Schistosomiasis

Author: MBLOGSTU

Introduction

Schistosomiasis is among the oldest known infections of man. Evidence for human infection with these parasites has been found in mummified remains from ancient Egypt and China.

Historically, schistosomiasis was confined to the tropical rural poor; its geographic distribution is determined primarily by that of the snail intermediate hosts. Human infection was characteristically chronic—resulting in significant end organ damage (e.g., liver fibrosis and kidney failure) in a small subset of those infected. During the twentieth century, the geographic distribution of schistosomiasis underwent significant changes.

Epidemiology

Schistosomiasis is one of the world’s major health problems. The World Health Organization estimates that at least 200 million people in 74 countries are infected, with at least 600 million more at risk.

Species Distribution:

• S. mansoni: Endemic throughout Africa and the Middle East.
• S. haematobium: Confined to Africa and the Middle East.
• S. japonicum and S. mekongi: Found only in Asia.

Transmission depends on human contact with fresh water; the presence of a specific snail species that completes the schistosome life cycle; and contamination of the water with human waste. In endemic areas, the highest prevalence and intensity of infection occurs in adolescents (10–16 years), with males generally having higher rates due to increased water contact. Variations in prevalence can even be marked in nearby communities. Specific high-risk occupations include farming, fishing, working in irrigation canals, and domestic activities (e.g., laundry) in open bodies of water.

Clinical Manifestations

Schistosomes are well adapted to the human host and typically establish a balanced host–parasite relationship. The majority of infected individuals are asymptomatic or experience mild, nonspecific symptoms; however, severe symptoms and major clinical sequelae occur in approximately 10–15% of cases without treatment.

Acute Schistosomiasis

Most infections are asymptomatic at first; in a small percentage of patients, immediate itching and urticaria may occur at the site of cercarial penetration (a reaction sometimes called “schistosome dermatitis”) which can progress into papular lesions lasting 5–7 days.

As schistosomulae migrate in the venous and arterial systems, minor mechanical and inflammatory changes may occur in the lung and liver, but these are generally asymptomatic. As the worms mature in the liver, migrate to small venules, and begin egg-laying, a second acute syndrome (Katayama fever) may develop 3–6 weeks after heavy exposure to cercariae. Symptoms include spiking fever with chills, myalgia, headache, diarrhea, fatigue, weight loss, nausea, vomiting, and cough.

Intestinal Schistosomiasis (S. mansoni, S. japonicum, S. mekongi)

Common symptoms of intestinal involvement include nonspecific abdominal pain, malaise, and intermittent diarrhea (which may alternate with constipation). Occasionally, stools may contain blood or mucus. A severe form involves colonic polyposis—often affecting young males—and may be accompanied by bloody diarrhea, protein-losing enteropathy, hypokalemia, and severe dehydration. Sigmoidoscopy and biopsy typically reveal intense granulomatous inflammation with parasite ova. Inflammatory masses may also be present, necessitating differentiation from malignancies.

Hepatosplenic Disease (S. haematobium, S. japonicum, S. mekongi)

Patients may develop diffuse, non-tender hepatic enlargement; splenomegaly is common when sensitive methods (e.g., ultrasound) are used. Palpable splenomegaly occurs less frequently, and massive splenomegaly (extending 8–12 cm below the left costal margin) is found in less than 5% of untreated cases.

Genitourinary Involvement (S. haematobium)

Early symptoms include urinary frequency and dysuria; however, hematuria is the classic presentation. Vesical involvement features intermittent terminal hematuria, dysuria, and urinary frequency. Some patients may also experience suprapubic or perineal pain with bladder distention. Chronic infection may lead to hydronephrosis caused by granulomas in the bladder wall, ureters, or urethra, and this can eventually result in pyelonephritis, recurrent urinary tract infections, and even progress to acute or chronic renal failure or squamous cell carcinoma of the bladder.

Association with Other Infections

Schistosomiasis can be associated with other infections such as Salmonella bacteremia, hepatitis, and cancer.

Laboratory Diagnosis

Because most individuals with schistosomiasis are asymptomatic, clinicians must maintain a high index of suspicion, particularly in patients with a relevant exposure history presenting with fever, eosinophilia, hepatosplenomegaly, anemia, hematuria, obstructive uropathy, or recurrent urinary tract infections.

Stool Diagnosis

S. mansoni, S. japonicum, S. intercalatum, and S. mekongi are typically diagnosed using stool samples. Several concentration methods can be used, including:

• Formol–ether or merthiolate–iodine–formol method
• Bell filtration method using ninhydrin as the stain
• The Kato–Katz thick smear technique (most common) – utilizes glycerol to clarify stool samples so eggs are easily visualized microscopically.

Urine Diagnosis

The eggs of S. haematobium are excreted in urine with diurnal periodicity, peaking between mid-morning and mid-afternoon. Urine samples collected during this period can be concentrated by sedimentation or filtered through cellulose to quantify egg excretion.

Biopsy

When stool examinations are negative, biopsy specimens (often rectal biopsy) showing mucosal inflammation with eosinophilic infiltration and a granulomatous response surrounding a viable egg are diagnostic of schistosomiasis. Crushing the specimen between slides and examining the entire sample increases sensitivity.

Ultrasound

Ultrasound is a major advance in the diagnosis of chronic schistosomiasis and is useful to assess the extent of organ involvement.

Serodiagnosis

Antibody-based serodiagnosis plays a supportive role in confirming exposure—especially in travelers with acute schistosomiasis (when eggs may not yet be seen in stool or urine). Additionally, detection of circulating schistosomal antigens in blood or urine can help identify active infection. Because circulating antigen levels drop after parasitologic cure, this method is attractive for use in endemic regions.