Parasitic infections in children: SAQ

This child has giardiasis, the most common intestinal parasitic infection in the developed world. Her clinical picture of chronic foul-smelling greasy diarrhoea with flatulence, bloating, weight loss, and a childcare outbreak is the classic presentation. The negative routine stool culture is expected because standard culture does not detect Giardia, and a specific stool antigen test is required. The absence of blood, mucus, and fever distinguishes this from invasive bacterial enteritis. ^[2]

Question 1 (10 marks)

What is the most likely diagnosis, what specific test would you request to confirm it, and why was the routine stool culture negative? ^[2]

The most likely diagnosis is giardiasis caused by Giardia lamblia. The clinical features that support this diagnosis are the chronic nature of the diarrhoea, its foul-smelling and greasy quality suggesting fat malabsorption, the associated flatulence and bloating, the weight loss crossing one centile, and the childcare outbreak suggesting a common faecal-oral source. The absence of blood, mucus, and fever argues against invasive bacterial causes such as Campylobacter, Salmonella, or Shigella. ^[2]

The specific test to confirm the diagnosis is a stool antigen test for Giardia, which detects Giardia-specific antigens in a single stool sample with a sensitivity of over ninety per cent. This is superior to stool microscopy for cysts, which requires three separate samples for adequate sensitivity, and it is far more practical in the outpatient setting. Polymerase chain reaction on stool is also highly sensitive but is generally reserved for multiplex panels rather than targeted testing. ^[2]

The routine stool culture was negative because standard bacterial stool culture is designed to grow Salmonella, Shigella, Campylobacter, and sometimes E. coli and does not detect protozoal parasites. Giardia lamblia is a protozoan that exists as a cyst in stool and a trophozoite in the small intestine, and it requires either antigen detection, microscopy for ova cysts and parasites, or molecular methods to identify. This is a common diagnostic pitfall: a child is labelled as having a negative stool workup when only bacterial culture was sent, and the Giardia antigen was never requested. ^[2]