Schistosomes are parasitic flatworms that cause a disease known as schistosomiasis, sometimes referred to as snail fever or bilharzia. This infection affects internal organs such as the liver and bladder. Schistosomiasis represents a considerable global health concern, particularly in tropical and subtropical regions. It is recognized as a neglected tropical disease, disproportionately affecting vulnerable populations.
The Parasite and Its Life Cycle
The schistosome life cycle involves two hosts: freshwater snails and mammals, including humans. The cycle begins when human feces or urine containing schistosome eggs contaminate freshwater sources. In optimal conditions, these eggs hatch in the water, releasing a larval form called miracidia.
These miracidia then swim and penetrate specific species of freshwater snails, which serve as intermediate hosts. Inside the snail, the miracidia undergo asexual reproduction, developing into two generations of sporocysts and eventually producing thousands of fork-tailed larvae called cercariae. These cercariae are released from the snail back into the freshwater, ready to infect a human host.
When humans come into contact with water containing these free-swimming cercariae, the larvae penetrate the skin, shedding their tails to become schistosomulae. These schistosomulae migrate through the bloodstream to the lungs, then to the heart, and finally mature in the liver’s portal vein system. Once mature, male and female adult worms pair up and migrate to specific locations within the body’s blood vessels, where they can live for several years.
Female worms begin laying eggs, producing hundreds to thousands daily. Some of these eggs pass out of the body in feces or urine, continuing the life cycle, while others become trapped in various tissues, leading to symptoms. Three main species infect humans: Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium.
How Infection Occurs and Symptoms
Human infection primarily occurs through skin contact with freshwater infested with cercariae, often during activities such as swimming, bathing, fishing, or washing clothes. Following exposure, some individuals may experience acute symptoms. One early manifestation can be “swimmer’s itch,” an itchy rash that develops at the site where the cercariae penetrated the skin.
A few weeks after infection, some people may develop Katayama fever, an acute illness characterized by symptoms like fever, cough, abdominal pain, diarrhea, and an enlarged liver and spleen. The more significant health issues arise from chronic infection, which can vary depending on the schistosome species and the affected organs. Symptoms are mainly caused by the body’s immune reaction to the worms’ eggs trapped in tissues, not the adult worms themselves.
Intestinal schistosomiasis, caused by S. mansoni or S. japonicum, can lead to chronic abdominal pain, diarrhea, and blood in the stool. In advanced cases, this can progress to liver enlargement, fluid accumulation in the abdominal cavity, and high blood pressure in the abdominal blood vessels. Urogenital schistosomiasis, caused by S. haematobium, is classically identified by blood in the urine, known as haematuria. Long-term urogenital infection can result in bladder and ureter fibrosis, kidney damage, and an increased risk of bladder cancer.
Diagnosis and Treatment
Diagnosing schistosomiasis involves identifying the parasite’s eggs in bodily fluid samples. For suspected Schistosoma mansoni or Schistosoma japonicum infections, microscopic examination of stool samples is performed. When Schistosoma haematobium is suspected, urine samples are examined for eggs. These microscopic examinations can be enhanced through concentration procedures.
In areas where Schistosoma mansoni is prevalent, a circulating cathodic antigen (CCA) test can also be used for diagnosis. For individuals in non-endemic or low-transmission regions, serological and immunological tests, which detect antibodies against the parasite, may indicate exposure and the need for further examination and treatment. The primary treatment for all forms of schistosomiasis is a single dose of the anthelmintic drug praziquantel. This medication is effective and well-tolerated, clearing the infection and reducing disease severity.
Prevention and Global Efforts
Preventing schistosomiasis involves both individual actions and broader public health initiatives. Individuals can reduce their risk by avoiding contact with freshwater in areas where the disease is known to occur. Practicing safe water habits, such as using clean water for bathing and domestic chores, and improving sanitation to prevent contamination of water sources, are also important.
Globally, significant efforts are underway to control and eliminate schistosomiasis. Mass drug administration (MDA) programs are central to these efforts, involving the periodic, large-scale treatment of at-risk populations with praziquantel. These programs aim to reduce the burden of the disease and prevent its progression. In addition to MDA, snail control measures and improvements in water and sanitation infrastructure play a role in interrupting the parasite’s life cycle. Education and community engagement are also important, empowering affected communities to adopt preventive behaviors and participate in control programs.