Schistosoma mansoni intermediate host — DTM&H MCQ

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ModerateEntomology and vector controlSchistosoma mansoni intermediate hostDTM&H

A child in Uganda has intestinal schistosomiasis after swimming in Lake Albert. Local mapping finds freshwater snails in vegetation near the shore. Which intermediate host is most likely for Schistosoma mansoni?

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Correct answer: D — Biomphalaria snail

Biomphalaria snail (option B) is correct because it is the specific freshwater snail genus that acts as the intermediate host for Schistosoma mansoni, sustaining the larval sporocyst stages and releasing infective cercariae into water. In East Africa, including the Lake Albert region of Uganda, species such as Biomphalaria pfeifferi and Biomphalaria sudanica colonise vegetated, slow moving freshwater margins, exactly the habitat described in the stem. The child's exposure (swimming near shoreline vegetation) fits the ecological niche of Biomphalaria, which prefers still or slow flowing water with aquatic plants for attachment and feeding. This snail-parasite pairing underlies focal transmission mapping used in schistosomiasis control programmes, since each Schistosoma species is tied to a distinct snail genus. Why the other options are wrong: E. Bulinus snail: this genus is the intermediate host for Schistosoma haematobium (urinary schistosomiasis), not S. mansoni, and is associated with a different snail ecology and disease presentation (haematuria, bladder pathology). A. Oncomelania snail: this operculate snail genus transmits Schistosoma japonicum, found in East Asia (China, Philippines, Sulawesi), not in African freshwater systems like Lake Albert. B. Cyclops copepod: this crustacean is the intermediate host for Dracunculus medinensis (guinea worm) and some Diphyllobothrium species, not for any Schistosoma species. C. Lymnaea snail: this genus is the intermediate host for Fasciola hepatica (liver fluke), a trematode with an entirely different life cycle and clinical picture (biliary disease), not schistosomiasis. Key point: each Schistosoma species has a species-specific snail intermediate host, and in African intestinal schistosomiasis (S. mansoni) that host is always Biomphalaria, distinguishing it from Bulinus (S. haematobium) and Oncomelania (S. japonicum).

Reference: CDC DPDx, Schistosomiasis: Biology (intermediate hosts by species), reviewed 2024, https://www.cdc.gov/dpdx/schistosomiasis/index.html