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In order to investigate the phenomenon of age-related immunity to visceral leishmaniasis, a 1 year prospective survey was carried out on 5671 people in a Leishmania infantum focus in north-west Iran. The average incidence rate 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 ratio 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 with a reduction in the seroconversion rate and an increase in the serorecovery rate. Even amongst people with no detectable cell-mediated immunity to Leishmania, the seroconversion rate decreased and the serorecovery rate increased with age. All current VL patients had a negative leishmanin skin test response. Hence, adults may develop protection against L. infantum through 2 processes, 1 dependent and 1 independent of acquired cell-mediated immunity.

Abstract. One explanation proposed for the widespread failure to control zoonotic visceral leishmaniasis by culling infected domestic dogs is that wild canids or humans play significant roles in transmission. The aim of this study was to determine the importance of domestic dogs as the reservoir hosts of visceral leishmaniasis in northwest Iran. A random sample of 3,872 children and 199 dogs in 38 villages was surveyed by the direct agglutination test. Dog ownership details among these households were collected by questionnaire. Parasites isolated from 16 patients and 12 dogs were characterized as Leishmania infantum MON-1. Average seroprevalence in dogs (21.6%) was much higher than in children (7%). Child seropositivity increased significantly with village dog density in absolute terms (P < 0.001) and in relation to dog/human ratios (P ‫ס‬ 0.028). Dog ownership within villages also was a significant risk factor for child seropositivity (P ‫ס‬ 0.003).

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