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The National Academies: What You Need To Know About Infectious Disease

What You Need To Know About Infectious Disease


Schistosomiasis, also called bilharzia, is a disease caused by a genus of trematodes, Schistosoma—commonly known as blood flukes. These parasitic worms that live in certain types of freshwater snails. The snails become infected when Schistosoma eggs hatch in freshwater contaminated by urine and feces from infected people and attach themselves inside the snails, where they begin to develop and multiply. It is also possible that the parasite has been spread through irrigation and dam-building projects designed to manage the flow of freshwater. At a certain point, the larval form of the parasite leaves the snail and enters the water, where it can survive for about 48 hours. 

When people come in contact with contaminated water—from swimming, bathing, washing, or wading, for example—they can become infected if the parasites penetrate the skin. During the course of several weeks, the parasites migrate through the person’s tissues and develop into adult worms. Mature worms mate inside the human host and the females produce eggs, some of which travel to the infected person’s bladder or intestines and are passed in urine or stool. Other eggs become trapped in body tissues, causing an immune reaction and progressive damage to organs. 

The worms are not found in the United States but more than 200 million people are currently infected by this disease worldwide, primarily in tropical and subtropical locations. The disease is second only to malaria as the most devastating parasitic disease.

Symptoms of schistosomiasis are caused by the body’s reaction to the eggs produced by the worms, not by the worms themselves. Eggs from the worms travel to the intestine, liver, or bladder. Those that are not shed during urination or defecation can become lodged in these areas, causing inflammation or scarring. During the early phase of infection, most people do not have any symptoms. For those who do, within days of being infected, a person may develop a rash or itchy skin. From 1 to 2 months after infection, the individual may develop other symptoms, including fever, chills, muscle aches, and cough. Children who are repeatedly infected can develop anemia, malnutrition, and learning disabilities. If left untreated, schistosomiasis can persist for years. Symptoms of chronic schistosomiasis include abdominal pain, enlarged liver, blood in the stool or urine, problems passing urine, and increased risk of bladder cancer. Eventually, after years of infection, the parasites can damage the liver, spleen, intestine, lungs, and bladder. In rare cases, eggs can be found in the brain or spinal cord, possibly causing seizures, paralysis, or spinal cord inflammation.

If left untreated, chronic schistosomiasis can result in death. In 2014, 258 million people required preventive treatment, and 61.6 million people were treated for the parasite. A safe and effective drug called praziquantel, typically prescribed for 1 to 2 days, is available to treat this disease. Although access to the drug in countries where it is most needed has been difficult, there have been increased efforts to rectify this problem. These efforts have had some impact.


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What do you know about infectious disease?

Which are larger?

  • Correct!

    Bacteria are 10 to 100 times larger than viruses.

  • Sorry, that’s incorrect.

    Bacteria are 10 to 100 times larger than viruses.

Infectious Disease Defined

Biological Agent (Terrorism)

A bacterium, virus, or other biological toxin used in bioterrorism or biological warfare.

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