Coccidian Intestinal Parasites
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Direct answer
Coccidian intestinal parasites are protozoa that produce oocysts and cause watery diarrhoea, and the three genera of exam interest are Cryptosporidium, Cyclospora and Cystoisospora. Their oocysts resist routine stains, so a modified acid-fast stain is the usual method of identification. Oocyst size separates the genera, at about 4 to 6 micrometres for Cryptosporidium, 8 to 10 for Cyclospora and 20 to 30 for Cystoisospora.
What you must remember
- Cryptosporidium parvum: Small, acid-fast oocysts cause profuse watery diarrhoea, especially in patients with HIV.
- Cyclospora cayetanensis: Oocysts are only variably acid-fast but autofluoresce under ultraviolet light, as do those of Cystoisospora.
- Cystoisospora belli: Large acid-fast oocysts; the organism was formerly called Isospora belli.
- Staining: A modified acid-fast stain reveals oocysts in stool smears, and UV autofluorescence helps with Cyclospora.
- Cryptosporidium treatment: Nitazoxanide is the drug used, with restoration of immunity as the main step in HIV.
- Cyclospora and Cystoisospora treatment: Trimethoprim-sulfamethoxazole is the standard regimen.
- Diagnosis by antigen or PCR: Stool antigen testing and PCR detect parasites when microscopy is negative.
- Severity: Prolonged, severe cryptosporidiosis occurs in patients with low CD4 counts.
Choosing the right stain and drug from the stool
These parasites cause self-limiting diarrhoea in immunocompetent hosts but a chronic illness in HIV. The steps below follow the path from a stool sample to a confident diagnosis and treatment.
- Clinical clue: Prolonged watery diarrhoea lasting more than a week, especially in a patient with HIV, suggests a coccidian infection. Dehydration and weight loss may follow.
- Stool sample: Send fresh stool and ideally several samples on different days, because oocyst shedding is intermittent and a single sample often misses infection.
- Staining: A modified acid-fast stain identifies Cryptosporidium and Cystoisospora oocysts. UV autofluorescence helps recognise Cyclospora and Cystoisospora, but not Cryptosporidium.
- Measuring the oocyst: Size separates the genera, with Cryptosporidium the smallest at 4 to 6 micrometres and Cystoisospora the largest at 20 to 30 micrometres.
- Treatment: Nitazoxanide is used for Cryptosporidium, trimethoprim-sulfamethoxazole for Cyclospora and Cystoisospora, and fluid replacement for all. Nitazoxanide works poorly in severe HIV, so restoring CD4 counts matters most.
Where the exam tries to catch you
Most mistakes come from treating the three genera as identical in staining and drugs.
- Trap: Calling all coccidian oocysts acid-fast in equal measure Correct: Cyclospora is only variably acid-fast, so UV autofluorescence and oocyst size are the more reliable clues.
- Trap: Using trimethoprim-sulfamethoxazole first for Cryptosporidium Correct: nitazoxanide is the drug used for Cryptosporidium, while trimethoprim-sulfamethoxazole is for Cyclospora and Cystoisospora.
- Trap: Treating HIV cryptosporidiosis with antiparasitic drugs alone Correct: antiretroviral therapy restores CD4 counts and is the mainstay in advanced HIV.
- Trap: Reporting a negative stool after one clear sample Correct: oocyst shedding is intermittent, so repeated samples or antigen testing are needed.
Frequently asked questions
What is the size of Cryptosporidium oocysts?
Cryptosporidium oocysts measure about 4 to 6 micrometres, making them the smallest of the coccidian oocysts tested in exams. Their small size and acid-fast staining are the key features on microscopy.
Which coccidian oocyst autofluoresces under UV light?
Cyclospora cayetanensis oocysts autofluoresce under ultraviolet light, as do those of Cystoisospora but not Cryptosporidium. Cyclospora oocysts measure about 8 to 10 micrometres and are only variably acid-fast.
Which drug treats Cystoisospora belli infection?
Cystoisospora belli infection responds to trimethoprim-sulfamethoxazole, which is the standard regimen for this genus. The same drug is used for Cyclospora cayetanensis infection, so both genera are grouped together in treatment questions. Supportive fluid replacement is given alongside.
Why are multiple stool samples needed to diagnose cryptosporidiosis?
Oocyst shedding is intermittent, so a single negative stool sample does not exclude cryptosporidiosis. Examining several samples collected on different days improves detection by microscopy, and antigen or PCR tests add further sensitivity.
Why does cryptosporidiosis become severe in HIV?
Cryptosporidiosis becomes severe in HIV because low CD4 T-cell counts fail to clear the parasite from the intestinal epithelium. Prolonged watery diarrhoea then leads to dehydration and weight loss, and antiretroviral therapy that restores CD4 counts is the main control measure.