Schistosoma/Schistosomiasis

Schistosomiasis is an infection caused by Schistosoma, a trematode. Schistosomiasis occurs in developing countries with poor sanitation. Freshwater snails are the intermediate host and are transmitted to humans through skin contact with contaminated fresh water. The clinical presentation occurs as a result of the host’s immune response to antigens from the eggs. Most patients are asymptomatic, but others may develop acute schistosome dermatitis, acute schistosomiasis syndrome, or chronic schistosomiasis. The diagnosis can be made with microscopic evaluation of the urine or feces, serology, or PCR. The treatment for schistosomiasis is praziquantel.

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General Characteristics and Epidemiology

General features of Schistosoma

  • Trematode 
  • Parasitic flatworm
  • Eggs:
    • Round or oval shaped
    • Hinged at 1 end
  • Adults:
    • Elongated
    • Tubular
    • 10–20 mm long
    • Males are shorter than females.
    • Oral and ventral suckers
Schistosoma eggs

Photomicrograph depicting a grouping of 3 different species of Schistosoma eggs: S. japonicum, S. mansoni, and S. haematobium.

Image: “21017” by CDC/Dr. Brodsky. License: Public Domain

Clinically relevant species

  • GI disease:
    • S. mansoni
    • S. japonicum
    • S. mekongi
    • S. intercalatum
  • Genitourinary tract disease: S. haematobium

Epidemiology

  • Worldwide annual incidence: 200–300 million
  • Men > women
  • Children and adolescents > adults
  • Mortality rate: 0.3 per 100,000 cases

Geographic distribution

  • S. mansoni:
    • Sub-Saharan Africa
    • South America
    • Caribbean
    • Middle East
  • S. japonicum:
    • China
    • Philippines
    • Thailand
    • Indonesia
  • S. haematobium:
    • Africa
    • Middle East
  • S. mekongi: Southeast Asia
  • S. intercalatum: Central and West Africa

Pathogenesis

Reservoir

  • Humans
  • Mammals:
    • Domesticated and wild animals
    • Livestock
  • Freshwater snails are intermediate hosts.

Transmission

Transmission occurs through skin contact with infected fresh water.

Host risk factors

Schistosoma occupies freshwater environments in areas with poor sewerage management.

  • Recreational exposures:  
    • Swimming
    • Playing in water or mud
  • Occupational exposures: 
    • Fishing
    • Laundry

Life cycle

  • Schistosoma larvae (miracidia) penetrate a snail (intermediate host) → asexual reproduction and maturation into cercariae → released from the snail into water
  • Cercariae penetrate human skin → become schistosomulae after shedding their forked tails
  • Migration into the bloodstream → infect the liver → maturation and reproduction
  • Adult worms move through the portal vessels to the mesenteric venules of the intestines.
  • The females lay eggs → penetrate the intestines → excreted in feces
  • Eggs hatch in water → cycle continues
  • Note: S. haematobium occupy the vesical venous plexus → eggs are excreted in the urine
The life cycle of Schistosoma

The life cycle of Schistosoma

Image by Lecturio.

Pathophysiology

Clinical disease is caused by:

  • Movement of eggs through the splanchnic venous system to:
    • Liver
    • Lungs
    • Spleen
    • Brain
    • Spinal cord
  • Immune response to proteins and carbohydrates secreted by the eggs leads to:
    • Bowel inflammation → blood loss and scarring
    • Periportal fibrosis → portal hypertension and esophageal varices
    • Bladder inflammation → pseudopolyps and bladder cancer

Clinical Presentation

Most patients are asymptomatic. Symptomatic individuals can present in several ways:

Acute schistosome dermatitis

Also known as “swimmer’s itch”:

  • Pruritic rash at the site of cercariae entry:
    • May be papular or urticarial
    • Due to a hypersensitivity reaction
  • Feet and lower legs are more often affected.
Swimmer's itch

A papular rash due to acute schistosome dermatitis:
The hypersensitivity reaction occurs when skin is penetrated by Schistosoma cercariae.

Image: “5249” by CDC. License: Public Domain

Acute schistosomiasis syndrome (Katayama fever)

Occurs due to a systemic hypersensitivity reaction 3–8 weeks after the initial infection:

  • Fever
  • Chills
  • Fatigue
  • Urticaria
  • Angioedema
  • Nonproductive cough
  • Myalgias
  • Arthralgias
  • Abdominal pain
  • Diarrhea

Chronic schistosomiasis

Presentation can vary depending on the number of eggs in the tissues, the infection site, and the immune response of the host.

