Giardia lamblia

Giardia lamblia is a flagellated protozoan with a distinctive pear-shaped trophozoite bearing two nuclei and four pairs of flagella — often described as having a face-like appearance under the microscope. It exists in two forms: the environmentally hardy cyst, which is the infectious stage, and the trophozoite, which is the form that attaches to intestinal mucosa.

Transmission is fecal-oral through contaminated water, and the classic exposure history is a hiker or camper drinking from a stream — hence the informal name beaver fever. Day care centers and institutional settings are other common sources. Ingested cysts excyst in the duodenum, and trophozoites attach to the brush border of the duodenum and jejunum by a ventral adhesive disc. They do not invade tissue, so the diarrhea is non-bloody and non-inflammatory; instead, villous injury and malabsorption of fat produce steatorrhea — bulky, greasy, foul-smelling stools — along with bloating, flatulence, and weight loss. Patients with IgA deficiency are especially vulnerable to prolonged infection.

Diagnosis is made by finding cysts or trophozoites in stool, or more commonly now by stool antigen detection or PCR. Metronidazole is the standard treatment, with tinidazole and nitazoxanide as alternatives. The contrast with Cryptosporidium is worth holding onto: both cause watery non-bloody diarrhea from contaminated water, but Giardia is a non-invasive flagellate producing fat malabsorption, while Cryptosporidium is an intracellular parasite whose acid-fast oocysts cause severe disease in AIDS patients.

USMLE Step 1 tests Giardia in the parasitology section and revisits it in pharmacology through metronidazole. Recognize the vignette pattern — camping or stream water, greasy floating stools, no blood, no fever — and pair it with the two-nuclei trophozoite image and metronidazole as the answer.