EXPERIMENTAL ACTIVATION OF INNATE IMMUNITY OF BIOMPHALARIA ALEXANDRINA AND ITS EFFECT ON SCHISTOSOMA MANSONI INFECTION
Samia E Etewa, Rania S Hamza and Ghada M Fathy
Department of Medical Parasitology, Faculty of Medicine, Zagazig University, Egypt.
Successful establishment of the larval forms of Schistosoma mansoni inside the snail intermediate host requires overcoming the host's internal defense mechanisms. These comprise humoral and cellular elements cooperating in recognition and destruction of invading organisms. Circulating and tissue hemocytes represent the primary mediators of the cellular defense reactions (innate immunity) in Biomphalaria snails. This study tried to find out whether the innate immunity of a highly susceptible B. alexandrina strain toward Schistosoma mansoni invasion could be activated experimentally or not in a trial to change susceptible snails to resistant ones to put an end to exposure cycle of S. mansoni inside them as a step in schistosomiasis control. It also investigated the correlation between the presence and numbers of hemocytes (in tissue lesions and in the circulating hemolymph) in B. alexandrina snails and the intensity of S. mansoni infection. Two snail groups with different shell sizes were used in the study. Another two groups with the same size as the tested groups were used as control groups. Circulating and tissue hemocytes were estimated in the tested groups before and after inoculation with S. mansoni whole antigen in the snails' cephalo-podal region. The effect of the hemocyte number on S. mansoni infection was estimated by parasitological and histopathological methods. There was a high significant increase in the number of circulating hemocytes between small (juvenile) and large sized (adult) snails before S. mansoni antigen inoculation. After Ag inoculation, there was a significant increase of the cell number in large sized snails as compared to small sized snails but without statistical significance with their control groups while after S. mansoni infection, this number began to show significant increase in all groups with the peak in the 6th week (P < 0.05). Furthermore, all the snails (juvenile and adults) previously activated with S. mansoni antigen shed less cercariae than their respective controls, but without statistical significance. In conclusion, young snails (more susceptible) have fewer number of circulating hemocytes as compared to older snails. Experimental antigenic activation induced local proliferation of tissue hemocytes, while it didn't increase the number of circulating hemocytes enough to interfere with the developing S. mansoni infection.