Schistosomiasis (bilharziasis)

Causative agent

• Schistosoma mansoni, S. haematobium, S. japonicum and more rarely S. mekongi and S. intercalatum

Distribution

• The risk of infection is greatest in Africa.
• Schistosoma mansoni is encountered in Africa, the Middle East as well as Central and South America, S. haematobium in Africa and the Middle East and S. japonicum in the East and Southeast Asia. S. mekongi (Southeast Asia) and S. intercalatum (Africa) are rarer.

Transmission

• Human infection with Schistosoma species is acquired after swimming or wading in freshwater that harbours cercariae (larvae), which are able to penetrate intact human skin. Once inside the body, the cercariae travel to the lungs and further to the liver. From the liver the worms travel either to the venous plexus of the urinary bladder (S. haematobium) or to the mesenteric veins (other Schistosoma species). There they lay eggs and may live for prolonged periods. Some of the eggs remain in the tissues and cause an immunological reaction that slowly damages the target tissue.
• The parasite eggs are shed into freshwater via the urine and faeces of an infected human. After maturing inside freshwater snails, the parasites are released back into the water as infectious cercariae.

Significance worldwide

• Schistosomiasis is, after malaria, the second most prevalent tropical disease in the world.
• More than 700 million people live in areas endemic for schistosomiasis.
• 240 million cases in sub-Saharan Africa, more than 200 000 deaths annually

Symptoms

• Cercarial dermatitis is a rare pruritic maculopapular allergic rash that may occur within a few hours of skin penetration.
• Acute schistosomiasis (Katayama's fever) is associated with the first infection by S. mansoni and particularly by S. japonicum.
  • Acute schistosomiasis is a hypersensitivity reaction that occurs 4–6 weeks after infection, i.e. at the time of initial egg release.
  • The symptoms include: fever, generalised symptoms, asthmatic symptoms, urticaria and diarrhoea, enlarged lymph nodes, hepatosplenomegaly and severe eosinophilia.
  • Usually the symptoms disappear in a few weeks even without treatment, but in the case of S. japonicum the symptoms may prove fatal without treatment.
• Chronic schistosomiasis starts 3–6 months after infection. The character of the symptoms depends on the species of schistosome causing the infection and the severity of infestation. Repeated exposure increases the number of worms in persons living in endemic regions.
• S. haematobium causes haematuria and urinary tract symptoms. The bladder wall will gradually become fibrosed and calcified, and its elasticity will diminish. Possible complications include hydroureter, hydronephrosis, renal failure and urinary bladder cancer. The infection may also cause different kinds of damage to the genital area.
• S. mansoni causes lethargy, abdominal pains and chronic, sometimes bloody diarrhoea. Hepatomegaly is possible, and some patients will develop an enlarged spleen, bleeding oesophageal varices or liver failure. Pulmonary fibrosis is also possible.
• S. japonicum causes symptoms similar to those caused by S. mansoni. The disease progress, however, is faster and the risk of liver fibrosis is greater. In the Far East, schistosomiasis is an important cause of epilepsy.

Diagnosis

• The diagnosis of acute schistosomiasis is based on clinical presentation and antibody detection from the blood; an antibody test will become positive about 4 weeks after infection.
• Schistosoma antibodies are included in the serum screening test for anti-helminth antibodies that is used for investigating eosinophilia. No eggs are present in the acute phase.
• Identification of eggs in urine, faeces or an endoscopically obtained tissue biopsy is used to diagnose chronic schistosomiasis.
• Haematuria is suggestive of an S. haematobium infection as are, in some cases, urinary tract ultrasound and x-ray findings; eggs may also be detected in the urine or a cystoscopically obtained biopsy sample.
• In chronic S. mansoni and S. japonicum infection, eggs are present in a stool parasite sample and a rectal biopsy sample even when the mucous membranes appear macroscopically normal.
• Eosinophilia is normal particularly in the early phases of the infection; it often disappears as the disease becomes chronic. After treatment, eosinophilia normally increases initially and disappears after a few months. This may be used both to confirm diagnosis and to monitor treatment response.

Treatment and prognosis

• A specialist in infectious diseases should be consulted regarding treatment.
• Acute schistosomiasis
  • Glucocorticoids in severe symptoms
  • Specific treatment with praziquantel (may require special license) only when eggs can be detected. It may be necessary to repeat the treatment even several times with 6–8-week intervals.
• Chronic schistosomiasis
  • Praziquantel 40 mg/kg, in S. japonicum infection 60 mg/kg, as a single dose (divided into two portions administered at an interval of 4–6 hours)
• Treatment result is checked after 3 months by examining a stool or urine sample for the presence of worm eggs, and possibly also a mucous membrane biopsy sample (urinary bladder or rectum). Blood eosinophilia concentration should also be checked.
• The changes are usually reversible when the treatment has been given early enough.

Prevention

• Prevention of water contamination with human faeces
• Destruction of the intermediate snail hosts
• Reduction of human contact with untreated fresh water in endemic regions
• Regular treatment of infected individuals
• Vigorous towel drying immediately after a brief exposure to water may help to reduce the risk of infection.

Related resources

• Cochrane reviews
• Other Internet resources
• Literature