Intestinal schistosomiasis:

  • Abdominal pain
  • Decreased appetite
  • Diarrhea
  • Intestinal bleeding
  • Complications:
    • Bowel strictures
    • Intestinal polyps and dysplasia

Hepatosplenic schistosomiasis:

  • Hepatosplenomegaly
  • Complications of portal hypertension:
    • Esophageal varices (bleeding risk)
    • Pulmonary hypertension (consequence of portosystemic collaterals allowing egg embolization to the lungs)
  • Note: Liver function is not compromised.

Genitourinary schistosomiasis:

  • Dysuria
  • Urinary frequency
  • Terminal hematuria (blood at the end of urine stream)
  • Hemospermia
  • Genital ulcerations
  • Complications:
    • Ureteral strictures
    • Bladder neck obstruction 
    • Hydronephrosis
    • Bladder cancer
    • Infertility (due to ovary or fallopian tube involvement)

Neuroschistosomiasis:

  • Acute myelopathy
  • Transverse myelitis
  • Intracerebral lesions
  • Multifocal encephalopathy

Diagnosis, Management, and Prevention

Diagnosis

  • Microscopic examination of urine and stool for eggs
  • Serologic testing:
    • ELISA
    • Indirect hemagglutination
    • Immunofluorescent antibody test
  • Antigen testing:
    • Schistosome circulating antigen
    • Circulating cathodic antigen
    • PCR
  • Parasite DNA testing (PCR)
  • CT and MRI may show nonspecific changes for neuroschistosomiasis.

Management

  • Anthelmintic therapy (praziquantel):
    • ↑ Calcium ion permeability in adult worms
    • Damage to the worm induces a host immune response.
  • Corticosteroids to reduce inflammation in:
    • Acute schistosomiasis syndrome
    • Neuroschistosomiasis

Prevention

Personal protection in endemic areas:

  • Avoid swimming or wading in freshwater.
  • Vigorously towel off after brief, accidental water exposure.
  • Wear protective clothing and footwear.
  • Boil bath water.

Control strategies:

  • Sanitation programs (dispose of human urine and feces away from freshwater sources)
  • Snail control
  • Community-based mass-treatment programs

Comparison of Similar Helminths

Table: Comparison of similar helminths and their associated diseases
OrganismEnterobius vermicularisToxocara canisAscaris lumbricoidesStrongyloides stercoralisSchistosoma mansoni
CharacteristicsNematodeNematodeNematodeNematodeTrematode
ReservoirHumansDogsHumans
  • Humans
  • Dogs
  • Cats
Humans
TransmissionFecal–oralFecal–oralFecal–oralSkin contact with contaminated soilSkin contact with contaminated water
Clinical
  • Pruritus ani
  • Abdominal pain and vomiting are less common.
  • Visceral larva migrans
  • Ocular larva migrans
  • Cough
  • Wheezing
  • Hemoptysis
  • Abdominal cramping
  • Nausea
  • Malnutrition
  • Cough
  • Wheezing
  • Abdominal pain
  • Diarrhea
  • Rash
  • Swimmer’s itch
  • Katayama fever
  • Chronic infections lead to granuloma formation, causing brain, lung, intestinal, and liver disease.
Diagnosis
  • Clinical
  • Cellophane tape test
  • Serology
  • Biopsy
Stool analysis
  • Stool analysis
  • Serology
  • Stool analysis
  • Serology
Management
  • Albendazole
  • Mebendazole
  • Pyrantel pamoate
  • Albendazole
  • Mebendazole
  • Albendazole
  • Mebendazole
  • Ivermectin
  • Albendazole
Praziquantel
PreventionGood hygiene
  • Good hygiene
  • Deworm dogs.
  • Proper disposal of dog feces
  • Good hygiene
  • Clean raw fruits and vegetables before consuming.
  • Wear shoes and protective clothing.
  • Improve sanitation.
  • Avoid swimming or bathing in contaminated water.
  • Drink boiled or bottled water.
  • Improve sanitation.

