Abstract
IntroductionAsymptomatic 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.MethodsWe 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.ResultsWe 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).DiscussionAlthough 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.
Highlights
Asymptomatic persons infected with the parasites causing visceral leishmaniasis (VL) usually outnumber clinically apparent cases by a ratio of 4–10 to 1
The main strategy to achieve this was early diagnosis and treatment, along with vector control measures. Key assumptions underlying this elimination strategy are that the disease is an anthroponosis and that active cases of VL and post-kala azar dermal leishmaniasis (PKDL) are the only reservoirs maintaining disease transmission
The study site is a rural area comprised of 50 villages with a total population of 85,333, in which 193 VL cases were reported over a 2 K year period (March 2007– December 2009)
Summary
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. The main strategy to achieve this was early diagnosis and treatment, along with vector control measures. Key assumptions underlying this elimination strategy are that the disease is an anthroponosis and that active cases of VL and post-kala azar dermal leishmaniasis (PKDL) are the only reservoirs maintaining disease transmission. More recently Stauch et al [5] pointed to the possible role that latent carriers of L. donovani infection could play in transmission, if they were infectious for sand flies. Given the biological differences between L. infantum and L. donovani, the different host, vector and epidemiological factors and the potential clinical and public health implications, these conclusions cannot necessarily be generalized to L. donovani
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