Schistosomiasis

Background

Schistosomiasis (also known as bilharzia) is caused by infection with parasitic worms called schistosomes. Humans are the primary hosts of 3 major species of schistosomes:

• S. mansoni and S. japonicum – cause bowel and liver complications.
• S. haematobium – mainly causes kidney and bladder complications, occasionally causes liver disease, and rarely causes skin disease.

Humans usually become infected via contact with freshwater lakes and streams in endemic areas:

• S. mansoni – parts of South America, the Caribbean, sub-Saharan Africa, and the Middle East
• S. haematobium – sub-Saharan Africa, North Africa, the Middle East, Turkey, and India
• S. japonicum – China, Indonesia, Thailand, and the Philippines

Schistosomiasis is an extremely common illness in endemic areas. It is estimated that more than 200 million people worldwide are infected. Although the majority of infected people are symptom-free, schistosomiasis causes more than 200,000 deaths per year worldwide.

Other schistosomes can also cause disease in humans (swimmer's itch – see below).

Schistosome lifecycle

Schistosomiasis is passed on by infected people urinating or defaecating into freshwater. Schistosoma eggs hatch in the water and develop inside particular snail species. These snails release thousands of larvae back into the water. Larvae can penetrate unbroken human skin when humans enter the water to bathe or swim. Within a few weeks, worms grow inside blood vessels and produce eggs. Some of these eggs migrate to the bladder or bowel and are released in the urine or faeces. Other eggs become trapped in body tissues, producing an immune reaction.

Clinical features

Schistosomiasis can cause acute and chronic complications.

Acute disease

• Also called Katayama fever – this is a systemic (whole body) hypersensitivity reaction which develops several weeks after exposure in some individuals. Symptoms include fever, muscle and joint aches, cough, diarrhoea, enlarged liver and spleen, and headache. Symptoms usually resolve without treatment over several weeks, but occasionally can be fatal.
• Human schistosomes can produce a rash within hours to days of penetration of the skin. The rash is usually mild with redness, urticaria, and/or itchy papules (small bumps). However, in many people skin penetration is symptom-free.
• “Swimmer's itch” can occur following penetration of the skin by non-human schistosomal larvae. These larvae can cause a severe dermatitis on areas of the body that have been under water. The rash is itchy, maculopapular (small red bumps), and may become vesicular (small fluid-filled blisters). Nonhuman schistosomes are unable to complete their lifecycle so this infection resolves spontaneously with no long-term complications. The reaction is more intense if repeat exposure occurs.

Chronic disease

Worldwide chronic schistosomiasis is far more common than the acute illness. Chronic symptoms arise months to years after exposure. Chronic features are due to an intense immune response against the schistosomal eggs, with granuloma (ball-like collection of immune cells) formation and scarring.

• Bowel involvement causes bloody diarrhoea, chronic pain, and iron deficiency anaemia. Polyps (small growths) may develop due to granuloma formation in the bowel wall.
• Granuloma formation in the liver can cause portal hypertension (raised blood pressure within the liver). This can lead to an enlarged spleen, ascites (fluid in the abdominal cavity), and oesophageal varices (bleeding in the oesophagus). Collateral blood vessels can form, which redirect blood flow from the liver to the lungs and allow egg migration. Granuloma formation in the lungs causes pulmonary hypertension (raised blood pressure within the lungs) and cor pulmonale (heart failure).
• Urinary tract involvement can cause bloody urine, pain when urinating, bladder polyps, obstruction of urine flow leading to kidney damage, and an increased risk of developing bladder cancer.
• In rare cases eggs have migrated to the skin producing firm red papules that can present some months to years after exposure to schistosomes.
• Eggs can also be deposited and produce complications in almost any organ, such as lungs, brain, spinal cord, muscles, genitals, and eyes.

Diagnosis of schistosomiasis

• Blood serology (detection of antibodies against schistosomiasis) and PCR testing (detection of schistosomal DNA) can confirm a diagnosis of schistosomiasis.
• Microscopic examination of the urine, stools, or biopsies from skin lesions or other tissues may reveal schistosomal eggs.
• Pitfalls – current diagnostic tests are unreliable in acute illness. Furthermore, serology cannot distinguish between a past infection and current disease.

Treatment of schistosomiasis

• All individuals with schistosomiasis should receive treatment even if they are symptom-free.
• The anthelmintic, praziquantel, is the drug most commonly used worldwide for the treatment of schistosomiasis (but is not registered in New Zealand).
• Severe Katayama fever may require treatment with systemic corticosteroids to manage the acute hypersensitivity reaction. Praziquantel is less effective for Katayama fever and can worsen symptoms due to antigen release. It may be useful to either delay treatment with praziquantel therapy or co-administer with corticosteroids and give a second course of praziquantel after 3-6 months.

Prevention of schistosomiasis

There is no vaccine currently available for schistosomiasis.

• Travellers should avoid exposure to freshwater in endemic areas – avoid bathing in ponds and lakes.
• Bath water should be heated for 5 minutes at 65°C. Water held in a storage tank for at least 48 hours should be safe for showering.
• Drink bottled water, or water that has been boiled for at least one minute.