Transmission of leprosy

Reservoir of infection

The human being is the only known reservoir of infection in leprosy except for the fact that naturally occurring disease with organisms indistinguishable from M.leprae has also been detected among wild armadillos in parts of the southern United States (Walsh et al, 1981). Up to 5% of armadillos in Louisiana have been found to have clinical disease with about 20% having serological evidence of M.leprae infection (Truman et al, 1986). The epidemiological significance of the armadillo is generlly considered to be negligible in spite of occasional cases reported among individuals giving history of handling armadillos (Lumpkin et al, 1983). Among human beings it is the lepromatous cases that carry the largest load of organisms, the maximum load reaching over seven billion organisms per gram of tissue. Patients with non-lepromatous cases carry a very much smaller bacillary load, probably not exceeding one million organisms in total. In additon to clinically identified cases, the occurrence of acid-fast bacilli (AFB) in the skin (Figueredo & Desai, 1949; Chatterjee, 1976) and nasal mucosa of healthy subjects (Chacko et al, 1979) have also been reported. The evidence that the AFB found on such 'carriers' is M.leprae is not conclusive, although there is some evidence that persons who carry such AFB have a higher chance of developing the disease as found during their follow-up (Chatterjee, 1976).

Portal of exit of M.leprae

The two portals of exit of M.leprae often described are the skin and the nasal mucosa. However, the relative importance of these two portals is not clear. It is true that the lepromatous cases show large numbers of organisms deep down in the dermis. However, whether they reach the skin surface in sufficient numbers is doubtful. Although there are reports of AFB being found in the desquamating epithelium of the skin, Weddell et al (1963a) have reported that they could not find any AFB in the epidermis even after examining a very large number of specimens from patients and contacts. Regarding the nasal mucosa, its importance has been recognized as early as 1898 by Schaeffer (1898), particularly tht of the ulcerated mucosa. The quantity of bacilli from nasal mucosal lesions in lepromatous leprosy has been demonstrated by Shepard (1960) as large, with counts ranging from 10 000 to 10 000 000. Pedley (1973) has reported that the majority of lepromatous patients showed leprosy bacilli in their nasal secretions as collected through nose blows. Davey & Rees (1974) have indicated that nasal secretions from lepromatous patients can yield as much as 1 million viable organisms per day.