Apicomplexan protozoan diarrheal illness (Cryptosporidium spp.) — self-limited in healthy hosts, devastating in advanced HIV.
Also known as: cryptosporidiosis, Cryptosporidium, Crypto
Overview
Diarrheal disease caused by Cryptosporidium parvum and C. hominis, apicomplexan protozoa that infect intestinal epithelium. Chlorine-resistant oocysts are infectious immediately upon shedding.
Epidemiology
~750,000 US cases annually (CDC estimates). Leading cause of recreational water outbreaks (chlorine-resistant). Worldwide major cause of childhood diarrhea mortality in low-income settings. Children, daycare attendees, and immunocompromised most affected.
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Question 1Infectious DiseaseMedium
A 9-year-old boy presents with 5 days of profuse, watery, non-bloody diarrhea, crampy abdominal pain, and low-grade fever. One week ago he spent an afternoon at a crowded outdoor water park, and three other children from the same group have developed similar symptoms. He is afebrile with mild dehydration on exam. A routine stool ova-and-parasite examination is unrevealing. Which of the following is the most appropriate next diagnostic test?
AModified acid-fast stain of stool
BStool toxin assay for C. difficile
CStool culture for enteric bacteria
DStool antigen immunoassay for Giardia
Reveal answer & full explanation
Correct answer: A — Modified acid-fast stain of stool
AModified acid-fast stain of stool✓
BStool toxin assay for C. difficile
CStool culture for enteric bacteria
DStool antigen immunoassay for Giardia
Why Modified acid-fast stain of stool is correct
The vignette is a classic recreational-water, chlorine-resistant outbreak of profuse, cholera-like watery diarrhea, which is Cryptosporidium until proven otherwise.
Cryptosporidium oocysts are small (4-6 micrometer) and easily missed on routine stool O&P, which is insensitive; a modified acid-fast stain reveals the characteristic acid-fast oocysts. Stool antigen EIA and multiplex PCR are equally acceptable targeted tests.
Recall the acid-fast 'CCC' enteric trio: Cryptosporidium, Cyclospora, and Cystoisospora.
Why the others are wrong
Stool antigen immunoassay for Giardia — Giardia is the wrong protozoan here; it causes greasy, malodorous diarrhea with bloating rather than the profuse cholera-like outbreak diarrhea, and it is not the chlorine-resistant water-park pathogen.
Stool culture for enteric bacteria — targets Salmonella, Campylobacter, and Shigella, which typically cause bloody, febrile dysentery, not the non-bloody watery diarrhea of a water-park outbreak.
Stool toxin assay for C. difficile — appropriate with recent antibiotic use or healthcare exposure, neither of which is present in this otherwise healthy child after recreational water exposure.
Question 2Infectious DiseaseMedium
A 7-year-old previously healthy boy is brought in with 5 days of profuse, watery, non-bloody diarrhea, crampy abdominal pain, and low-grade fever. He has had no recent antibiotics and no sick contacts at home, but several other children developed similar symptoms over the same week. On exam he has dry mucous membranes and diffuse mild abdominal tenderness. A stool multiplex PCR panel detects Cryptosporidium. Which of the following is the strongest risk factor for this infection?
AEating imported fresh raspberries
BPetting goats at a county fair
CSwimming at a public water park
DDrinking pasteurized cow's milk
Reveal answer & full explanation
Correct answer: C — Swimming at a public water park
AEating imported fresh raspberries
BPetting goats at a county fair
CSwimming at a public water park✓
DDrinking pasteurized cow's milk
Why Swimming at a public water park is correct
Cryptosporidium oocysts are chlorine-resistant and infectious immediately upon shedding, making recreational water (pools, water parks, splash pads, lakes) the leading source of cryptosporidiosis outbreaks per CDC surveillance.
A cluster of watery diarrhea after shared recreational water exposure, especially in children, is cryptosporidiosis until proven otherwise, and the 2-10 day incubation period fits this timeline.
Fecal-oral spread through ingested contaminated recreational water explains the simultaneous illness in several children sharing the same point source.
Why the others are wrong
Eating imported fresh raspberries is the classic exposure for Cyclospora cayetanensis (imported produce such as berries and basil), not Cryptosporidium, and it would not explain a localized cluster after shared recreational water exposure.
Drinking pasteurized cow's milk is not a recognized source, because pasteurization reliably kills oocysts; unpasteurized milk and untreated surface water carry the real risk.
Petting goats at a county fair reflects animal contact (calves, sheep, goats), a genuine but weaker route; recreational water carries far higher relative risk and better fits a multi-child point-source outbreak. (Note that recent antibiotics, which this patient denies, would instead point toward Clostridioides difficile.)
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Immunocompromise: HIV with CD4 <100 — severe chronic disease
MSM (oral-anal contact)
Pathophysiology
Oocysts ingested → sporozoites released in small bowel → invade intestinal epithelium and develop within parasitophorous vacuoles. Cause villous blunting, secretory diarrhea, and inflammation. Cell-mediated immunity is critical for clearance — defective in advanced HIV → unrelenting chronic infection. Biliary tract, pancreas, and respiratory epithelium can be involved in immunocompromised hosts.
Clinical presentation
Symptoms
Incubation 2-10 days
Profuse watery, non-bloody diarrhea (often described as 'cholera-like')
Outbreak of watery diarrhea after swimming at a water park — cryptosporidium until proven otherwise. AIDS patient with CD4 <50 and chronic watery diarrhea — also high suspicion.
Respiratory involvement in severe immunocompromise
Reactive arthritis
PANCE pearls
Cryptosporidium oocysts are CHLORINE-RESISTANT — recreational water outbreaks remain a major source despite standard pool chlorination. Hyperchlorination or UV needed.
AIDS-related cryptosporidiosis: ART (restoring CD4 >100) is the most effective therapy. Anti-cryptosporidial drugs alone rarely cure.
Suspect biliary cryptosporidiosis in HIV with RUQ pain — MRCP shows beaded bile ducts (sclerosing cholangitis pattern).
Public health: report outbreaks; advise infected persons not to swim for 2 weeks after symptom resolution.
References
CDC — Cryptosporidiosis: Diagnosis and Treatment Information for Public Health and Medical Professionals
DHHS OI — Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents with HIV — Cryptosporidiosis section
IDSA 2017 — Shane et al., Infectious Diseases Society of America Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea (Clin Infect Dis)
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