Merck Manual

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Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision May 2020| Content last modified May 2020
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Cryptosporidiosis is an intestinal infection caused by Cryptosporidium, a protozoan parasite. 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

  • Ingesting parasites from soil or an item that has been contaminated with eggs

  • Ingesting parasites acquired from close 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. Because of the enormous number of Cryptosporidium that an infected person or animal can shed (10 million to 100 million) in their stool and the ability of just a few ingested organisms to start an infection, the infection can easily spread person-to-person, for example, in day care centers or petting zoos.

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 advanced 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.

Cryptosporidiosis is also associated with undernutrition in children living in areas with poor sanitation.


  • 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), when diagnostic tests of stool do not identify a cause for persistent diarrhea. 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, hot tub, water park, stream, river, lake or ocean, or tap water during international travel to areas where sanitation is poor. Safer sex practices are important in preventing infection from anal contact.

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 "tested and certified by NSF/ANSI Standard No. 53 or No. 58 for cyst removal/reduction" are likely to be effective. Other types of filters may not be.


  • For people with normal immune systems, treatment is with nitazoxanide

  • For people with AIDS, treatment of the HIV infection, plus sometimes nitazoxanide to help relieve symptoms

  • If diarrhea is severe, fluids and an antidiarrheal drug

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

Nitazoxanide does not cure 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. While nitazoxanide does not cure cryptosporidiosis in people with AIDS, it may decrease diarrhea and other symptoms. It is not clear how effective nitazoxanide is in people with other causes of weakened immune systems, but it may help some people. Unless the immune system problem is corrected, diarrhea may continue throughout life in others.

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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