Schistosomiasis is one of the deleterious parasitic diseases in many developing countries. One important approach to control the disease is to eliminate its intermediate hosts. Therefore, methanol extract from Callistemon citrinus leaves was evaluated against biological & biochemical aspects of Schistosoma mansoni infected and uninfected B. alexandrina snails,
in addition to examination the histological changes in tissues of the snails' digestive gland. The plant extract exhibited considerable molluscicidal potency against B. alexandrina snails. The egg-laying capacity and reproductive rate (R0) of S. mansoni infected and uninfected snails were suppressed post exposure to LC25 of methanol extract for 24h/week for 4 successive weeks. Reduction rate of R0 for snails infected only and those infected treated with LC25 were 65.8% and 70.9%, respectively. Infection rates of B. alexandrina snails with S. mansoni and cercarial production/infected snail were reduced as a reflection of their exposure to LC10 of the plant extract either pre-, during or post their exposure to miracidia. The total number of hemocytes of S. mansoni infected and uninfected snails was elevated post 24h of exposure to the plant extract. However, it was reduced after 4 weeks of successive exposure to the extract, but the large granulocytes were elevated. Also, exposure of B. alexandrina snails (infected and uninfected) to the plant extract caused deleterious morphological changes in their hemocytes. The activities of antioxidant enzymes LPO, CAT & SOD
were elevated in tissue homogenate of uninfected snails after one week of exposure to the extract, while that of GSH was decreased. Moreover, the plant extract deteriorated the histological characters of digestive gland cells in treated snails ranged from degeneration and necrotic of its tubules, atrophy of different cell types and hyaline substance in enlarged digestive lumen..