Cryptosporidium spp. !

Parasite
Exam priority: medium

A tiny gut parasite spread by water, even chlorinated, and by contact. It causes watery diarrhea, which can be severe in people with AIDS.

Updated 2026-11-01

Also called: Cryptosporidium hominis, Cryptosporidium parvum, crypto

At a glance

Shape: Round oocyst with four sporozoites
Family: Cryptosporidiidae
Type: Apicomplexan protozoan (coccidian), intracellular but extracytoplasmic
Stain: Modified acid-fast: red oocysts on a blue-green background
Size: Oocyst about 4 to 6 µm

Morphology

Small round oocysts, acid-fast on the modified stain.
On biopsy they appear as dots along the epithelial brush border.
Trichrome does not stain them well.

Transmission

Drinking or recreational water contaminated with feces; oocysts tolerate chlorine.
Person-to-person (daycare) and contact with calves and other animals.
Contaminated raw food.
Contact precautions for diapered or incontinent patients.

Reservoir: Humans (C. hominis); Cattle, especially calves (C. parvum)

Isolation precautions

What the care team adds to standard precautions.

Standard

Life cycle

Sporulated oocysts are swallowed; they are infective as soon as they are passed.
In the small intestine they release sporozoites that invade the surface of epithelial cells.
They multiply asexually and then sexually inside a vacuole beneath the cell membrane.
Thin-walled oocysts form and reinfect the same host (autoinfection).
Thick-walled oocysts leave in feces and are immediately infectious.

Virulence factors

Intracellular but extracytoplasmic location, protected under the membrane.
Autoinfection by thin-walled oocysts: persistent infection in immunodeficiency.
Chlorine-resistant oocysts and a low infectious dose.
Villus atrophy and secretory diarrhea.

Diseases it causes

Cryptosporidiosis: watery diarrhea lasting one to two weeks in immunocompetent people. Cryptosporidiosis and other intestinal coccidia (Cyclospora, Cystoisospora) →
Acute gastroenteritis in children and outbreaks from water or pools. Acute (viral) gastroenteritis →
Severe chronic diarrhea and wasting in people with HIV and low CD4; it can involve the biliary tree. HIV infection and AIDS →

Clinical picture

Profuse watery diarrhea, cramps, nausea and low fever.
Immunocompetent: resolves on its own within 1 to 3 weeks.
AIDS: chronic diarrhea with dehydration, malabsorption and cholangitis.

Diagnosis

Modified acid-fast (Kinyoun) stain of stool.
Stool antigen (immunoassay, rapid tests) or direct immunofluorescence.
PCR in gastrointestinal panels.
It must be requested specifically: a routine ova-and-parasite exam often does not look for it.

Treatment

Rehydration is the basis. Nitazoxanide is the approved antiparasitic for immunocompetent patients. In people with HIV, what helps most is antiretroviral therapy that restores CD4 counts.

Drug class only; doses and regimens are set by the clinician from the current guideline.

Prevention

Boiled or filtered water; pool chlorine does not kill it quickly.
No swimming in pools while ill with diarrhea and for two weeks afterward.
Handwashing with soap and water (alcohol gel does not work well against oocysts).
People with HIV: avoiding untreated water and contact with calves.

Vaccine: There is no vaccine.

Test yourself

Think of the answer, then tap the question to see it.

Which stain is used to see Cryptosporidium in stool?

The modified acid-fast (Kinyoun) stain, which shows red oocysts on a blue-green background.

Why does Cryptosporidium cause outbreaks in chlorinated pools?

Because its oocysts tolerate usual chlorine levels and the infectious dose is low.

How does cryptosporidiosis present in a patient with AIDS and very low CD4?

As chronic watery diarrhea with dehydration, wasting and sometimes cholangitis; it improves when antiretrovirals restore immunity.

Which antiparasitic is approved for cryptosporidiosis?

Nitazoxanide, together with rehydration.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Cryptosporidiosis (CDC, 2024); About Crypto Infections | Cryptosporidium ("Crypto") | CDC (CDC, 2024).

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Representative model, for educational purposes only.