What is Schistosomiasis?
A Parasitic Disease Acquired From Fresh Water and Spread by Snails
Jun 1, 2007
What is schistosomiasis?
Schistosomiasis is a disease caused by tiny parasites that live in the blood. The worms themselves don’t cause significant chronic health problems; however, females produce hundreds, sometimes thousands of eggs a day, and it’s these eggs that cause serious disease.
In the tiny blood vessels around the urinary bladder and intestine, Schistosoma eggs leave the blood and move through the tissue into the bladder and intestine and are then passed from the body—this is how the infection spreads to others. Many eggs, however, get swept away with the flow of blood and are deposited in other parts of the body, like driftwood. They obstruct blood flow and do damage to organs such as the liver, kidneys, heart, brain, and even reproductive organs. More than 200,000,000 people are infected with schistosomes globally, and up to 800,000 die of schistosomiasis each year.
What are schistosomes?
Schistosomes are flukes, or trematodes. Several different species infect humans and they are acquired through contact with water that has been contaminated by human feces or urine:
- When human feces or urine that contains schistosome eggs enters fresh water, the eggs hatch releasing larvae called miracidia. Miracidia search in the water for particular species of freshwater snails.
- If they find a suitable snail, miracidia invade the snail’s body and begin to multiply inside it. Eventually, many tiny parasites called cercariae are released back into the water.
- If the cercariae find a human washing, wading, swimming, fishing, or doing anything else in the water, they have found their next host. They attach themselves to the skin and creep around for a short period of time.
- Cercariae soon penetrate the skin and enter the blood, traveling to the liver, where they spend time maturing into adult worms.
- Schistosoma worms travel to the blood vessels around the intestine or bladder—each species has a distinct preference for a particular location.
- Male and female worms pair up and females produce characteristic eggs with spines.
- Eggs move through the tissue and into the bladder or intestine and are passed in urine or stool. If they arrive in fresh water, they will hatch and start the cycle all over again.
Where are schistosomes found in the body?
Three species of Schistosomes cause the majority of schistosomiasis in humans:
- Schistosoma mansoni worms live in the blood vessels around the large intestine.
- Schistosoma japonicum worms live in the blood vessels around the small intestine.
- Schistosoma haematobium worms inhabit blood vessels near the bladder.
- The cercariae of schistosomes that normally infect birds will penetrate the skin of human swimmers, causing “swimmers itch,” but these parasites do not develop further in a human host.
What are the symptoms of schistosomiasis?
A few worms will often cause no symptoms. Some people experience an itchy rash after skin penetration, particularly if there has been prior exposure; however many people experience no symptoms until egg production begins. Acute schistosomiasis is characterized by bloody diarrhea with fever, chills, headache, muscle aches, abdominal pain and fatigue. In prolonged disease, sufferers experience bloody feces or urine, and organ damage depending on where eggs have accumulated and affected blood flow and organ function—intestinal damage, liver disease, enlargement of the spleen, brain damage, right-sided heart failure, high blood pressure etc. In populations where schistosomiasis occurs regularly, many people experience few overt symptoms beyond bloody urine or stool and a profound fatigue.
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Sources:
Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Clinical Parasitology 9th ed. Philadelphia: Lea and Febiger, 1984.
Garcia, Lynn S. and David A. Bruckner. Diagnostic Medical Parasitology 3rd ed. Washington: ASM Press, 1997.
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
