Life Under the Microscope: Giardia lamblia

Giardiasis, also known as “beaver fever”, is caused by an infection with the gastrointestinal parasite, Giardia lamblia. This parasite was first discovered by the “Father of the Microscope”, Antoine van Leeuwenhoek, and later described by and named after Alfred Mathieu Giard and Vilém Dušan Lambl.


Giardia lamblia, as visualized by Trichrome staining

G. lamblia is considered a strict anaerobe, surviving in environments that lack oxygen, causing it to thrive in the gastrointestinal tract of humans. These infections occur by the fecal-oral route through contaminated water, such as ponds, and streams. While many of those infected with the parasite display gastrointestinal symptoms (abdominal pain, cramping, diarrhea, etc.) that last on average two to four weeks, others can become carriers.

The life cycle of the parasite is similar to others. First, the parasite is a cyst, and is able to survive in extreme hot and cold weather. The cyst is then ingested by the host, and within the gastrointestinal tract trophozoites are released. These trophozoites multiply by binary fission, actively feeding and moving. As the trophozoites move towards the colon, they turn into cysts once again and are passed in the feces of infected individuals.

Several methods exist to detect G. lamblia, typically performed in public health laboratories. Traditionally, microscopic analysis of stool using wet mount or Trichrome stain has been performed. When stained, the trophozoite form of the parasite appears as a “smiling face”, with the two stained nuclei appearing as the eyes and the median bodies as the mouth. While microscopic analysis is the gold standard for identification of parasites, antigen testing for G. lamblia has a greater sensitivity (>90% compared to 90% for microscopic analysis with three stool samples).

Giardiasis is commonly treated with the anti-parasitic drug furazolidone, which has an efficacy of greater than 90% and is tolerated well by the patient.


Immunoassay for the detection of fecal parasites Giardia and Cryptosporidium that detect antigens of the parasites using antibodies present in the assay. On the far left, the specimen is negative for both Cryptosporidium and Giardia. In the middle, the specimen is negative for Cryptosporidium and positive for Giardia. In the far right, the specimen is positive for Cryptosporidium and negative for Giardia.