Schistosoma mansoni: melatonin enhances efficacy of cercarial and soluble worm antigens in the induction of protective immunity against infection in the hamster.

Based on the beneficial influence of melatonin administration on the course of schistosomiasis and on its possible action on the immune system, we aimed in this study to establish an immunization program using Schistosoma mansoni adult worm antigen (SWAP) and cercarial antigen (CAP) alone or concurrently with melatonin treatment, for 30 successive days, in an attempt to enhance their efficacy against the infection in hamsters. Results showed that the worm reduction percentages were 53.8%, 67.01%, 56.4% and 99.3% for CAP, CAP+melatonin, SWAP, SWAP+melatonin, respectively, indicating that melatonin enhanced efficacy of SWAP but only produced a slight increase in efficacy of CAP. Highly significant reductions in egg load in the liver and alteration in the oogram pattern with a high percentage of immature eggs and few dead eggs were recorded in the groups that received melatonin treatment suggesting a possible role for melatonin in the regulation of egg production and development. On the other hand, melatonin clearly improved the oxidative status in the immunized groups. No antibody (Ab) response was recorded in the groups immunized with SWAP+melatonin while low Ab level was seen in the other melatonin-treated group. In addition to the antioxidant properties of melatonin, our results suggested that the early and continuous melatonin administration may result in immunomodulatory actions which in turn enhanced the efficacy of SWAP and CAP in different ways. This indicates the importance of further investigation of the mechanisms of melatonin action and the possible application in a vaccination program.