TRANSMISSION AND PROTECTION IN LEPROSY - INDICATIONS OF THE ROLE OF MUCOSAL IMMUNITY

Recent advances in treatment have achieved a large drop in the prevale nce of active leprosy cases, but the incidence is at best decreasing s lowly. Most people within leprosy-endemic populations have been expose d to MJ,Mycobacterium leprae, but few develop disease and it seems lik ely that the majority of the population develops protective immunity. If the site of initial infection is in the nose, dissemination of baci lli around the body to skin and nerve implies that the initial infecti on is bacilliferous and it has been shown that nasal M. leprae are det ectable by polymerase chain reaction (PCR) of nasal swabs. Since saliv ary anti-M. leprae IgA (sMLIgA) levels are correlated with protection, (5) we have surveyed groups of leprosy patients, contacts and the gene ral population for both their sMLIgA and nasal PCR positivity. A total of 304 subjects were enrolled in the study: PCR and mucosal challenge tests were performed in 204 of these individuals. sMLIgA was present in 66% of treated patients, 76% of leprosy workers and 72% of healthy contacts. However, only 33% of indigenous subjects were sMLIgA+, in co ntrast to the earlier studies showing 74% positivity.(5) PCR for M. le prae was present in both household contacts (2%) and indigenous contro ls (5%). In a subsequent follow-up study, nasal swabs were taken from 97 of those studied in the first series: three PCR+ individuals follow ed up after one year became negative, while of the remaining 94 PCR-in dividuals retested, 2 became positive. Of 112 subjects retested with t he mucosal challenge test for sMLIgA: 22 converted from positive to ne gative and 12 From negative to positive. These results suggest that th ere is widespread subclinical transmission of M. leprae with transient infection of the nose resulting in the development of a mucosal immun e response, despite the fact that few individuals will develop clinica l disease. This may explain the current lack of effect of multidrug th erapy (MDT)control programmes on incidence, although the reduction in general population immunity is consistent with some effect of MDT on t ransmission.