Stool Ova, Cysts and Parasites

Stool examination for ova, cysts and parasites (O&P) detects the eggs, larvae, cysts and trophozoites of intestinal helminths and protozoa by microscopy.

Methods include direct wet mount (for motile trophozoites), concentration by formalin ethyl acetate sedimentation or flotation to find eggs and cysts, permanent stains (trichrome, iron haematoxylin) for protozoa, and modified acid fast stain for Cryptosporidium, Cyclospora and Cystoisospora. Antigen and nucleic acid tests for Giardia, Cryptosporidium and Entamoeba histolytica have higher sensitivity. Quantitative egg counts (Kato-Katz) measure worm burden.

Over 1.5 billion people, about 24% of the world population, are infected with soil transmitted helminths (Ascaris, Trichuris, hookworm), mostly in sub-Saharan Africa, East Asia and the Americas. Schistosomiasis affects over 200 million people. Giardia infects about 280 million people a year. In high income countries the test is used for returned travellers, migrants, children in day care and immunocompromised patients.

Indications. Persistent diarrhoea (over 14 days), diarrhoea after travel to an endemic area, eosinophilia of unknown cause, anaemia in an endemic area, abdominal pain, weight loss or malabsorption, perianal itching, immunocompromise with diarrhoea (HIV, transplant), outbreaks (day care, institutions), and screening of migrants and refugees.

Specimen. Fresh stool in a clean container with no urine, water, oil, barium or antacids in the preceding 5 to 7 days, since these interfere with microscopy. For trophozoites, examine within 30 minutes to 1 hour or preserve in formalin and polyvinyl alcohol. Three specimens collected on alternate days increase sensitivity from about 50 to 60% to over 90%, because shedding is intermittent. For pinworm, use cellophane tape swab from the perianal skin in the morning before washing. For strongyloides, agar plate culture and serology are more sensitive.

Normal. No ova, cysts or parasites seen. Non-pathogenic protozoa (Entamoeba coli, Endolimax nana, Iodamoeba bütschlii) are reported but do not need treatment, though they show faecal oral exposure.

Helminths.

• Ascaris lumbricoides: large fertilised eggs, with abdominal pain, obstruction in heavy infection, and pulmonary migration.
• Trichuris trichiura: barrel shaped eggs with polar plugs, with colitis and rectal prolapse in heavy infection.
• Hookworm (Ancylostoma, Necator): thin shelled eggs, with iron deficiency anaemia and hypoalbuminaemia.
• Strongyloides stercoralis: rhabditiform larvae, which cause autoinfection and fatal hyperinfection in patients on corticosteroids. Repeated stool examination or culture is needed because sensitivity of a single stool is under 50%.
• Taenia saginata and solium: proglottids and eggs. T. solium also causes neurocysticercosis.
• Schistosoma mansoni and japonicum: lateral spined and small spined eggs. S. haematobium eggs are found in urine.
• Enterobius vermicularis: eggs on tape test.
• Hymenolepis nana, Fasciola, Clonorchis and Opisthorchis, Diphyllobothrium: eggs of characteristic size and shape.

Protozoa.

• Giardia lamblia: cysts and trophozoites, with malabsorptive diarrhoea. Antigen testing is more sensitive than microscopy.
• Entamoeba histolytica: cysts and trophozoites with ingested red cells in dysentery. Microscopy cannot distinguish from non-pathogenic E. dispar, so antigen or PCR is needed.
• Cryptosporidium, Cyclospora, Cystoisospora: oocysts on modified acid fast stain, with prolonged diarrhoea in the immunocompromised.
• Blastocystis hominis and Dientamoeba fragilis: of uncertain pathogenicity, and treated only in the symptomatic with no other cause.

Egg counts. Eggs per gram of faeces (EPG) classify intensity: for Ascaris, light 1 to 4999, moderate 5000 to 49 999, heavy 50 000 or more. For Trichuris, light 1 to 999, moderate 1000 to 9999, heavy 10 000 or more. For hookworm, light 1 to 1999, moderate 2000 to 3999, heavy 4000 or more.

Eosinophilia. A raised eosinophil count with a negative stool examination in a returned traveller needs serology (strongyloides, schistosomiasis, filariasis, toxocariasis), as many tissue dwelling helminths do not shed eggs in stool.

Critical values and findings needing immediate action.

• Strongyloides larvae in a patient on or about to start corticosteroids or other immunosuppression: treat before immunosuppression, because hyperinfection is often fatal.
• Taenia solium eggs or proglottids with seizures or headache: neuroimaging for neurocysticercosis.
• Entamoeba histolytica with fever, right upper quadrant pain or colitis: liver abscess and fulminant amoebic colitis need urgent care.
• Heavy Ascaris with obstruction or hepatobiliary involvement.
• Cryptosporidium in an HIV patient with a low CD4 count.

Interferences and pitfalls.

• One stool misses many infections, and three samples on separate days are needed when suspicion is high.
• Contamination with barium, mineral oil, bismuth, antibiotics (tetracycline, metronidazole) or antidiarrhoeals reduces yield, and testing is delayed for 1 to 2 weeks.
• Urine or water destroys trophozoites.
• Requesting a general O&P does not include Cryptosporidium or Cyclospora, which need a special stain, and the request must say so.
• Microscopy is operator dependent, and training matters.
• Egg negative infection is possible in early (prepatent) disease, with single sex worm infection, and with tissue dwelling parasites.
• Entamoeba histolytica and E. dispar look identical.
• Overuse of testing in patients without risk factors yields little.

Special groups.

• Children: higher infection rates, with growth faltering, anaemia and cognitive effects in heavy helminth infection.
• Pregnancy: hookworm anaemia and deworming after the first trimester in endemic areas.
• Migrants and refugees: screening for strongyloides and schistosomiasis before immunosuppression.
• Immunocompromised patients: Cryptosporidium, Cystoisospora, microsporidia and strongyloides hyperinfection.
• Regions: mass deworming programmes reduce intensity, and mixed infections are common.

Follow up tests. Repeated stool samples, full blood count with eosinophil count, ferritin, albumin, strongyloides, schistosomiasis and filarial serology, Giardia and Cryptosporidium antigen, Entamoeba histolytica serology and PCR, urine microscopy for Schistosoma haematobium, perianal tape test, HIV test, neuroimaging for neurocysticercosis, and liver imaging for amoebic abscess or fascioliasis.