Giardia lamblia: Life Cycle, Diseases, Lab Diagnosis

Last updated on June 1st, 2021

Giardia lamblia also known as Giardia intestinalis, or Giardia duodenalis is a protozoan flagellates. 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.
Cyst and Trophozoite of Giardia lamblia showing Encystation-excystation cycle
Cyst and trophozoite of Giardia lamblia showing encystation-excystation cycle

Morphology: Exists in two form

  • 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 (source: CDC)
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 2 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, can survive several months in cold water.)
    • Trophozoites are found in the 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 malabsorption of protein and fat.
    • Mild steatorrhoea (passage of yellowish greasy stools in which there is an excess of fat)
  • Approximately half of those who are 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

Faecal 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 under-diagnosis.
    • 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 for the demonstration of Giardia trophozoite and cysts.
      • Fresh diarrhoeic specimen: Try to find Giardia lamblia trophozoites. Generally difficult to detect as they attach themselves to the wall of the intestine. 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. 

About Acharya Tankeshwar 473 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.