Age, acquired immunity and the risk of visceral leishmaniasis: a prospective study in Iran

In order to investigate the phenomenon of age-related immunity to visceral leishmaniasis, a 1 year prospective survey was carried out on 5671 people i n a Leishmania infantum focus in north-west Iran. The average incidence rat e of infection since 1985 was 2.8%/year with all ages equally at risk. One in 13 infections in children led to visceral leishmaniasis (VL), and this r atio decreased significantly with age. Seroprevalence also dropped rapidly with age, suggesting that the same process may affect both clinical outcome and the humoral immune responses. Cell-mediated immunity was associated wi th a reduction in the seroconversion rate and an increase in the serorecove ry rate. Even amongst people with no detectable cell-mediated immunity to L eishmania,, the seroconversion rate decreased and the serorecovery rate inc reased with age. All current VL patients had a negative leishmanin skin tes t response. Hence, adults may develop protection against L. infantum throug h 2 processes, 1 dependent and 1 independent of acquired cell-mediated immu nity.