Giardiasis

Infectious Disease

OverviewWhoPresentationMechanismWorkupManagementPivots

Illness script · Infectious Disease

Giardiasis

Intestinal infection by the flagellated protozoan Giardia lamblia causing malabsorptive diarrhea, transmitted via fecal-oral route.

This illness script for Giardiasis covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.

Updated Sep 20, 2026All scripts

01

Predisposing factors

  • Drinking untreated stream/mountain water ('beaver fever') — classic exposure
  • International travel to endemic areas (developing countries)
  • Daycare centers — children and staff at high risk
  • Immunodeficiency, especially IgA deficiency and hypogammaglobulinemia
  • Campers, hikers, backpackers
  • Men who have sex with men (fecal-oral transmission)

02

Presentation

  • Onset 1–3 weeks after exposure (incubation period)
  • Prolonged watery, foul-smelling, greasy/fatty diarrhea (steatorrhea)
  • Bloating, flatulence, abdominal cramps — prominent features
  • Nausea, anorexia, weight loss over weeks
  • Absence of fever, blood, or mucus in stool distinguishes from invasive pathogens
  • Can become chronic/relapsing if untreated

03

Pathophysiology

  • Cyst ingested; excysts in duodenum → trophozoite form
  • Trophozoites attach to small intestinal brush border via ventral disk
  • Disrupts microvilli → malabsorption of fat and fat-soluble vitamins
  • Does NOT invade mucosa → no bloody diarrhea, no fever typically

04

Diagnostics

  • Stool antigen immunoassay (ELISA) — first-line, >90% sensitivity
  • Stool O&P (ova and parasites) — gold standard; send ×3 samples (cysts/trophozoites)
  • String test (Enterotest) or duodenal aspirate if stool tests negative but high suspicion
  • No fecal leukocytes — key pearl (non-invasive pathogen)
  • Stool antigen preferred over O&P in most clinical settings for speed/sensitivity

05

Management

  • Metronidazole 250 mg TID × 5–7 days — first-line
  • Tinidazole 2 g single dose — equally effective, better adherence
  • Nitazoxanide — alternative, especially in children
  • Treat all symptomatic patients; treat asymptomatic carriers in daycare/food-handler settings
  • Supportive hydration; no role for anti-motility agents early on

06

Clinical pivots

How to separate this script from the look-alikes that show up on exams and on the wards.

  • Cryptosporidiosis

    Crypto causes profuse watery diarrhea mainly in immunocompromised (HIV <200 CD4) with no effective treatment; Giardia is self-limited in immunocompetent and responds to metronidazole.

  • Bacterial gastroenteritis (e.g., Salmonella, Campylobacter)

    Bacterial causes are acute with fever, fecal leukocytes, and bloody diarrhea; Giardia is subacute/chronic, afebrile, non-bloody, with steatorrhea.

  • Celiac disease

    Celiac also causes steatorrhea and malabsorption but has no infectious exposure, positive anti-tTG IgA antibodies, and villous atrophy on biopsy.

  • Entamoeba histolytica

    Amebic colitis causes bloody dysentery with mucus and invasive extraintestinal disease (liver abscess); Giardia produces fatty non-bloody diarrhea without invasion.

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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.