eMedicine Specialties > Emergency Medicine > Infectious Diseases

Schistosomiasis

Author: Amy J Behrman, MD, Associate Professor, Department of Emergency Medicine, Director, Division of Occupational Medicine, University of Pennsylvania School of Medicine
Contributor Information and Disclosures

Updated: Apr 2, 2008

Introduction

Background

Schistosomiasis (also known as bilharzia, bilharziasis, bilharziosis or snail fever) is a human disease syndrome caused by infection from one of several species of parasitic trematodes of the genus Schistosoma. Schistosomiasis is a major source of morbidity and mortality for developing countries in Africa, South America, the Caribbean, the Middle East, and Asia. Tourism to and immigration from endemic areas can result in schistosomiasis cases presenting anywhere in the developed world. Acute schistosomiasis among travelers may be increasing, and rates of emigration from endemic areas are increasing. In the complete absence of routine pre-symptomatic screening of these groups in developed countries, it is increasingly likely that patients with acute or chronic schistosomiasis will present to EDs with a variety of complaints.

Most human schistosomiasis is caused by Schistosoma haematobium, Schistosoma mansoni, or Schistosoma japonicum. Less prevalent species such as Schistosoma mekongi and Schistosoma intercalatum may also cause systemic human disease. Less importantly, other schistosomes with avian or mammalian primary hosts can cause severe dermatitis in humans (eg, swimmer's itch secondary to Trichobilharzia ocellata).

The geographic distribution and pathophysiology of schistosomiasis reflect the unique life cycle of these parasites. Schistosomes infect susceptible freshwater snails in endemic areas, usually with specific species of schistosomes infecting specific species of snails. The infected snails release cercariae 4-6 weeks after infection. The cercariae are fork-tailed free-swimming larvae approximately 1 mm in length. They can survive in freshwater up to 72 hours, during which time they must attach to human skin or to that of another susceptible host mammal or die.

Successful cercariae attach to human hosts utilizing oral and ventral suckers. They then migrate through intact skin to dermal veins and, over the next several days, to the pulmonary vasculature. During this migration, the cercariae metamorphose, shedding tails and outer glycocalyces while developing double-lipid-bilayer teguments that are highly resistant to host immune responses.

The organisms, now called schistosomula, incorporate host proteins, including major histocompatibility complexes (MHCs) and blood group antigens, in their integuments. Their metabolism shifts to glycolysis. The worms then migrate through the pulmonary capillaries to the systemic circulation, which carries them to the portal veins where they mature. Within the portal vasculature, male and female adults pair off, with the thin female entering and remaining in the gynecophoric canal of the stockier 8-mm male worm. Together they migrate along the endothelium, against portal blood flow, to the mesenteric (S mansoni, S japonicum) or vesicular (S haematobium) veins where they begin to produce eggs.

The eggs, which are highly antigenic and can induce an intense granulomatous response, migrate through the bowel or bladder wall to be shed via feces or urine. During this time (approximately 10 d) they begin to mature into miracidia. Eggs that are not shed successfully may remain in the tissues or be swept back to the portal circulation (from the mesenteric vessels) or to the pulmonary circulation (from the vesicular vessels via the inferior vena cava [IVC]).

The free-swimming miracidia that are shed into freshwater survive 1-3 weeks, during which time they must infect a susceptible snail to complete the life cycle. Within the infected snail, 2 generations of sporocysts multiply, mature into free-swimming cercariae, and exit the snail to seek a human host and begin a new cycle.

Pathophysiology

Acute schistosomiasis

Acute schistosomiasis (Katayama fever) is a systemic, serum sicknesslike illness that develops after several weeks in some, but not most, individuals with new schistosomal infections. It may correspond to the first cycle of egg deposition and is associated with marked peripheral eosinophilia and circulating immune complexes. It is most common with S japonicum and S mansoni infections and is most likely to occur in heavily infected individuals after primary infection. Symptoms usually resolve over several weeks, but the syndrome can be fatal. Early treatment with cidal drugs may exacerbate this syndrome and necessitate concomitant glucocorticoid therapy. Tourists and travelers are most likely to present to EDs in this country with this syndrome; a history of contact with freshwater such as swimming, boating, rafting, or water skiing should be obtained.

Mild maculopapular skin lesions may develop in acute infection within hours after exposure to cercariae. Significant dermatitis is rare with the major human schistosomal pathogens, probably because the invading and developing cercariae are minimally immunogenic. However, abortive human infection with schistosomal species that rely on other primary hosts may cause marked dermatitis or swimmer's itch. This self-limited process may recur more intensely with subsequent exposures to the same species.

