Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Oct 2022
Topic Resources

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

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 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.

Did You Know...

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

Symptoms of Cryptosporidiosis

Cryptosporidiosis symptoms may begin abruptly about 7 days after people are infected and consist mainly of abdominal cramps and 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 [11 to 15 liters] of watery stool per day in people with AIDS) and may persist for a long time.

Diagnosis of Cryptosporidiosis

  • 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 Endoscopy Endoscopy is an examination of internal structures using a flexible viewing tube (endoscope). In addition to examinations, doctors can use endoscopy to do biopsies and give treatment. Endoscopes... read more ) 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.

Prevention of Cryptosporidiosis

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.

Treatment of Cryptosporidiosis

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 cryptosporidiosis in people who have AIDS. The most important therapy is to treat the HIV infection Antiretroviral Treatment of Human Immunodeficiency Virus (HIV) Infection Antiretroviral medications used to treat human immunodeficiency virus (HIV) infection aim to do the following: Reduce the amount of HIV RNA (viral load) in the blood to an undetectable amount... read more (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. Doctors may also try paromomycin, or a combination of paromomycin and azithromycin, to help decrease symptoms.

It is not clear how effective nitazoxanide is in people with other causes of a weakened immune system Overview of Immunodeficiency Disorders Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more , but it may help some 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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Drugs Mentioned In This Article

Generic Name Select Brand Names
Azasite, Zithromax, Zithromax Powder, Zithromax Single-Dose , Zithromax Tri-Pak, Zithromax Z-Pak, Zmax, Zmax Pediatric
Anti-Diarrheal, Imodium A-D, Imodium A-D EZ Chews , K-Pek II
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