Schistosomiasis
Schistosomiasis, also known as bilharzia, is a parasitic disease caused by trematode worms of the genus Schistosoma. The infection occurs when larval forms of the parasite, released by freshwater snails, penetrate human skin during contact with infested water. Once inside the host, schistosomes mature in the venous system, with adult worms residing in the mesenteric or pelvic veins depending on the species. The pathogenesis of schistosomiasis is characterized by an immune-mediated response to the deposition of parasite eggs in tissues, leading to granulomatous inflammation and subsequent fibrosis. Chronic infection can result in significant morbidity, including hepatic, intestinal, urogenital, and, less commonly, pulmonary and neurological complications. The health impacts range from acute symptoms such as fever, rash, and eosinophilia (Katayama syndrome) to chronic sequelae like portal hypertension, hematuria, bladder cancer, and infertility, depending on the Schistosoma species and the organs affected.
This type is primarily caused by Schistosoma mansoni, Schistosoma japonicum, and Schistosoma mekongi. The adult worms inhabit the mesenteric veins of the intestines, and eggs deposited in the intestinal wall provoke granulomatous inflammation. Clinical manifestations include abdominal pain, diarrhea, and blood in the stool, with chronic disease potentially leading to hepatosplenic involvement, periportal fibrosis, portal hypertension, and splenomegaly. Severe cases may result in life-threatening complications such as esophageal varices and ascites.
Predominantly caused by Schistosoma haematobium, this form affects the pelvic venous plexus, particularly around the bladder. The eggs induce mucosal ulceration and granulomatous reactions in the urinary tract, leading to hematuria, dysuria, and, over time, fibrosis of the bladder and ureters. Chronic urogenital schistosomiasis is associated with an increased risk of squamous cell carcinoma of the bladder, obstructive uropathy, and infertility, particularly in women, due to involvement of the genital tract.
This severe manifestation results from chronic infection, most commonly with S. mansoni or S. japonicum. The persistent immune response to eggs trapped in the liver leads to periportal fibrosis, causing portal hypertension and its sequelae such as splenomegaly, variceal bleeding, and hypersplenism. Despite significant portal hypertension, hepatic synthetic function is often preserved until late in the disease.
Ectopic egg deposition in the central nervous system, most frequently seen with S. japonicum (cerebral) and S. haematobium or S. mansoni (spinal), can cause neurological complications. Cerebral involvement may present with seizures, focal neurological deficits, or increased intracranial pressure, while spinal schistosomiasis can result in transverse myelitis, paraparesis, or bladder dysfunction. These forms are rare but potentially devastating.
Schistosomiasis is a major neglected tropical disease, affecting over 230 million people worldwide, with more than 90% of cases occurring in sub-Saharan Africa. The disease is endemic in 78 countries spanning Africa, the Middle East, South America, the Caribbean, and parts of Asia. Transmission is closely linked to freshwater sources contaminated with human excreta and the presence of specific snail intermediate hosts. School-aged children and young adults are most at risk due to frequent water contact. Morbidity is substantial, with an estimated 200,000 deaths annually attributed to schistosomiasis and its complications. Control efforts, including mass drug administration and snail control, have reduced prevalence in some regions, but reinfection remains common in highly endemic areas. Migration and travel have led to imported cases in non-endemic regions, making awareness important for clinicians worldwide.
The diagnosis of schistosomiasis relies on a combination of clinical suspicion, epidemiological exposure, and laboratory confirmation. Microscopic identification of characteristic schistosome eggs in stool (for intestinal species) or urine (for S. haematobium) remains the gold standard. Concentration techniques, such as the Kato-Katz method for stool and filtration for urine, improve sensitivity, particularly in low-intensity infections. Serological assays detecting anti-schistosome antibodies are useful in travelers and in low-endemicity settings but cannot distinguish active from past infection. Circulating antigen detection assays (e.g., circulating cathodic antigen) are increasingly employed for active infection diagnosis, especially in mass screening programs. Imaging modalities such as abdominal ultrasound can assess hepatosplenic involvement, while cystoscopy and biopsy may be required for urogenital disease with atypical presentations. Eosinophilia and elevated IgE levels may support the diagnosis but are non-specific. Definitive diagnosis is based on demonstration of eggs or schistosome antigens in biological specimens from individuals with compatible exposure history and clinical features.
Praziquantel is the primary pharmacological treatment for schistosomiasis and is administered as an oral agent effective against all major Schistosoma species. The drug acts by increasing the permeability of the parasite's cell membranes to calcium ions, resulting in paralysis and death of the adult worms. Praziquantel is generally well tolerated and is recommended for both individual therapy and mass drug administration programs. Treatment regimens typically involve a single-day dose, which may be repeated in cases of persistent infection or heavy worm burden. The drug is suitable for use in both adults and children, and its widespread availability has made it a cornerstone in global schistosomiasis control strategies.
| Structure | Generic Name | CAS Registry Number | Molecular Formula | Molecular Weight |
|---|---|---|---|---|
 | praziquantel (Rec INN; USAN; BAN) | 55268-74-1 | C19 H24 N2 O2 | 312.406 |
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