Description

• Flatworm infection (trematodes) of the genus Schistosoma
• Commonly presents as a swimmer’s itch and maculopapular rash
• Considered to be a neglected tropical disease (NTD)
• Katayama fever (acute schistosomiasis) is a systemic reaction to the parasite in the bloodstream.
• Chronic disease is primarily caused by tissue migration of Schistosoma eggs. Immune response causes inflammation and scarring; primarily occurs in gastrointestinal and/or genitourinary tracts

Epidemiology

Prevalence

• >230 million people infected worldwide (1), most of whom are children.
• Schistosomiasis is endemic in Africa (primarily) but also in Asia and South America (1).

Etiology and Pathophysiology

• Schistosoma mansoni (Africa, South America), Schistosoma japonicum (China, Philippines, Indonesia), and Schistosoma haematobium (sub-Saharan Africa, Middle East) are the most common organisms in human schistosomiasis (1). Two other species may also cause disease: Schistosoma intercalatum (Central Africa) and Schistosoma mekongi (Laos and Cambodia).
• Infection occurs in warmer climates (<1,800 m elevation) due to temperature requirements of the reservoir (snails).
• Adult worms live in the human mesenteric veins (S. mansoni and S. japonicum) or perivesicular veins around the bladder (S. haematobium) (2).
• A fully matured female releases hundreds to thousands of eggs daily (2). Eggs migrate through the blood vessel walls and into the surrounding tissue by secreting proteolytic enzymes and gradually making their way into the intestinal lumen (S. mansoni and S. japonicum) and into the bladder (S. haematobium) lumen (2). Eggs are then excreted in feces or urine (2).
• On contact with fresh water, miracidia are released from the egg and seek out species-specific intermediate freshwater snail hosts (2). Within the snail, miracidia multiply asexually (2).
• After 4 to 6 weeks, free-swimming cercarial larvae are released with a lifespan of <48 hours (2).
• After contacting human skin or mucosal surfaces, cercariae penetrate through the tissue and into the bloodstream, eventually migrating to the portal vein. Over the next 4 to 6 weeks, they mature, mate, and migrate to their final destination (mesenteric or venous plexus of the bladder) (2).
• Eggs entrapped in the tissues during migration cause chronic disease through an inflammatory response that produces fibrosis and calcification (2).
• Severity of symptoms relates to the burden of infection and host immune response (2).
• Genitourinary disease (S. haematobium) is caused by irritation of the bladder and/or ureteral walls (2).
• Gross and microscopic hematuria is common, especially in children. Ureteral stenosis can cause hydronephrosis and eventual renal failure. Bladder cancer is increased in patients with schistosomal infections, either due to chronic inflammation or altered carcinogen (tobacco, etc.) susceptibility (2).
• Deposition of eggs in the female reproductive tract can lead to infertility (2).
• Hepatic periportal inflammation, especially in early disease, can cause hepatosplenomegaly. Years of chronic inflammation can lead to fibrosis, portal hypertension, and splenomegaly or varices (1).
• Neuroschistosomiasis (the most serious form of schistosomal infection) can occur when eggs or adult worms cause meningeal inflammation (2).
• Genital schistosomiasis (S. haematobium) has been associated with HIV infection in sub-Saharan African women (1).
• Egg excretion may take 40 to 50 days after initial infection.

Risk Factors

Exposure to contaminated freshwater in endemic areas

General Prevention

• Avoid drinking, bathing, or swimming in untreated freshwater in endemic areas.
• Boil water for at least 1 minute prior to drinking or use appropriately filtered water.
• Water held in storage for 48 hours may generally be used for bathing.
• Iodine treatment may not rid water of all larvae.
• Proper community-based sanitation. Control of the freshwater snails that serve as intermediate hosts is not as effective; environmental effects of chemicals used to eliminate snails can have unintended consequences (2).
• Mass treatment of high incidence populations is helpful. Retreatment is often necessary as recurrence is high (53%).
• Vaccine is currently under development.

Commonly Associated Conditions

• Salmonella coinfection (free in the body as well as sequestered in the parasite) is common and can further reduce immunologic functioning and make treatment difficult.
• Schistosomiasis infection worsens the prognosis for those infected by HIV/AIDS, TB, Hepatitis B and C, and malaria.