Giardiasis is infection with the flagellated protozoan Giardia duodenalis (G. lamblia, G. intestinalis). Infection can be asymptomatic or cause symptoms ranging from intermittent flatulence to chronic malabsorption. Diagnosis is by identifying the organism in fresh stool or duodenal contents, by assays for Giardia
(See also Overview of Intestinal Protozoan and Microsporidia Infections.)
Giardia trophozoites firmly attach to the duodenal and proximal jejunal mucosa and multiply by binary fission. Some organisms transform into environmentally resistant cysts that are spread by the fecal-oral route.
Giardia infection is prevalent throughout the world, particularly in areas with poor sanitation. Waterborne transmission is the major source of infection (1), but transmission can also occur by ingestion of contaminated food or by direct person-to-person contact. In 2018, there were 15,579 giardiasis cases in the United States reported to the Centers for Disease Control and Prevention (CDC) (see CDC: Giardiasis NNDSS Summary Report for 2018).
Giardia cysts remain viable in surface water and are resistant to routine levels of chlorination in drinking water. Thus, mountain streams as well as chlorinated but poorly filtered municipal water supply systems have been implicated in waterborne epidemics. Infections are also associated with childcare, especially involving children in diapers; close contact with family or household members who have giardiasis; ingestion of water or ice from untreated or improperly treated water from lakes, streams, or wells; backpackers, hikers, and campers who drink unsafe water or fail to practice good hand hygiene; ingesting water while swimming or playing in lakes, ponds, rivers, or streams; or exposure to feces through sexual contact.
There are 8 genetic groups (assemblages) of G. duodenalis. Two infect humans and animals; the others infect only animals. The clinical manifestations appear to vary with genotype.
General reference
1. Schnell K, Collier S, Derado G, et al: Giardiasis in the United States - an epidemiologic and geospatial analysis of county-level drinking water and sanitation data, 1993-2010. J Water Health 14(2):267–279, 2016. doi: 10.2166/wh.2015.283.
Symptoms and Signs of Giardiasis
Many cases of giardiasis are asymptomatic. However, asymptomatic people can pass infective cysts.
Symptoms of acute giardiasis usually appear 1 to 14 days (average 7 days) after infection. They are usually mild and include watery malodorous diarrhea, abdominal cramps and distention, flatulence, eructation, intermittent nausea, epigastric discomfort, and sometimes low-grade malaise, fatigue, and anorexia. Acute giardiasis usually lasts 1 to 3 weeks. Giardiasis is often accompanied by acquired lactose-intolerance. Malabsorption of fat and sugars can lead to significant weight loss in severe cases. Neither blood nor white blood cells are present in stool.
A subset of infected patients develop chronic diarrhea with foul stools, abdominal distention, and malodorous flatus. Substantial weight loss and fatigue may occur. Chronic giardiasis occasionally causes failure to thrive in children.
Diagnosis of Giardiasis
Enzyme immunoassay for antigen or molecular test for parasite DNA in stool
Microscopic examination of stool
Enzyme immunoassay to detect parasite antigen in stool is more sensitive than microscopic examination. Characteristic trophozoites or cysts in stool are diagnostic, but parasite excretion is intermittent and at low levels during chronic infections. Thus, microscopic diagnosis may require repeated stool examinations.
Sampling of the upper intestinal contents can also yield trophozoites but is seldom necessary.
Molecular tests for parasite DNA in stool are available.
Treatment of Giardiasis
Dehydration due to diarrhea is a particular risk for pregnant women and infants. Rehydration therapy is especially important for these groups.
Even after parasitologic cure, patients may experience lactose intolerance, irritable bowel syndrome, or fatigue that last for weeks to months. If Giardia is not detected in 3 stool exams with concentration, or by repeat stool test, it is highly probable that the patient is no longer infected. Guidelines for managing recurrent giardiasis infections and treatment failures are available (see CDC: Giardia: Diagnosis and Treatment Information for Medical Professionals).
Prevention of Giardiasis
Prevention of giardiasis requires
Appropriate public water treatment
Hygienic food preparation
Appropriate fecal-oral hygiene
Thorough handwashing after contact with feces
Water can be decontaminated by boiling. Giardia cysts resist routine levels of chlorination. Disinfection with iodine-containing compounds is variably effective and depends on the turbidity and temperature of the water and duration of treatment. Some handheld filtration devices can remove Giardia cysts from contaminated water, but the efficacy of various filter systems has not been fully assessed.
(See also Centers for Disease Control and Prevention [CDC]: Parasites - Giardia: Prevention & Control and CDC Yellow Book: Giardiasis).
Key Points
The major source of giardiasis is waterborne transmission, including via fresh-appearing mountain streams and poorly filtered municipal water supplies.
Giardia cysts resist routine levels of chlorination, and disinfection with iodine-containing compounds is variably effective.
Enzyme immunoassay to detect parasite antigen in stool is preferred because it is more sensitive than microscopic examination.
Symptoms may persist after parasite clearance.
