasal turbinate biopsies, 19.5% (22/113) in blood samples, and seropositivity of 62.8% (71/113); with increasing incidences towards the multibacillary (MB) pole of the clinical spectrum. Positivity among HHCs: 49% (51/104) for nasal swabs, 53.8% (56/104) for nasal biopsies, 6.7% (7/104) for blood, and 18.3% (19/104) for anti-PGL-I. During the follow-up of 5-7 years, out of 104 HHCs, 7 developed leprosy (6.7%). Risk for the disease outcome was estimated comparing results of HHCs who develop leprosy with those not affected. Neither nasal passage nor mucosa positivity was determinant of later disease onset; howsoever, blood presence increased the risk for disease development [RR/LR+ 5.54 (95% CI, 1.30 - 23.62)], as well did the seropositivity [LR+ 3.69 (95%CI, 1.67 - 8.16); RR 5.97 (95%CI, 1.45 - 24.5)].

CONCLUSIONS:  Our findings strongly suggest that the aerosol route of infection and transmission is predominant, and HHCs contribute to the infection risk to themselves and most probably to others.