Cryptosporidium parvum is a microscopic, single-celled parasite that causes the diarrheal illness cryptosporidiosis. The disease, often called “Crypto,” affects the intestinal tract of mammals, including humans. While several species of Cryptosporidium can infect people, C. parvum is one of the most common. The parasite resides and multiplies within the cells lining the intestines, causing gastrointestinal symptoms.
Lifecycle and Transmission of the Parasite
The lifecycle of Cryptosporidium parvum begins with ingesting its infectious stage, a hardy, thick-shelled structure called an oocyst. These oocysts are passed in the feces of an infected person or animal and are immediately infectious. Ingesting even a small number of oocysts is enough to establish an infection, as the tough outer shell allows the parasite to survive for extended periods in the environment, including through freezing temperatures.
Once swallowed, the oocyst travels to the small intestine, where it excysts and releases four infectious sporozoites. These sporozoites invade the epithelial cells lining the intestine. Inside these cells, the parasites undergo reproductive cycles to produce more oocysts. Two types are formed: thick-walled oocysts shed in the stool to contaminate the environment, and thin-walled oocysts that can cause autoinfection by re-infecting the same host.
Transmission to humans occurs through the fecal-oral route. Swallowing contaminated water is a common mode of infection, including from recreational sources like pools and lakes, or from drinking water supplies. People can also get infected by eating uncooked, contaminated food or through direct contact with infected individuals or animals.
Symptoms of Cryptosporidiosis
After the parasite is ingested, symptoms of cryptosporidiosis appear within 2 to 10 days. The most prominent symptom is profuse, watery diarrhea, often accompanied by severe stomach cramps or abdominal pain. Other common effects include nausea, vomiting, fever, loss of appetite, and weight loss.
Some individuals may experience dehydration due to significant fluid loss. In people with healthy immune systems, the illness is self-limiting. Symptoms last for one to two weeks, though they can come and go for up to 30 days before resolving.
The infection can be more serious for those with compromised immune systems, such as people with HIV or cancer patients. For these individuals, symptoms can be more severe and prolonged. Chronic diarrhea can lead to dangerous dehydration, malnutrition, and wasting, potentially becoming a life-threatening condition.
Diagnosis and Medical Treatment
Confirming cryptosporidiosis requires a healthcare provider to request a stool sample for laboratory analysis. Diagnostic methods include microscopic examination, which uses special stains to make the oocysts visible, or a sensitive enzyme immunoassay that detects parasite antigens.
For many people with a healthy immune system, the infection resolves on its own without specific medication. Treatment focuses on supportive care to manage symptoms. The primary goal is to prevent dehydration by drinking plenty of fluids, and a healthcare provider should be consulted before using anti-diarrheal medications.
For more severe infections, a doctor may prescribe an antiparasitic drug. The FDA-approved medication for cryptosporidiosis is nitazoxanide, which can reduce the duration of diarrhea in children and adults. Its effectiveness is less clear for people with weakened immune systems, for whom managing the underlying immune deficiency is a primary part of treatment.
Prevention and Water Safety
Preventing cryptosporidiosis centers on good hygiene and avoiding contaminated water and food. Because the parasite is transmitted through fecal matter, thorough handwashing with soap and water is a primary prevention method. This is especially important after using the toilet, changing diapers, before preparing or eating food, and after contact with animals. Alcohol-based hand sanitizers are not effective against Cryptosporidium.
A significant challenge is the parasite’s high resistance to chlorine, meaning oocysts can survive for days in properly chlorinated swimming pools, spas, and water parks, making them common sources of outbreaks. To reduce risk, do not swallow water while swimming. Individuals who have had diarrhea should wait at least two weeks after symptoms stop before entering a pool.
Do not drink untreated water from lakes, rivers, or streams. During outbreaks or if a well is contaminated, boiling water for at least one minute is an effective way to kill the parasite. Alternatively, specific water filters can remove Cryptosporidium. Consumers should look for filters certified under NSF/ANSI Standard 53 or Standard 58 for “cyst removal” or “cyst reduction,” or filters with an absolute pore size of 1 micron or smaller.