Chronic schistosomiasis

The pathology of chronic schistosomiasis, which is far more common than the acute form of the infection, results from egg-induced immune response, granuloma formation, and associated fibrotic changes. Although cercarial and adult worms are minimally immunogenic, schistosomal eggs are highly immunogenic and induce vigorous circulating and local immune responses. (Eggs may require an intense immune response to aid their migration through the body.)

Egg retention and granuloma formation in the bowel wall (usually S mansoni or S japonicum) may cause bloody diarrhea, cramping, and, eventually, inflammatory colonic polyposis. Patients with heavy bowel wall involvement have an increased rate of recurrent salmonellal infection, generally with positive blood cultures and negative stool cultures.

Unshed eggs, which are swept back to the portal circulation, lodge there and induce granulomatous reactions in the portal tracts. Heavy infestations are more likely to produce hepatic disease. Eventually, severe periportal fibrosis in a characteristic pipestem pattern ("Symmer's pipestem fibrosis") may occur. Although hepatocellular function is spared, periportal fibrosis can lead to portal hypertension with the usual potential sequelae, including splenomegaly, ascites, esophageal variceal bleeding, and development of portosystemic collaterals. Through these collaterals (or directly from the IVC in the case of bladder wall schistosomiasis), eggs can reach the pulmonary circulation. The resulting pulmonary granulomatosis and fibrosis can lead to pulmonary hypertension and frank cor pulmonale with a high mortality rate. Co-infection with hepatitis B or hepatitis C can accelerate hepatic dysfunction and raise the risk for hepatocellular carcinoma beyond that seen with hepatitis alone. In addition,gallbladder cancer may be associated with schistosomal infection.

Egg retention and granuloma formation in the urinary tract (S haematobium) can lead to hematuria, dysuria, bladder polyps and ulcers, and even obstructive uropathies. S haematobium infection is also associated with an increased rate of bladder cancer, usually squamous cell rather than transitional cell.

Ectopic egg deposition can lead to additional clinical syndromes including involvement of skin, lung, brain, muscle, adrenal glands, genitalia, and eyes. Central nervous system involvement can result in transverse myelitis (best described for S haematobium and S mansoni) and/or cerebral disease (most common with S japonicum infection).

Frequency

United States

Acute and chronic schistosomiasis infections are not common in the United States. Although it is estimated that 400,000 infected persons have immigrated to this country, neither susceptible snail species nor chronically infected human reservoirs sufficient to infest freshwater exist. However, pathogenic schistosomes can survive and replicate in human hosts for years and even decades. Therefore, persons who have traveled or immigrated may present to EDs with active cases of acute or chronic schistosomiasis and/or the associated end-organ complications described.

International

Globally, schistosomiasis is a major source of morbidity and mortality. The unique schistosomal life cycle limits endemic areas to tropical and subtropical zones, but these areas exist around the world. Geographic spread continues because of water resource engineering issues in developing countries and the migration of infected populations. At least 200 million people in at least 74 countries have active schistosomal infection.¹ Of these, approximately 120 million people have symptoms, and 20 million are severely ill. Disease prevalence is heterogeneous in vulnerable locales and tends to be worse in areas with poor sanitation, increased freshwater irrigation usage, and heavy schistosomal infestation of human and/or snail populations.

Over the past decade, targeted interventions combining snail control, improved water supply quality, and treatment of infected persons, particularly children, have shown success in diverse endemic areas including China, Brazil, Egypt, and, most recently, some areas of sub-Saharan Africa. Nevertheless, the human cost of schistosomal infections remains high, and schistosomiasis contributes to comorbidity with other infections such as hepatitis, human immunodeficiency virus (HIV), and malaria in endemic regions.
 
Schistosomal species vary with geographic region: S mansoni and S haematobium infections predominate in sub-Saharan Africa. S mansoni is endemic in parts of South America and the Caribbean. S japonicum is common in China, Indonesia, and the Philippines. Although freshwater lakes and streams are usually identified as the source, man-made reservoirs and irrigation systems are increasingly implicated in some countries.

