Schistosoma intermediate host — DTM&H MCQ
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Correct answer: D — Bulinus freshwater snail
The correct answer is D, Bulinus freshwater snail. Urinary (urogenital) schistosomiasis is caused by Schistosoma haematobium, and this parasite's cercariae develop specifically within Bulinus species snails found in African freshwater lakes such as Lake Malawi. <cite index="0-2">Some snails act as intermediate hosts (vectors) for parasitic flatworms (flukes) that cause neglected tropical diseases, such as schistosomiases</cite>, and for S. haematobium this host genus is Bulinus, which releases infective cercariae that penetrate intact skin during freshwater exposure such as swimming. Terminal spined ova on urine microscopy and terminal haematuria are the classic clinical clues pointing to S. haematobium rather than S. mansoni, which is transmitted by a different snail genus. Why the other options are wrong: C. Anopheles mosquito: This is the vector for malaria (Plasmodium species), not schistosomiasis, and mosquitoes are not involved in the schistosome life cycle at all. E. Simulium blackfly: This transmits Onchocerca volvulus causing onchocerciasis (river blindness), a filarial disease unrelated to schistosome transmission. B. Cyclops copepod: This crustacean is the intermediate host for Dracunculus medinensis (guinea worm) and some fish tapeworms, not for schistosomes. A. Biomphalaria freshwater snail: This snail genus transmits Schistosoma mansoni, which causes intestinal schistosomiasis with terminal spineless (lateral spined) ova, not the terminal spined ova of urinary disease described here. Key point: Terminal spined ova and terminal haematuria indicate S. haematobium, whose intermediate host is Bulinus, whereas Biomphalaria transmits S. mansoni with lateral spined ova and intestinal disease.
Reference: PMC (National Center for Biotechnology Information), Nuclear genome of Bulinus truncatus, an intermediate host of the carcinogenic human blood fluke Schistosoma haematobium, 2022, https://pmc.ncbi.nlm.nih.gov/articles/PMC8861042/