Question
Cryptosporidium parvum.
Answer
Cryptosporidium parvum is a coccidian protozoan parasite, an important cause of watery diarrhoea, particularly significant as an opportunistic pathogen in AIDS patients, in whom it can cause severe, chronic, life-threatening diarrhoea with substantial fluid loss.
Transmission: faecal-oral route, via ingestion of the environmentally resistant oocyst in contaminated water (notably resistant to standard chlorination, making it an important cause of waterborne outbreaks) or food, or by direct contact with infected animals/persons.
Pathogenesis: ingested oocysts release sporozoites that invade the intestinal epithelium, developing within an intracellular but extracytoplasmic parasitophorous vacuole beneath the brush border membrane; the resulting mucosal damage and villous atrophy impair absorption, producing watery diarrhoea; disease severity and duration are markedly influenced by host immune status — self-limiting in immunocompetent individuals but chronic and severe in advanced AIDS (typically at CD4 counts below 100 cells/μL).
Laboratory diagnosis: modified Ziehl-Neelsen (modified acid-fast) staining of stool — demonstrates small, round, acid-fast oocysts (4–6 μm) that stain pink-red against a blue-green background, distinguishing them from other faecal debris; this modified (weaker acid-alcohol decolorization) technique is specifically required, since standard ZN staining conditions used for mycobacteria are too harsh and can decolorize the oocysts; stool antigen ELISA and PCR are also available, offering greater sensitivity.
Treatment: largely supportive (rehydration) in immunocompetent hosts, since the infection is usually self-limiting; nitazoxanide has some efficacy; in AIDS patients, restoration of immune function via antiretroviral therapy is the most effective approach, since specific antiparasitic treatment is often inadequate without immune reconstitution.