Age related efficiency of the leishmanin skin test as a marker of immunity to human visceral leishmaniasis.

This study aimed at investigating whether the efficiency of the leishmanin skin test (LST) to evaluate the immune status of individuals exposed to Leishmania (L.) infantum is age-related. It was conducted in two districts of the governorate of Kairouan, an endemic region for L. infantum infection in Tunisia. Healthy individuals (n = 119) were selected according to two criteria: no current or past history of visceral or cutaneous leishmaniasis, and their age range: 1-6 years (group I), 7-14 years (group II), and 20-66 years old (group III). Assessments comprised LSTs, in vitro lymphoproliferative response, and interferon-gamma (IFN-gamma) productions induced by soluble leishmanial antigens (SLA). LST recorded an overall of 89.07% and 89.9% concordance with T cell proliferation and IFN-gamma production induced by SLA, respectively. Using in vitro tests as gold standards, LST was found more sensitive for screening individuals from group I (96% and 100%, considering T cell proliferation and IFN-gamma production results, respectively), than group II (91% and 97%) and group III (70% and 74%,). Conversely, LST was less specific in group I (84% and 77%) than group II (100% and 94%) and group III (100% for both in vitro tests). Our results suggested that the strength of LST resided in its higher sensitivity, to unravel asymptomatic injections and cell mediated immunity to L. infantum parasite in infants and its higher specificity for screening adult individuals. Negative LST in adults and positive LST in children < 5 years, the population at risk of developing visceral leishmaniasis, remain the weaknesses of LST and should be interpreted with caution.