Definition
Giardiasis is caused by the flagellate protozoan Giardia lamblia (also called Giardia intestinalis/duodenalis). It is spread by the faeco-oral route and is a common cause of persistent, non-bloody diarrhoea, particularly after travel.
Pathophysiology
Ingested cysts excyst in the small bowel, releasing trophozoites that adhere to the duodenal and jejunal mucosa. This impairs the absorptive surface, producing malabsorption and steatorrhoea. A secondary lactose intolerance may develop and persist for several weeks after treatment.
Risk factors
- Foreign travel
- Swimming in, or drinking, untreated water from a river or lake
- Male-to-male sexual contact
Clinical presentation
- Often asymptomatic
- Non-bloody diarrhoea — characteristically with steatorrhoea (fatty, offensive, floating stools)
- Bloating and abdominal pain
- Flatulence
- Lethargy
- Weight loss
- Malabsorption and lactose intolerance can occur
Investigations
- Stool microscopy for trophozoites and cysts — sensitivity only ~65%, so send repeated samples if suspicion is high
- Stool antigen detection assay — greater sensitivity and faster turnaround than conventional microscopy (the preferred test)
- PCR assays are increasingly available
Management
- Metronidazole is the treatment of choice
- Tinidazole is an effective single-dose alternative
- Consider treating symptomatic close contacts; advise that post-infectious lactose intolerance may linger
MRCP-specific traps
- Diarrhoea is non-bloody — bloody diarrhoea points elsewhere (e.g. amoebiasis, Shigella, Campylobacter).
- Giardia is a classic cause of malabsorption/steatorrhoea, not just watery diarrhoea.
- Unlike helminth infections, giardiasis does not typically cause eosinophilia.
- A single negative stool microscopy does not exclude it (65% sensitivity) — use the antigen assay.
Summary
Giardiasis is a faeco-orally transmitted protozoal infection causing non-bloody diarrhoea, steatorrhoea and malabsorption. Diagnose with a stool antigen assay (microscopy is only ~65% sensitive) and treat with metronidazole.
infectionsgastroenterologyprotozoamrcp part 1