Giardia lamblia: Life Cycle, Diseases, Lab Diagnosis

Giardia lamblia also known as Giardia intestinalis, or Giardia duodenalis is a protozoan flagellate. It is transmitted by the fecal-oral route and causes the diarrheal illness called giardiasis (popularly known as beaver fever).

General Characteristics

  • Geographical distribution: worldwide; found in the soil, water, or surfaces contaminated with feces of infected human/animal.
  • Habitat: Duodenum and upper part of jejunum of humans.

Morphology: Exists in two form

Trophozoite and Cyst forms of Giardia
Cyst and Trophozoite of Giardia lamblia showing Encystation-excystation cycle
  • Cyst: Oval cyst is thick-walled with four nuclei and several internal fibers. Each cyst gives rise to two trophozoites during excystation in the intestinal tract.
  • Trophozoite: Pear-shaped with two nuclei, four pairs of flagella, and a suction disk are present.

Giardia is protected by an outer shell that allows it to survive outside the body for long periods of time and makes it tolerant of chlorine disinfection.

Life Cycle of Giardia lamblia

Life Cycle of Giardia Lamblia
Life Cycle of Giardia lamblia (source: CDC)
  • Infection occurs by the ingestion of cysts in contaminated water, food, or by the fecal-oral route (hands or fomites)
  • In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites).
  • Trophozoites multiply by longitudinal binary fission remaining in the lumen of the proximal small bowel where they can be free or attached to the mucosa by a ventral sucking disk.
  • Encystation occurs as the parasites transit toward the colon.
  • Both cysts and trophozoites can be found in the feces
    • The cyst is the stage found most commonly in non-diarrheal feces (The cysts are hardy and can survive several months in cold water.)
    • Trophozoites are found in diarrheal stools.

Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible.


  • Many species of mammals, as well as humans, act as reservoirs.
  • Risk groups:
    • Hikers who drink untreated stream water
    • Male homosexuals
    • Children in daycare centers
    • Patients in mental hospitals
    • IgA deficiency greatly predisposes to symptomatic infection
  • The trophozoite causes inflammation of the duodenal mucosa, leading to the malabsorption of protein and fat.
    • Mild steatorrhoea (passage of yellowish greasy stools in which there is an excess of fat)
  • Approximately half of those infected are asymptomatic carriers who continue to excrete the cyst for years.

Clinical Findings

Giardia infection can cause a variety of intestinal symptoms, which include;

  • Anorexia
  • Flatulence (gas)
  • Upset stomach or nausea/vomiting
  • Stomach or abdominal cramps
  • Watery (non-bloody), foul-smelling diarrhea
  • Greasy stools that tend to float
  • Dehydration (loss of fluids)

Giardiasis can cause weight loss and failure to absorb fat, lactose, vitamin A, and vitamin B12. In children, severe giardiasis might delay physical and mental growth, slow development, and cause malnutrition.

Laboratory Diagnosis

Fecal specimens containing Giardia lamblia may have an offensive odor and are pale-colored, fatty, and float in water.

  • Ova and parasite (O+P) examination
    • Giardia cysts can be excreted intermittently, so many cases (>50%) of giardiasis will be missed with a single O+P examination, resulting in underdiagnosis.
    • Multiple stool collections (i.e., three stool specimens collected on separate days) increase test sensitivity.
    • Use of concentration methods increases sensitivity.
    • Microscopical examination of freshly passed stools is used to demonstrate Giardia trophozoite and cysts.
      • Fresh diarrhoeic specimen: Try to find Giardia lamblia trophozoites. Generally difficult to detect as they attach themselves to the intestine wall. A Giemsa or Field’s stained fecal smear should be examined if giardiasis is suspected but no trophozoites are detected in a wet mount of the feces.
      • Formed fecal specimen: Look for the Giardia lamblia cyst.
  • Fecal immunoassays that are more sensitive and specific can be used:
    • An ELISA test that detects a Giardia cyst wall antigen in the stool can be used.
  • String test (Entero-Test):
    • Swallowing a weighted piece of string until it reaches a duodenum.
    • Trophozoites adhere to the string and can be visualized after the withdrawal of the string.
  • Polymerase Chain Reaction (PCR) can be used to identify the subtypes of Giardia lamblia. 

References and further readings

  • Sastry A.S. & Bhat S. (2014) Essentials of Medical Parasitology. Jaypee Brothers Medical Publishers (P) Ltd
  • Gracia, L.S. (2016). Diagnostic Medical Parasitology. ASM Press.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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