Medicine & USMLE

Giardia lamblia

  1. Giardia lamblia
  2. Toxoplasma gondii
  3. Entamoeba histolytica
  4. Cryptosporidium
  5. Naegleria fowleri
  6. Trypanosoma brucei
  7. Plasmodium Overview
  8. Plasmodium Disease (Malaria)
  9. Babesia
  10. Trypanosoma cruzi
  11. Leishmania
  12. Trichomonas vaginalis
  13. Enterobius vermicularis (pinworm)
  14. Ascaris lumbricoides (giant roundworm)
  15. Strongyloides stercoralis (threadworm)
  16. Ancylostoma and Necator
  17. Trichinella spiralis
  18. Trichuris trichiura (whipworm)
  19. Toxocara canis
  20. Onchocerca volvulus
  21. Loa loa
  22. Wuchereria bancrofti
  23. Taenia solium
  24. Diphyllobothrium latum
  25. Echinococcus granulosus
  26. Schistosoma
  27. Clonorchis sinensis
  28. Sarcoptes scabiei (Scabies)
  29. Pediculus humanis and Phthirus pubis (Lice)


Giardia lamblia is a parasite that is usually transmitted in a fecal-oral fashion, classically through the ingestion of contaminated water. Due to this, Giardia typically affects hikers, campers, and people who do outdoor activities, who are more likely to drink from contaminated water sources.  Infection with Giardia lamblia presents as Giardiasis, which manifests as non-bloody fatty diarrhea or steatorrhea. Individuals with IgA deficiency are at especially high risk for developing Giardia infections. Diagnosis of Giardia is usually performed by a stool ova and parasite exam, and patients should be treated  with metronidazole.

Key Points

  • Giardia lamblia
    • Characteristics
      • Parasite
    • Transmission
      • Fecal-oral transmission
        • Consumption of trophozoites or cysts released in stool of infected people
        • Usually in contaminated water (e.g. outdoors, camping, hiking, traveler)
          • Small inoculum needed (<10 organisms)
    • Presentation: Giardiasis
      • Abdominal pain, bloating, flatulence
      • Non-bloody steatorrhea
        • Foul-smelling fatty yellow diarrhea
        • Due to malabsorption due to colonization of jejunum and ileum
        • Non-bloody as parasite does not invade intestinal wall
      • Increased risk with IgA deficiency
        • Secretory IgA normally prevents adherence of microbes to GI mucosa
        • May be caused by selective IgA deficiency or X-linked agammaglobulinemia
    • Diagnosis
      • Stool O&P (ova and parasite exam)
        • Specific use of stool microscopy; trophozoites and cysts seen in stool
      • ELISA for antigen detection is also used
    • Treatment
      • Metronidazole