Key facts
- What it is: a microscopic protozoan parasite that forms tough, chlorine-resistant oocysts.
- Why summer: the leading cause of pool, splash-pad and water-park outbreaks.
- Incubation: about 7 days (range 2–10); hallmark is watery diarrhea.
- Testing catch: routine O&P misses it — needs acid-fast staining, antigen or PCR.
Cryptosporidium (“Crypto”) is a microscopic parasite and the leading cause of waterborne recreational outbreaks. Its oocysts are unusually tough — they survive the chlorine levels used in pools and drinking water — which is why a single ill swimmer can seed a pool or splash-pad outbreak. You get it by swallowing oocysts from contaminated water, from animals (calves, livestock), person-to-person, or from food.
Symptoms
The hallmark is watery diarrhea, often with abdominal cramping, nausea or vomiting, low-grade fever, loss of appetite and dehydration. Symptoms usually begin about a week after exposure and last from a few days to a few weeks; they can wax and wane before resolving. In healthy people it’s usually self-limiting; in immunocompromised people (especially advanced HIV) it can be severe and chronic.
Why routine stool tests miss it
This is the important part. Standard O&P microscopy does not reliably detect Cryptosporidium — the oocysts are easily missed on a routine wet mount and require a modified acid-fast stain, a direct fluorescent antibody or antigen test, or a molecular PCR panel. The CDC advises clinicians to specifically request Crypto testing because it isn’t part of a standard exam. Multiplex PCR panels give the highest sensitivity. Learn more about how parasite testing works.
Treatment
Most healthy people recover without drug treatment — supportive care with fluids is the mainstay. Nitazoxanide is the only FDA-approved medication and is prescribed for those who need it (approved for age 1 and up with healthy immune systems). For immunocompromised patients, nitazoxanide is less reliably effective and restoring immune function is central; these cases need specialist care.
Prevention & recovery
Because Crypto is chlorine-resistant, the CDC’s summer message is simple: don’t swim (or let children swim) while sick with diarrhea, and stay out of the water for two weeks after diarrhea stops; don’t swallow pool water; and wash hands after using the toilet or handling animals. Cryptosporidiosis is a nationally notifiable disease in the U.S. and Canada. After the infection clears, some people have lingering gut symptoms — see rebuilding your gut after an infection. The GI-MAP detects Cryptosporidium by qPCR on its parasite panel.
The GI-MAP™ detects Cryptosporidium by qPCR — the parasite routine stool tests miss.
Order Your GI-MAP™ Test → How it worksFrequently asked questions
How do you get Cryptosporidium?
By swallowing oocysts, most often from contaminated recreational or drinking water; also person-to-person, from animals, or food. It survives pool chlorine.
What are the symptoms?
Watery diarrhea, cramping, nausea, low-grade fever and dehydration, starting about a week after exposure and lasting days to weeks.
Does a routine stool test detect it?
Often not — it needs a special acid-fast stain, antigen test or PCR, specifically requested. GI-MAP detects it by qPCR.
How is it treated?
Most healthy people recover with fluids; nitazoxanide is the FDA-approved drug for those who need it. Immunocompromised people need medical care.
Sources & further reading
- CDC — Cryptosporidium clinical testing & diagnosis.
- CDC — Cryptosporidium treatment.
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