By Zheng Qi 2020-11-15

Schistosomiasis, also widely known as bilharzia, is a neglected tropical acute and chronic parasitic disease caused by flatworms of the genus Schistosoma. Unlike many other parasitic infections that enter the human body through contaminated food or direct ingestion, this disease follows a unique transmission route tied closely to freshwater ecosystems and human daily activities. The infection begins when microscopic larval forms called cercariae, actively released by infected freshwater snails into surrounding water, penetrate unprotected human skin during routine contact—whether people are wading in streams, washing clothes at riverbanks, swimming in village ponds, or simply stepping through shallow water while working in rice fields. This makes the disease far more intertwined with local lifestyles than many common public health conditions.

The global distribution of schistosomiasis is heavily concentrated in tropical and subtropical regions, where warm water temperatures create ideal breeding conditions for the specific snail species that act as the parasite’s intermediate host. It thrives most severely in marginalized, poor communities that lack reliable access to safe, treated drinking water and adequate sanitation infrastructure. In these settings, open defecation and the absence of properly constructed sewage systems create constant opportunities for human waste to flow directly back into nearby rivers, lakes, and irrigation channels, closing the loop of transmission. According to the World Health Organization, at least 90% of all people worldwide who require urgent preventive and curative treatment for schistosomiasis reside on the African continent, where limited public health resources and widespread dependence on untreated natural freshwater sources keep transmission cycles persistent across generations.

The full life cycle of the parasite is a carefully adapted sequence that links human behavior, freshwater environments, and snail biology. When a person already living with schistosomiasis contaminates a freshwater source with their excreta, the parasite eggs contained in their urine or faeces hatch quickly once they come into contact with water, releasing free-swimming miracidia that seek out and infect local snails. Inside the snail host, the parasites multiply asexually over several weeks, eventually producing thousands of the infectious cercariae that are later released back into the water to find new human hosts. Once these larvae penetrate human skin, they travel through the bloodstream, mature into long-lived adult worms, and settle in the small blood vessels surrounding the intestines or bladder. Here, male and female worms pair up, and the female worms begin releasing hundreds of eggs every single day. Only a portion of these eggs are successfully passed out of the body through faeces or urine to continue the external life cycle. The remaining eggs become trapped in surrounding organ tissues, triggering chronic, ongoing immune reactions that gradually cause scarring, inflammation, and progressive damage. Over years of untreated infection, this can lead to severe complications including enlarged liver, kidney failure, bladder cancer, and permanent damage to the intestines, lungs, and even the central nervous system.

Today, schistosomiasis remains one of the most impactful yet underrecognized public health threats, affecting hundreds of millions of people and trapping many vulnerable communities in cycles of poverty and illness.

Edited by: Secretariat of the Asia-Pacific Network for Drug and Diagnostics Innovation
Produced by: Zheng Qi
Reviewed by: Zhang Haobing
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