Giardia Antigen

Giardia intestinalis, also known as G. lamblia and G. duodenalis, is a flagellated protozoan parasite that lives in contaminated streams, lakes, and municipal water resevoirs. Giardiasis is the most commonly reported intestinal parasitic infection in the United States with an annual incidence of 1.2 million cases per year. 

Giardia infection is transmitted by the fecal-oral route and results from ingestion of Giardia cysts in fecally contaminated food or water or through person to person or animal to person transmission. Drinking untreated water from lakes and rivers, swimming in contaminated water, having contact with some animal species, and sexual practices involving fecal contact increase the risk of infection.

Giardiasis is often detected in international travelers and among internationally adopted children. Transmission to close contacts of infected persons can also occur, especially in child-care settings. People with occupational exposure to human waste are also at risk.

Cysts are infectious immediately upon being excreted in feces. They are environmentally hardy and tolerant to chlorine. Infectious dose is very low; ingestion of 10 cysts can cause infection. Infected persons have been reported to shed more than 100 cysts in their stool per day and to excrete cysts for months.

Symptoms usually begin 7 to 14 days after infection and consist of abdominal cramping, watery diarrhea, malodorous steatorrhea, nausea, vomiting, low-grade fever, and weight loss. Symptoms resolve in most patients after several weeks. However, up to 20% of patients remain chronically infected if not treated. 

Giardiasis should be included in the differential diagnosis when a patient experiences diarrhea lasting more than 3 days. Giardiasis is generally a self-limited illness characterized by diarrhea, abdominal cramps, bloating, weight loss, and malabsorption. Asymptomatic infection is also common. 

While an O&P exam can detect Giardia, the sensitivity is low due to sporadic shedding of the parasite. As many as 7 or 8 specimens may be required for Giardia to be excluded by O&P exam. 

Enzyme immunoassay for Giardia antigen in the stool has a sensitivity of 95% and is the test of choice for giardiasis. Specimen requirement is 10 g of a random stool sample submitted in a screw-capped, plastic container with 10 % formalin.

Specimen requirement is a fresh diarrheal stool sample from inpatients or a stool sample preserved in 10% buffered formalin. A single specimen is sufficient. 

The reference value for the enzyme immunoassay is a negative result.

Giardia is also included in multiplex gastrointestinal panels that utilize polymerase chain reaction, to simultaneously test for parasitic, viral, and bacterial pathogens that cause diarrhea. One example is the FilmArray Gastrointestinal Panel that detects 22 pathogens. Please refer to the article entitled: “Gastrointestinal Pathogen Panel.”

References

Garcia LS, et al. Practical guidance for clinical microbiology laboratories: Laboratory diagnosis of parasites from the gastrointestinal tract. Clin Microbiol Rev. 2017;31(1):e00025-17.

Rosenblatt JE, Sloan LM, Schneider SK. Evaluation of an enzyme-linked immunosorbent assay for the detection of Giardia lamblia in stool specimens. Diagn Microbiol Infect Dis. 1993;16(4):337-341