What is schistosomiasis?


What is schistosomiasis?

What is schistosomiasis?

Schistosomiasis is a tropical disease caused by a parasite. The parasite is transmitted through contact with fresh water contaminated with the parasite’s larvae.

What is schistosomiasis?

  • Schistosomiasis (also known as bilharzia) is a tropical disease affecting around 240 million people worldwide.
  • It is caused by parasitic worms called schistosomes. These live in fresh water, such as rivers and lakes, in sub-tropical and tropical regions of the world.
  • There are three main species of schistosome that cause schistosomiasis in humans:
    • Schistosoma haematobium
    • Schistosoma mansoni
    • Schistosoma japonicum

Eggs from the three species of schistosoma that cause disease in humans. From left to right: Schistosoma mansoni, Schistosoma haemotobium, Schistosoma japonicum.
Image credit: Imgbuddy

  • Other species of schistosome can cause disease in other animals. For example, Schistosoma bovis infects domestic cattle, impacting on their health and commercial productivity.
  • People become infected with schistosomes if they come into contact with the larval forms of the parasite, for example, when washing or playing in contaminated fresh water.
  • Schistosomiasis particularly affects poor communities without clean water, adequate sanitation or readily-available medical treatment. These factors put them at a higher risk of developing schistosomiasis.
  • In sub-Saharan Africa, more than 200,000 deaths per year are due to schistosomiasis.

Children bathe in a lake contaminated with schistosome larvae in Zambia.
Image credit: Wellcome Photo Library

How is schistosomiasis transmitted?

  • Individuals get infected when they come into contact with fresh water contaminated with the schistosome parasite.
  • Fresh water becomes contaminated with the parasite when individuals infected with schistosomiasis urinate or defecate in the water, releasing the parasite’s eggs, which later hatch.
  • The parasite larvae then develop and multiply inside different freshwater snails, including Bulinus and Biomphalaria species, before re-entering the water.
  • The parasite can survive for up to 48 hours after it enters the water. During this time it swims around looking for a human host.
  • When the schistosome parasite finds a host in the water, it burrows into their skin.
  • Within several weeks the parasite matures and the females produce eggs. Some of these eggs can then migrate to the bladder or intestine where they are passed into the urine or faeces and excreted.
  • The highest levels of schistosome transmission are in communities living near freshwater lakes and rivers.
  • In South America and Africa, migration to urban areas is introducing schistosomiasis to new locations.
  • Increasing population sizes is also leading to higher rates of transmission as demand for water increases with more people sharing limited freshwater supplies.

Biomphalaria snails, one of the species of freshwater snail that act as an intermediate host for schistosome larvae.
Image credit: Oregon State University via Flickr

Schistosomiasis life cycle

  • The schistosome eggs are excreted into the water system in the urine or faeces of an infected human.
  • The eggs hatch to release the free-swimming larval stage of the parasite, called miracidia, into the surrounding water.
  • The miracidia burrow into the tissue of a small, freshwater snail such as Biomphalaria.
  • In the snail, the miracidia undergo several rounds of asexual replication and mature into the next larval stage, called cercariae. These are then released back into the water where they swim around looking for a human host.
  • Once the cercariae come into contact with a human host they burrow into the skin. The cercariae lose their tails while burrowing into the skin and become schistosomulae.
  • These schistosomulae then enter the blood stream and migrate in the blood through the lung to the person’s liver where they mature into adult worms.
  • The adult worms then pair with a mate of the opposite sex and move along blood vessels against the blood flow to the blood vessels near the bowel, rectum or bladder.
  • Once they reach the blood vessels near the bowel, rectum or bladder the worms begin to lay their eggs. About half of these eggs will be released in the human’s urine and faeces but some will stay in the body and circulate back to the liver where they cause inflammation.
    • Schistosoma mansoni and Schistosoma japonicum generally migrate to the bowel or rectum and release eggs in the faeces.
    • Schistosoma haematobium tends to migrate to the bladder and release eggs in the urine.

Illustration showing the life cycle of the schistosome parasite.

Illustration to show the life cycle of the schistosome parasite.
Image credit: Genome Research Limited

What are the symptoms of schistosomiasis?

