Faecal parasite detection

Role

Faecal (stool) microscopy for ova, cysts, and parasites remains a standard investigation for suspected intestinal parasitic infection, though increasingly complemented or superseded by PCR-based stool panels for improved sensitivity and faster turnaround for specific organisms
Relevant to a broad range of presentations - chronic diarrhoea, unexplained eosinophilia, and travel/migration history to endemic regions all prompt consideration of stool parasite testing

Traditional microscopy requires an experienced observer and has variable sensitivity depending on parasite load, shedding pattern (intermittent for some organisms), and specimen handling - multiple stool samples (typically 3, collected on separate days) improve sensitivity given intermittent ova/cyst shedding for several organisms

Specific organism suspicion should guide test selection beyond routine microscopy - e.g. specific antigen testing or PCR for Giardia/Cryptosporidium (where microscopy sensitivity is comparatively lower), or serology for tissue-invasive parasites (e.g. Strongyloides) not reliably detected in stool
Eosinophilia should prompt specific consideration of helminth infection, since many protozoal infections do not cause eosinophilia while tissue-invasive helminths characteristically do

Organism-specific antiparasitic treatment once identified (e.g. metronidazole/tinidazole for Giardia, albendazole/mebendazole for many helminths)
Strongyloides screening is particularly important before immunosuppression (e.g. before corticosteroids or biologic therapy) in patients with relevant travel/residence history, given the risk of life-threatening hyperinfection syndrome if unrecognised

Strongyloides hyperinfection syndrome risk before immunosuppression (a critical screening indication), Giardia and Cryptosporidium (lower microscopy sensitivity, antigen/PCR preferred), eosinophilia as a helminth infection clue
Strongyloides infection can remain latent for decades and cause fatal hyperinfection syndrome if the patient is later immunosuppressed - screening (serology, given the limitations of stool microscopy for this organism) should be actively considered in anyone with relevant residence/travel history before starting significant immunosuppressive therapy

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