Mortality/Morbidity

• Acute schistosomiasis is associated with a mortality rate of up to 25% in some series. Although most individuals with chronic schistosomiasis have few or no symptoms, significant morbidity can develop.
• Complaints are difficult to quantitate because of the geographic distribution of this infection in underdeveloped nations and the frequency of comorbid conditions such as hepatitis. Hepatosplenic disease with portal hypertension is the most common long-term serious outcome, followed by cardiopulmonary involvement, obstructive nephropathy, bacteremia, and malignancy. Female genital infection can contribute to pregnancy complications including reports of related ectopic pregnancy.
• End-stage hepatosplenic disease with variceal bleeding, pulmonary hypertension with cor pulmonale, and central nervous system disease are associated with high mortality rates. Carcinoma of the urinary tract, liver, and gallbladder may cause death. Although effective antihelminthic treatment exists, it may not reverse fibrosis and may not be readily available in endemic areas.

Race

• The frequency of infection among individuals of specific races is based on the geographic distribution of endemic schistosomiasis in large tropical and subtropical regions of Africa, Asia, the Middle East, and the Caribbean. However, all humans appear equally susceptible if exposed to infested freshwater.
• The frequency of some complications appears to vary geographically during infection with the same worm species (eg, ascites is more common in the Middle East than in Brazil).

Sex

• In some endemic areas, rates of symptomatic infection vary between the sexes. This appears to reflect lifestyle differences between men and women regarding the likelihood of exposure to contaminated freshwater by activities such as bathing, working on irrigation equipment, or drinking from streams or irrigation ditches.
• S haematobium causes genital lesions in 30% of women who are infected. Vulval lesions may increase the risk of HIV transmission.

Age

The prevalence and severity of schistosomal infections vary with age.

• Children and adolescents are infected most often and are infested most heavily. As noted above, infection rates and severity may vary with gender-specific activity at all ages.
• Infections peak in individuals aged 10-19 years. In some areas, the prevalence in this group may approach 100%.
• In persons older than 19 years, the prevalence of active infection and egg counts slowly declines in populations living in endemic areas. (End-stage complications may persist or worsen.) This decline in active infection may reflect that individuals have an increasing host immune response or a decreasing exposure to contaminated water as they age. Reinfection, particularly after high exposure, is possible.

Clinical

History

• Patients with acute schistosomiasis (Katayama fever) present several weeks after contact with infested water. Obtaining a careful travel history, including drinking water sources and recreational activities, is important. Symptoms are likely secondary to immune complex formation following egg deposition in tissues; the illness resembles serum sickness.  
  • Fever
  • Headache
  • Malaise
  • Arthralgias/myalgias
  • Cough
  • Bloody diarrhea
  • Right upper quadrant (RUQ) pain
  • Rash - Urticarial and/or papular (Rash is the only symptom in patients with cercarial dermatitis due to exposure to less pathogenic species.)
• Patients with symptomatic chronic schistosomiasis may present months to years after primary exposure. A complete lifelong travel history is important for diagnosis. Many patients do not have a clear history of acute schistosomiasis. Many patients have few or mild symptoms. Individuals with symptoms may present with nonspecific complaints reflecting their level of infection, the primary location of egg production for the schistosomal species involved (eg, mesenteric, bladder wall), the extent of hepatosplenic involvement, the extent of cardiopulmonary involvement, and the presence of ectopic sites (eg, CNS). Those who present to the ED setting are most likely to complain of the following:  
  • Bloody diarrhea
  • Abdominal pain, RUQ pain, cramping
  • Hematemesis (with portal hypertension)
  • Ascites (with portal hypertension)
  • Hematuria, dysuria
  • Vulvar or perianal lesions
  • Dyspnea on exertion (with pulmonary hypertension)
  • Fatigue (with pulmonary hypertension)
  • Cough (with pulmonary hypertension)
  • Palpitations (with pulmonary hypertension)
  • Atypical chest pain (with pulmonary hypertension)
  • Seizures and/or mental status changes (with cerebral involvement)
  • Paralysis (with spinal cord involvement)

Physical

Physical findings vary with the stage of illness, worm burden, worm location, and end-organ involvement.

• Acute schistosomiasis 
  • Fever
  • Hepatosplenomegaly
  • RUQ tenderness
  • Urticaria
  • Lymphadenopathy
  • Bloody stool
• Chronic schistosomiasis 
  • Hepatosplenomegaly
  • Abdominal tenderness
  • Heme-positive stool and/or bloody diarrhea
  • Ascites
  • Pedal edema (with right heart failure)
  • Seizures, focal neurologic signs, and/or altered mental status (with cerebral infection)
  • Paralysis (with spinal core involvement)
  • Vulvar or perianal lesions, which may be hypertrophic, ulcerated, or fistulous (Lesions may mimic sexually transmitted diseases.)

Causes

Most human schistosomiasis is caused by S haematobium, S mansoni, or S japonicum. Less prevalent species, such as S mekongi and S intercalatum, may also cause systemic human disease.

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