Question
Giardiasis.
Answer
Giardiasis is caused by Giardia lamblia (also called G. intestinalis or G. duodenalis), a flagellated protozoan parasite, and is one of the most common causes of parasitic diarrhoea worldwide, particularly affecting children and travellers.
Transmission: faecal-oral route, via ingestion of the resistant cyst form in contaminated water (a common cause of waterborne outbreaks, since cysts resist routine chlorination) or food, or via direct person-to-person contact (e.g., in daycare settings).
Life cycle: ingested cysts excyst in the duodenum/upper small intestine, releasing the motile, flagellated, pear-shaped trophozoite form (with two nuclei giving a characteristic “owl’s-eye” or “old man’s face” appearance, and a ventral adhesive disc used to attach to the intestinal mucosa without invading it); trophozoites multiply by binary fission and cause disease by mechanically coating the mucosal brush border (impairing fat/nutrient absorption) rather than by tissue invasion; some trophozoites encyst again to form cysts, which are excreted in stool and are immediately infectious to a new host.
Clinical features: ranges from asymptomatic carriage to acute or chronic diarrhoea — typically foul-smelling, greasy, non-bloody (steatorrhoeic) stools, abdominal cramps, bloating, flatulence, and, in chronic cases, malabsorption and weight loss.
Laboratory diagnosis: stool microscopy for cysts (typically in formed stool) and/or trophozoites (typically in loose/watery stool) — multiple samples may be needed due to intermittent shedding; stool antigen detection (ELISA) — more sensitive than microscopy; string test (Entero-test) — recovers duodenal fluid/mucus for trophozoite examination when stool exam is negative but suspicion remains high; PCR where available.
Treatment: metronidazole (or tinidazole) is the treatment of choice.