Schistosomiasis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Schistosomiasis is an infection acquired through contact with fresh water infested with the infectious larval form of Schistosoma flat worms (trematodes). Although schistosomiasis is a tropical disease, travelers, students, immigrants, veterans, and tourists who previously lived in or visited regions where schistosomiasis is endemic, present worldwide. Early disease is usually asymptomatic, unless katayama fever, an acute immune complex disease, occurs Late disease is symptomatic and includes hepatosplenic schistosomiasis (presinusoidal portal hypertension), urinary and urogenital schistosomiasis (urinary obstruction, genital symptoms), schistosomal glomerulopathy (chronic immune complex deposition in the kidney), and ectopic disease in areas such as the lungs and central nervous system (CNS). Diagnostic methods include visualization of Schistosoma eggs in formed stool, urine, and crushed biopsy tissues; serologic assays; and urinary antigen testing. Scarring patterns characteristic of hepatosplenic and urogenital disease may be seen on ultrasonography. Praziquantel is first-line treatment.
Historical Perspective
Schistosomiasis is known as bilharzia or bilharziosis in many countries, after German physician Theodor Bilharz, who first described the cause of urinary schistosomiasis in 1851. The first doctor who described the entire disease cycle was Pirajá da Silva in 1908. It was a common cause of death for Ancient Egyptians in the Greco-Roman Period.[1]
Classification
Schistosomiasis may be classified into intestinal and urogenital schistosomiasis based on the organ involvement.
Pathophysiology
The pathogenesis of acute human schistosomiasis is mainly related to egg deposition and liberation of antigens of adult worms and eggs. A strong inflammatory response characterized by high levels of pro-inflammatory cytokines, such as interleukins 1 and 6 and tumor necrosis factor-α, and by circulating immune complexes participates in the pathogenesis of the acute phase of the disease. Schistosomes have a typical trematode vertebrate-invertebrate lifecycle, with humans being the definitive host. The life cycles of all five human schistosomes are broadly similar. Infection can occur by penetration of the human skin by cercaria or following the handling of contaminated soil. Cercaria gets transformed into migrating schistosomulum stage in the skin. The incubation period for acute schistosomiasis is usually 14-84 days. Both the early and late manifestations of schistosomiasis are immunologically mediated. The major pathology of infection occurs with chronic schistosomiasis in which retention of eggs in the host tissues is associated with chronic granulomatous injury.
Causes
Schistosomiasis is caused by Schistosoma. The major intestinal schistosomes include S.japonicum, S.mekongi, S.mansoni, S.intercalatum. The major urogenital schistosome include S.haematobium.
Differentiating Schistosomiasis from Other Diseases
Schistosomiasis must be differentiated from tapeworm infections that cause abdominal pain, fever, chills, cough, and muscle aches such as like diphyllobothriasis, hymenolepiasis, and taeniasis.
Epidemiology and Demographics
More than 600 million persons are exposed to Schistosoma parasites, 200 million persons are infected, and 20 million symptomatic cases of schistosomiasis are reported worldwide. All age groups are vulnerable to Schistosoma infection, but school-aged children and adolescents living in endemic areas tend to have the highest intensity of disease. There is no racial predilection to schistosomiasis. Schistosomiasis affects men and women equally.
Risk Factors
The most potent risk factor in the development of schistosomiasis is skin exposure to contaminated fresh water (wading, swimming, washing, or working in fresh water that is infested with cercariae). Other risk factors include travel to endemic areas.
Screening
Routine screening of travelers for schistosomiasis is not recommended. Screening is recommended only to guide mass public health treatment programs to targeted villages in endemic areas.
Natural History, Complications and Prognosis
If left untreated, most of the patients with schistosomiasis may progress to develop ulceration or cancer of the bladder, liver or kidney failure. Common complications of schistosomiasis include hematuria, malnutrition, intestinal polyps, hydronephrosis, glomerulonephritis, bladder polyps, bladder cancer, infertility, ectopic pregnancy, renal failure, and cor-pulmonale. Depending on the extent of the disease progression at the time of diagnosis, the prognosis of schistosomiasis may vary. However, the prognosis is generally regarded as good with treatment.
Diagnosis
History and Symptoms
The majority of patients with schistosomiasis in early phase are asymptomatic, unless katayama fever, an acute immune complex disease, occurs. Late Schistosomiasis is symptomatic and includes hepatosplenic schistosomiasis (presinusoidal portal hypertension), urinary and urogenital schistosomiasis (urinary obstruction, genital symptoms), schistosomal glomerulopathy (chronic immune complex deposition in the kidney), and ectopic disease in areas such as the lungs and central nervous system (CNS).
Physical Examination
Common physical examination findings of schistosomiasis include generalized lymphadenopathy, hepatosplenomegaly, rash, fever, right upper quadrant tenderness, urticaria, bloody stool.
Laboratory Findings
Microscopic identification of eggs in stool or urine is the most practical method for diagnosis. The stool exam is the more common of the two. For the measurement of eggs in the feces of presenting patients the scientific unit used is epg or eggs per gram. Stool examination should be performed when infection with S. mansoni or S. japonicum is suspected, and urine examination should be performed if S. haematobium is suspected.
Medical Therapy
Schistosomiasis is readily treated using a single oral dose of the drug Praziquantel. While Praziquantel is safe and highly effective in curing an infected patient, it does not prevent re-infection by cercariae and is thus not an optimum treatment for people living in endemic areas. As with other major parasitic diseases, there is ongoing and extensive research into developing a vaccine that will prevent the parasite from completing its life cycle in humans.
References
- ↑ "Proceedings of the 13h Annual History of Medicine Days", a medical historical paper from University of Calgary. March 2004.