Add-On: Parasitology Module

Microscopic examination identifies intestinal parasites through direct visualisation of ova, cysts, trophozoites, and larvae in concentrated and permanently stained faecal specimens. The technique relies on morphological identification under oil immersion, distinguishing pathogenic protozoa (Giardia lamblia, Entamoeba histolytica, Cryptosporidium species, Cyclospora cayetanensis) and helminths (roundworms, tapeworms, flukes) from non-pathogenic commensals. Multiple specimens increase diagnostic yield because shedding is often intermittent.

Direct microscopy remains the reference method for many intestinal parasites, particularly helminths and protozoa that are not reliably detected by antigen or molecular assays. In patients with chronic gastrointestinal symptoms, travel history, or immunosuppression, parasitology identifies treatable infections that overlap clinically with functional bowel disorders and inflammatory conditions. A negative result does not exclude light infection or parasites that require specialised stains or culture.

Enzyme immunoassay detects Giardia-specific antigens in stool, offering higher sensitivity than microscopy for this flagellated protozoan that colonises the small intestine. Giardia causes acute or chronic diarrhoea, bloating, and malabsorption through direct mucosal damage and altered brush-border function; cyst excretion is intermittent, so antigen testing captures more cases than a single microscopy specimen.

Giardiasis is a common cause of persistent diarrhoea in returning travellers, children in day-care settings, and immunocompromised patients. Antigen detection is more sensitive and practical than microscopy for routine screening, and a positive result directs antimicrobial therapy (metronidazole, tinidazole, or nitazoxanide).

Enzyme immunoassay detects Cryptosporidium oocyst antigens in stool. This coccidian protozoan infects small-intestinal epithelium, causing watery diarrhoea that is self-limited in immunocompetent hosts but severe and persistent in immunosuppressed patients. Oocysts are small and easily missed on routine O&P; antigen testing markedly improves detection.

Cryptosporidiosis is an important differential in acute diarrhoea following exposure to contaminated water, in HIV/AIDS patients with CD4 counts below 200 cells/µL, and in other immunosuppressed states. Detection guides supportive care in immunocompetent patients and targeted antimicrobial or immune reconstitution strategies in immunocompromised hosts.

Blastocystis hominis is a polymorphic protist commonly found in human stool; its pathogenic role remains contested. Multiple subtypes exist, and some studies associate higher parasite loads and specific subtypes with gastrointestinal symptoms, while others regard it as a commensal. Microscopy and PCR both detect it, with PCR offering higher sensitivity.

The clinical significance of Blastocystis detection is debated. Many carriers are asymptomatic, but some practitioners consider it in patients with unexplained bloating, diarrhoea, or irritable-bowel-like symptoms, particularly when other causes have been excluded. Treatment trials (metronidazole or nitazoxanide) are sometimes attempted, though evidence of benefit is inconsistent.

Dientamoeba fragilis is a flagellate protozoan (often misclassified as an amoeba) that inhabits the colon. It lacks a cyst stage and is thought to be transmitted within helminth eggs. Molecular detection is more sensitive than microscopy, as the trophozoites are fragile and degenerate rapidly in stool.

Dientamoeba is associated with chronic diarrhoea, abdominal pain, and flatulence in some patients, though asymptomatic carriage also occurs. Detection by PCR in symptomatic individuals with no other identified pathogen may warrant a trial of therapy (metronidazole, iodoquinol, or paromomycin).