  • The symptoms of schistosomiasis are caused by the body’s immune system reacting to the schistosome eggs.
  • The eggs can lodge in a number of areas around the body causing inflammation (swelling). This can then lead to the formation of tissue masses called granulomas and stiffening of the tissue called fibrosis. This can lead to a range of symptoms depending on which organ they are located in.
  • Schistosomiasis has both an ‘acute’ and a ‘chronic’ (meaning long-term/persistent infection) phase.
  • In acute schistosomiasis, symptoms are caused by the immune system reacting to the parasite and are generally short-term and mild.
  • Symptoms of acute schistosomiasis can develop a few weeks after the schistosome parasite first burrows into the skin of its human host. These symptoms are in response to the first parasite eggs getting trapped in the liver and spleen.
  • These acute symptoms are generally flu-like, including a high temperature and muscle aches, but can also include a skin rash, cough and abdominal pain.
  • Sometimes acute schistosomiasis symptoms can be so mild they go unnoticed.
  • In areas of the world where individuals do not have access to medical care and already have a weak immune system schistosomiasis can become chronic. This means that the schistosomiasis persists for a long time in the body.
  • Chronic schistosomiasis can develop months or even years after the initial infection and cause long-term health problems. Damage to the organs after chronic infection is irreversible.
  • Symptoms of chronic schistosomiasis result from the inflammation and scarring of various tissues and organs caused by the body’s immune response to the schistosome eggs:
    • Pain when urinating – schistosome eggs lodged in the urinary tract cause inflammation and result in symptoms similar to urinary tract and bladder infections. In rare cases this has been found to predispose individuals to bladder cancer. In females, chronic urinary tract infections are also associated with damage to the cervix, fallopian tubes and vagina.
    • Blood in the urine – this is the result of a severe bladder inflammation caused by eggs lodged in blood vessels in the wall of the bladder.
    • Bloody diarrhoea – schistosome eggs can lodge in the blood vessels of the intestine causing inflammation, and, consequently, diarrhoea.
    • Chest pain – this is the result of larval parasites moving though the lung tissue and later schistosome eggs becoming trapped in the blood vessels that supply the lungs. These result in inflammation, granuloma formation and fibrosis.
    • Liver failure –the immune response to schistosome eggs lodged in the blood vessels supplying the liver causes the formation of granulomas and scar tissue. These can eventually stop the liver from performing its normal functions leading to a build-up of toxins in the body.
    • Seizures (such as a stroke or fit) and paralysis – this can be the result of schistosome eggs lodging in the spinal cord or brain, causing inflammation.
  • Children who are repeatedly infected with schistosomiasis (due to constant exposure to contaminated water) can develop anaemia, malnutrition and learning disabilities.
  • The intensity and prevalence of schistosomiasis infection usually rises with age, peaking at around 15-20 years old.
  • As people get older, although the prevalence of infection tends to stay the same, the number of parasites in the body (parasite burden) has been seen to decrease.

Schistosome eggs (four ovals in middle) in the bladder surrounded by a granuloma (dark pink area).
Image credit: Wellcome Images

How is schistosomiasis diagnosed?

  • There are a number of tests that can be used to diagnose someone with schistosomiasis:
    • Under a microscope, urine and faeces samples can be studied for the presence of live schistosome eggs.
    • Blood tests can show if an individual has anaemia or if their liver or kidney function has been affected. These may be signs of schistosomiasis.
    • A chest X-ray can show if the lungs are damaged by fibrosis and inflammation which may be due to parasite larvae migrating to the lungs or eggs getting trapped in lung tissue.
    • An ultrasound scan can show if there is damage to the liver or heart. This damage may be due to schistosome eggs migrating to these organs and causing inflammation.
    • A colonoscopy (looking at the bowel with a camera) or cystoscopy (looking at the bladder with a camera) can be used to show if eggs or inflammation are visible in the bladder or bowel.

How is schistosomiasis treated?

  • Almost all people who receive treatment for schistosomiasis will get better.
  • The primary drug for treating all species of schistosomiasis is called praziquantel.
  • Praziquantel is the frontline treatment for schistosomiasis and has few side effects.
  • It is given in tablet form and taken as a single or double dose on the same day.
  • Praziquantel kills the adult worms by causing severe spasms and paralysis in their muscles. Their remains are then broken down naturally by the body.
  • The parasites can be killed at any time after they become adult worms (usually six weeks after infection), including in patients with chronic infection.
  • If treated too early, while the parasite is still immature, praziquantel is not effective at clearing infection.
  • Older drugs are still widely used in the treatment of schistosomiasis. These include metrifonate, which is effective against S. haematobium, and oxamniquine, which is effective against S. mansoni.
  • Mass treatment of entire communities is not considered cost-effective unless there is evidence that at least 75 per cent of the population is infected.

How can schistosomiasis be prevented?

  • Schistosomiasis is difficult to avoid in communities with poor sanitation and limited clean water for drinking and bathing.
  • Screening of whole communities can be carried out by examining samples of urine or faeces under the microscope for evidence of schistosome eggs.
  • The risk of schistosomiasis can be reduced by improving water quality through piped drinking water supplies and more efficient sewage disposal. It can be further reduced by educating communities to avoid swimming in freshwater rivers and lakes.
  • The World Health Organization (WHO) has a strategy to help control the parasite in high-risk countries. This involves regular treatment of at risk groups with antihelminthic drugs such as praziquantel, albendazole and ivermectin, used either alone or in combination. This is called preventative chemotherapy.
  • Water supplies can also be treated with chemicals to reduce the number of snails, thus removing the ‘intermediate host’ that allows the parasite to develop into cercariae. However, there is a risk that this will harm other species of animal in the water.
  • There is currently no vaccine available to prevent schistosomiasis.

A poster from 1945 used by the United States War Department to inform soldiers about how to avoid Snail Fever (Schistosomiasis).
Image credit: US National Library of Medicine

This page was last updated on 2021-07-21

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