Latent Infection with Leishmania donovani in Highly Endemic Villages in Bihar,India

Introduction: asymptomatic persons infected with the parasites causing visceral leishmaniasis (vl) usually outnumber clinically apparent cases by a ratio of 4-10 to 1. we describe patterns of markers of leishmania donovani infection and clinical vl in relation to age in bihar,india. methods: we selected eleven villages highly endemic for leishmania donovani. during a 1-year interval we conducted two house to house surveys during which we collected blood samples on filter paper from all consenting individuals aged 2 years and above. samples were tested for anti-leishmania serology by direct agglutination test (dat) and rk39 elisa. data collected during the surveys included information on episodes of clinical vl among study participants. results: we enrolled 13,163 persons; 6.2% were reactive to dat and 5.9% to rk39. agreement between the tests was weak (kappa = 0.30). among those who were negative on both tests at baseline,3.6% had converted to sero-positive on either of the two tests one year later. proportions of sero-positives and sero-converters increased steadily with age. clinical vl occurred mainly among children and young adults (median age 19 years). discussion: although infection with l. donovani is assumed to be permanent,serological markers revert to negative. most vl cases occur at younger ages,yet we observed a steady increase with age in the frequency of sero-positivity and sero-conversion. our findings can be explained by a boosting effect upon repeated exposure to the parasite or by intermittent release of parasites in infected subjects from safe target cells. a certain proportion of sero-negative subjects could have been infected but below the threshold of antibody abundance for our serologic testing.