Transmission of leprosy
Sub-clinical infection in leprosy
In spite of the fact that as yet there is no simple immunological test to identify sub-clinical infection with sufficient specificity and sensitivity, evidence accumulated in the past few years clearly indicate that sub-clinical infection does occur in leprosy as in many other communicable diseases. This evidence has mainly come from limited studies with in vitro tests for cell-mediated immunity (CMI) such as the lymphocyte transformation test (LTT) and serological tests for detecting humoral antibodies such as phenolic glycolipid I-based ELISA. In addition to the above, skin tests with various preparations of lepromin, and more recently with soluble antigens from M.leprae, have also provided useful information on the occurrence of sub-clinical infection, although the specificity of these tests, particularly of integral lepromin, has been rather questionable. Zuniga et al (1982), using a soluble skin test antigen prepared by the Convit method, have found that skin test positivity in a part of Venezuela was 19% among the general population (non-contacts), 36% among contacts outside the household and 48% among household contacts. The gradation of reactivity clearly suggests the correlation between exposure and possible sub-clinical infection. However, in India (Gupte et al, 1990) no difference was seen in the distribution of skin test reactions to soluble antigens among cases, contacts and general population.
Transmission by contact
The term 'contact' in leprosy is generally not clearly defined. All that we know at present is that individuals who are in close association or proximity with leprosy patients have a greater chance of acquiring the disease. It is with reference to this observation that the early workers appear to have used the term 'contact' as method of transmission. However, it is the definition of contact by later workers with qualifications such as 'skin to skin', 'intimate', 'repeated', etc. that has made it appear as if the disease could be acquired only under such conditions, and that the transmission involved some kind of 'inunction' or rubbing in of the organisms from the skin of affected persons into the skin of healthy subjects. Certainly, there is no proof that transmission takes place only through such inunction. In general, closeness of contact is related to the dose of infection which in turn is related to the occurrence of disease. Of the various situations that promote close contact, contact within the household is the only one that is easily identified. In that area the relative risk for contacts was about four times that of non-contacts. The actual incidence among contacts and the relative risk for them appear to vary considerably in different studies. Attack rates for contacts of lepromatous leprosy have varied from 6.2 per 1000 per year in Cebu (Doull et al, 1942) to 55.8 per 1000 per year in a part of South India (Noordeen & Neelan, 1978).