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By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

Cryptosporidiosis is an intestinal infection caused by Cryptosporidium, a protozoan. The main symptoms are abdominal cramping and diarrhea.

  • People acquire the infection by consuming contaminated water or food or by having contact with infected people or animals.

  • Abdominal cramping and watery diarrhea may begin suddenly, sometimes accompanied by nausea, vomiting, fever, and weakness.

  • Doctors diagnose the infection by examining or analyzing a stool sample for signs of the parasite.

  • Adequate sanitation and hand washing can help prevent spread of the infection, as can boiling water before drinking it.

  • Healthy people often recover on their own, but they may require treatment with an antiparasitic drug.

  • People with AIDS or a weakened immune system may continue to have diarrhea even when they are treated with an antiparasitic drug.

Cryptosporidium parasites are protozoa that infect people and many kinds of animals throughout the world.

Cryptosporidiosis is acquired by

  • Ingesting Cryptosporidium parasites in water or food contaminated by human or animal feces containing the parasite's eggs

  • Having contact with soil or an item that has been contaminated with the parasite

  • Having contact with an infected person or animal

The thick-walled eggs of Cryptosporidium are very hardy and are frequently present in swimming pools, hot tubes, water parks, lakes, and rivers around the world. The parasite is not killed by freezing or by the usual levels of chlorine in swimming pools or drinking water.

After people swallow the eggs, the eggs move to the intestine, where they release an immature form of the parasite, which enters the cells that line the intestine. The parasite matures, multiplies, and produces eggs. People then pass the eggs in their feces. Only a small number of Cryptosporidium eggs are required to cause infection.

Cryptosporidiosis is a common cause of diarrhea among children living in developing areas where sanitation is poor. It occasionally occurs among travelers to such areas. Cryptosporidiosis also causes diarrhea outbreaks in the United States. In Baker City, Oregon, 2,780 people became sick when the city's water supply was contaminated with Cryptosporidium in 2013. Outbreaks have occurred in other cities and in day care centers.

People with a weakened immune system, particularly those with AIDS, are prone to cryptosporidiosis and are more likely to have severe, persistent disease.

Did You Know...

  • People with a healthy immune system may not need treatment for cryptosporidiosis.


Cryptosporidiosis symptoms may begin abruptly about 7 days after people are infected and consist mainly of abdominal cramps and profuse, watery diarrhea. Nausea, vomiting, loss of appetite, fever, and weakness may also occur. Symptoms usually last 1 to 2 weeks, then subside. People may pass eggs in their stool for several weeks after symptoms have disappeared.

In people with a weakened immune system, symptoms of cryptosporidiosis may begin gradually, and the diarrhea can vary from mild to severe (as much as 3 to 4 gallons of watery stool per day in people with AIDS) and may persist for a long time.


  • Stool tests

To diagnose cryptosporidiosis, a doctor sends a stool sample to be tested for a protein released by the parasite (antigen testing) or for the parasite's genetic material (DNA).

Another approach is to examine stool under a microscope for Cryptosporidium eggs, but this method is less sensitive, and several stool samples may be needed to find the parasite. Specialized techniques can be used to increase the chances of identifying the eggs.

Doctors may use a flexible viewing tube (endoscope) to examine the upper part of digestive tract, including the first part of the small intestine (duodenum). Doctors may use this procedure to obtain a sample of tissue to be examined and analyzed (biopsied). Cryptosporidium, if present, can be seen in a biopsy sample from the intestine.


Cryptosporidiosis prevention involves adequate sanitation and hand washing, particularly in health care facilities and day care centers and after contact with soil, animals, or infected people. People should not drink or swallow water that could be contaminated, such as that from a swimming pool, stream, water park, or lake or in an area where sanitation is poor.

When public health departments discover a localized outbreak of the disease, they typically advise people to

  • Boil drinking water (including water for toothbrushing and food washing)

  • Eat only cooked foods

  • Avoid unpasteurized milk and juice

Tap water filters that use reverse osmosis or have the words "absolute 1 micron" or "tested and certified by NSF/ANSI Standard No. 53 or No. 58 for cyst removal/reduction" are effective. Other types of filters may not be.


  • For people with persistent diarrhea, nitazoxanide

  • For people with AIDS, treatment of the HIV infection

  • If diarrhea is severe, fluids and an antidiarrheal drug

People with a healthy immune system typically recover on their own. If such people have persistent diarrhea, nitazoxanide (an antiparasitic drug) may help speed recovery.

For people who have AIDS, the most important therapy is to treat the HIV infection (with antiretroviral drugs). When such treatment strengthens the weakened immune system, diarrhea is usually reduced. It is not clear how effective nitazoxanide is in immunocompromised people. Unless the immune system problem is corrected, diarrhea may continue throughout life.

People with severe diarrhea may require treatment with fluids, given by mouth or by vein, and antidiarrheal drugs such as loperamide. However, loperamide may not help people with AIDS.

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