Schistosomiasis, often called snail fever or bilharzia, is a neglected tropical disease caused by parasitic flatworms of the genus Schistosoma. It is considered the second-most socioeconomically devastating parasitic disease after malaria. Schistosoma mansoni is a significant pathogen among the species that infect humans, responsible for causing intestinal schistosomiasis in millions worldwide. The disease persists in tropical and subtropical regions, affecting communities lacking access to safe water and adequate sanitation.
The Parasite and Its Geographical Distribution
S. mansoni is a trematode, or blood fluke, belonging to the family Schistosomatidae. Unlike most flukes, Schistosoma species are dioecious, meaning they have distinct male and female sexes that must pair for reproduction. The adult worms are slender, and the female resides within a specialized groove on the male’s body called the gynecophoral canal.
This parasite causes the intestinal form of human schistosomiasis. The adult worms live primarily within the mesenteric veins, the blood vessels surrounding the large intestine. They are found predominantly in the inferior mesenteric veins, where the female lays hundreds of eggs daily.
The geographical distribution of S. mansoni is widespread and linked to the presence of its specific intermediate host snail. Endemic regions include much of Africa, the Middle East, parts of South America, and various Caribbean islands. Countries like Brazil, Venezuela, Suriname, and numerous sub-Saharan African nations face a significant burden from this infection.
The Complex Life Cycle and Transmission
The life cycle of S. mansoni requires both a human host and an aquatic snail intermediate host. The cycle begins when eggs are released into freshwater environments, typically through human feces. Once in the water, the eggs hatch to release a ciliated larval stage known as a miracidium.
The miracidium swims until it finds and penetrates a specific freshwater snail, usually from the genus Biomphalaria. Inside the snail’s tissue, the miracidium undergoes asexual reproduction, transforming first into a mother sporocyst and then into numerous daughter sporocysts.
After several weeks, the daughter sporocysts produce thousands of fork-tailed larvae called cercariae, which are the infective stage for humans. These cercariae are released from the snail back into the water. Human infection occurs when the skin is exposed to contaminated water, allowing the cercariae to actively penetrate the skin.
Upon penetration, the cercariae shed their tails and become schistosomulae, the next larval stage. The schistosomulae migrate through the circulatory system, traveling via the lungs and heart before reaching the liver. In the liver, they mature into adult worms and then migrate to the mesenteric veins, where they begin laying eggs.
Manifestations of Schistosomiasis
The primary pathology of S. mansoni infection is caused not by the adult worms, but by the eggs trapped within the host’s tissues. While many eggs migrate through the intestinal wall to be passed in the feces, a significant number are swept away in the bloodstream and lodge in organs, most notably the liver.
This lodgment triggers an immune response, leading to the formation of granulomas—localized clumps of immune cells surrounding the trapped egg. The eggs secrete substances that initiate this granulomatous reaction, which is a protective mechanism that also causes tissue damage. The disease manifests in two main stages.
The acute stage, known as Katayama fever, may develop weeks to months after infection, coinciding with the start of egg production. Symptoms are systemic and include fever, fatigue, malaise, muscle aches, and sometimes a cough or rash. This stage is a hypersensitivity reaction to the migrating schistosomulae and the initial egg deposition.
The chronic stage results from years of persistent egg deposition and granuloma formation, leading to severe organ pathology. In intestinal schistosomiasis, chronic inflammation causes abdominal pain, diarrhea, and bloody stools. The most severe consequence is hepatosplenic schistosomiasis, where liver granulomas cause periportal fibrosis, often called Symmers’ fibrosis.
This progressive fibrosis impairs blood flow through the liver, resulting in portal hypertension—an increase in blood pressure in the portal vein system. Portal hypertension causes the enlargement of the spleen (splenomegaly) and the formation of fragile blood vessels (varices) in the esophagus and stomach. Rupture of these varices can lead to life-threatening gastrointestinal hemorrhage.
Diagnosis, Treatment, and Global Control Efforts
Diagnosis of S. mansoni infection relies on the microscopic examination of stool samples to identify the parasite’s characteristic eggs. The eggs are distinguishable by their oval shape and a prominent, sharp lateral spine. Because egg excretion can be intermittent, concentration techniques or repeated examinations are sometimes necessary to confirm light infections.
Serological tests, which detect antibodies against the parasite, are also used, especially in travelers or those with low-intensity infections. However, antibody tests cannot differentiate between a current active infection and a past, successfully treated one. Imaging techniques, such as ultrasound, are useful for assessing the extent of liver and spleen damage in chronic cases.
The drug of choice for treating schistosomiasis is Praziquantel, an anthelmintic medication highly effective against the adult worms. For S. mansoni, the recommended dosage is typically a single-day treatment regimen of 40 mg/kg of body weight, administered in two divided doses. Praziquantel works by increasing the worm’s cell membrane permeability to calcium ions, causing muscle paralysis and detachment from the host’s blood vessel walls.
Global control efforts focus on a multi-pronged approach aimed at reducing disease burden and interrupting transmission.
Mass Drug Administration (MDA)
Mass Drug Administration (MDA) programs are the primary strategy for controlling morbidity. These preventive chemotherapy campaigns involve treating at-risk populations, often targeting school-aged children, with Praziquantel.
Integrated Control Strategies
Integrated control strategies complement drug treatment by targeting other parts of the parasite’s life cycle. Improving water, sanitation, and hygiene (WASH) infrastructure helps prevent eggs from reaching freshwater sources. Additionally, molluscicides—chemicals designed to kill the intermediate host snails—are employed in specific areas to reduce the Biomphalaria population and limit the transmission cycle.