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?
ARepeat stool ova-and-parasite examination
BModified acid-fast stain of stool
CStool antigen immunoassay for Giardia
DStool culture for enteric bacteria
EStool toxin assay for C. difficile
Reveal answer & full explanation
Correct answer: B — Modified acid-fast stain of stool
ARepeat stool ova-and-parasite examination
BModified acid-fast stain of stool✓
CStool antigen immunoassay for Giardia
DStool culture for enteric bacteria
EStool toxin assay for C. difficile
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.
Repeat stool ova-and-parasite examination — the traditional teaching of sending serial O&P specimens makes a repeat exam tempting after one negative result, but routine O&P is insensitive for the small Cryptosporidium oocysts, so repeating it adds little; the parasite must be specifically targeted, as with the modified acid-fast stain.
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?
APetting goats at a county fair
BEating undercooked ground beef
CSwimming at a public water park
DEating imported fresh raspberries
EHandling a classroom pet turtle
Reveal answer & full explanation
Correct answer: C — Swimming at a public water park
APetting goats at a county fair
BEating undercooked ground beef
CSwimming at a public water park✓
DEating imported fresh raspberries
EHandling a classroom pet turtle
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 among several children over the same week is typical of a cryptosporidiosis outbreak, for which shared recreational water is the most common source, and the 2-10 day incubation period fits this timeline.
Fecal-oral spread through ingested contaminated recreational water would explain 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, which the stool PCR identified here.
Petting goats at a county fair - reflects animal contact (calves, sheep, goats), a genuine but weaker route; recreational water causes far more outbreaks per CDC surveillance and is the most likely shared source when several children fall ill in the same week. (Note that recent antibiotics, which this patient denies, would instead point toward Clostridioides difficile.)
Eating undercooked ground beef - although cattle are a Cryptosporidium reservoir, beef itself is not a recognized vehicle; the classic risk of undercooked ground beef is Shiga toxin-producing Escherichia coli, which causes bloody diarrhea rather than this boy's profuse non-bloody stools.
Handling a classroom pet turtle - reptiles such as turtles are a classic source of nontyphoidal Salmonella, which has caused outbreaks among children who handle classroom or household reptiles, so it fits a pediatric cluster; turtles are not a recognized source of human cryptosporidiosis, whose leading outbreak source is chlorine-resistant oocysts in shared recreational water.
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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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