Differential Diagnosis

  • Gastroenteritis: inflammation of the stomach and intestines, which is commonly caused by infections from bacteria, viruses, or parasites. Clinical features include abdominal pain, diarrhea, vomiting, fever, and dehydration. Diagnostic testing with stool analysis or culture is not always required, but can help determine the etiology. Most cases are self-limited; therefore, the only required treatment is supportive therapy (fluids). 
  • Inflammatory bowel disease (IBD): chronic inflammation of the GI tract due to a cell-mediated immune response to the GI mucosa. Crohn’s disease and ulcerative colitis are types of IBD. Symptoms include diarrhea, abdominal pain, weight loss, and extraintestinal manifestations. Diagnosis includes imaging, endoscopy, and biopsy. Management involves steroids, aminosalicylates, immunomodulators, and biologic agents.
  • Viral hepatitis: liver inflammation caused by infection from a hepatitis virus. Patients may present with a viral prodrome of fever, anorexia, and nausea. Right upper quadrant abdominal pain, jaundice, and transaminitis also occur. Diagnosis is made with viral serologic testing to differentiate viral hepatitis from schistosomiasis. Management of acute hepatitis is supportive.
  • Acute pancreatitis: inflammation of the pancreas. Patients present with a sudden onset of severe epigastric pain, which is typically sharp and radiates to the back. Acute pancreatitis is associated with alcohol abuse and gallstones. Diagnosis is based on abdominal pain, lipase elevation, and/or imaging (reveals pancreatic edema). Management includes bowel rest, pain control, and IV fluid hydration.
  • Contact dermatitis: an erythematous, papular dermatitis often with areas of vesiculation (due to direct skin exposure to an offending irritant with a direct cytotoxic effect). Diagnosis is based on history and physical exam findings. Management includes avoidance of offending irritants, adoption of protective measures, and the use of emollients and moisturizers. Topical steroids are the 1st-line intervention.
  • Urinary tract infection: a wide spectrum disease ranging from simple cystitis to severe pyelonephritis. Depending on the location of the infection, patients can present with dysuria, urinary urgency/frequency, suprapubic pain, and/or fever. Urinalysis and culture establish the diagnosis. Management options include oral or IV antibiotics. 
  • Diphyllobothriasis: an intestinal parasitic infection caused by Diphyllobothrium, a cestode, and acquired by ingestion of larvae in undercooked or raw fish from cold-water lake regions. Patients present with abdominal discomfort, diarrhea, glossitis, and anemia.  Diphyllobothriasis is diagnosed with microscopic examination of the stool for ova and parasites. Management includes anthelmintic therapy such as praziqunatel.
  • Taeniasis: a tapeworm infection occurring after consumption of raw or undercooked beef or pork. Patients are generally asymptomatic, but may present with the passage of proglottids. Taenia solium presentation may rarely include seizures from neurocysticercosis. Diagnosis requires a high index of suspicion, a stool analysis (for ova and parasites), or imaging. Management includes anthelmintic therapy.

References

  1. Schistosomiasis, Fact Sheet No 115; February 2010. World Health Organization. Retrieved April 15, 2021, from http://www.who.int/mediacentre/factsheets/fs115/en/
  2. Gryseels B, De Vlas SJ. Worm burdens in schistosome infections. Parasitol Today 1996; 12:115.
  3. Chistulo L, Loverde P, Engels D. Disease Watch: Schistosomiasis. TDR Nature Reviews Microbiology. 2004. 2:12:
  4. Nicolls DJ, Weld LH, Schwartz E, et al. Characteristics of schistosomiasis in travelers reported to the GeoSentinel Surveillance Network 1997-2008. Am J Trop Med Hyg 2008; 79:729.
  5. Clerinx, J., and Soentjens, P. (2019). Schistosomiasis: Epidemiology and clinical manifestations. In Baron, E.L. (Ed.), UpToDate. Retrieved April 15, 2021, from https://www.uptodate.com/contents/schistosomiasis-epidemiology-and-clinical-manifestations
  6. Soentjens, P., and Clerinx, J. (2021). Schistosomiasis: Diagnosis. In Baron, E.L. (Ed.), UpToDate. Retrieved April 15, 2021, from https://www.uptodate.com/contents/schistosomiasis-diagnosis
  7. Soentjens, P., and Clerinx, J. (2021). Schistosomiasis: Treatment and prevention. In Baron, E.L. (Ed.), UpToDate. Retrieved April 15, 2021, from https://www.uptodate.com/contents/schistosomiasis-treatment-and-prevention
  8. Ahmed, S.B. (2020). Schistosomiasis (Bilharzia). In Bronze, M.S. (Ed.), Medscape. Retrieved April 15, 2021, from https://emedicine.medscape.com/article/228392-overview
  9. Lackey, E.K., and Horrall, S. (2021). Schistosomiasis. [online] StatPearls. Retrieved April 15, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK554434/
  10. Pearson, R.D. (2020). Schistosomiasis. [online] MSD Manual Professional Version. Retrieved April 15, 2021, from https://www.msdmanuals.com/professional/infectious-diseases/trematodes-flukes/schistosomiasis
  11. Schistosoma. (n.d.). The Australian Society for Parasitology. Retrieved April 15, 2021, from http://parasite.org.au/para-site/text/schistosoma-text.html
  12. Colley, D.G., Bustinduy, A.L., Secor, W.E., and King, C.H. (2014). Human schistosomiasis. Lancet, 383(9936), 2253–2264. https://doi.org/10.1016/S0140-6736(13)61949-2

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