Overview
Giardia lamblia is a single-celled parasite that lives in the upper small intestine. It causes giardiasis, an acute, watery diarrhea, in an estimated 280 million people every year. Asymptomatic infections are equally common.[1] It is the most frequently identified protozoan cause of intestinal infection, and in some areas where it is endemic, infection rates approach 90%.[2]
Giardia is unusual among intestinal pathogens. It does not invade the gut wall,[1] produces no known toxin, and does not trigger a strong inflammatory response.[2] Yet infection has been linked to problems that last for years, including irritable bowel syndrome and chronic fatigue.[3]
Classification
- Phylum: Metamonada
- Family: Hexamitidae
- Genus: Giardia
- Species: G. lamblia (also called G. intestinalis or G. duodenalis)
The three species names refer to the same organism, which is why they appear interchangeably in medical literature and lab reports. Giardia is a biological oddity: its active form, the trophozoite, lacks conventional mitochondria, and its primitive organelles and metabolism have led to proposals that Giardia is among the most primitive eukaryotes. The two major genetic groups that infect humans are so different that they may warrant separate species designations.[4]
How Giardia Spreads
Giardia alternates between two forms: trophozoites, which attach to the lining of the small intestine and multiply, and hardy cysts, which pass in stool and survive in the environment. Infection begins when cysts are swallowed, most often through contaminated water. Food and direct contact with infected people or animals can also spread it. Giardia infects many other mammals as well as humans.[4]
Waterborne outbreaks can be large. A 2004 waterborne outbreak in Bergen, Norway, led to more than 1,200 laboratory-confirmed infections, a group researchers then followed for years.[3]
Giardia and the Gut Microbiome
Because Giardia produces no known toxin, researchers have looked for other explanations for its symptoms. One is dysbiosis. In mice, Giardia colonization of the small intestine caused a gut-wide shift in bacterial communities, with expansion of aerobic Proteobacteria and loss of anaerobic Firmicutes throughout both the small intestine and colon. The authors proposed that giardiasis could be, in part, an "ecological disease."[2] This is animal research, and how closely it reflects human infection is still being studied.
Intriguingly, recent data have also suggested that Giardia infection may, in some settings, be protective against other diarrheal diseases.[1] Its relationship with human health is more complex than simple pathogen and victim.
Long-Term Effects: IBS and Chronic Fatigue
The Bergen outbreak cohort has provided long-term evidence that an intestinal infection can be followed by lasting symptoms. Researchers compared infected people with matched controls:
| Time after infection | IBS (infected vs controls) | Chronic fatigue (infected vs controls) |
|---|---|---|
| 3 years | 46.1% vs 14.0% (adjusted RR 3.4)[5] | 46.1% vs 12.0% (adjusted RR 4.0)[5] |
| 6 years | 39.4% in infected group (adjusted RR 3.4)[3] | 30.8% in infected group (adjusted RR 2.9)[3] |
Both conditions became less common between years three and six, suggesting that these complications are very long-lasting but slowly self-limiting.[3] Giardiasis has also been linked to food allergies after the infection resolves.[1]
If you developed persistent digestive symptoms after a diarrheal illness, see our pages on IBS and chronic fatigue syndrome, and our guide to probiotics for IBS.
Diagnosis and Treatment
Giardiasis is diagnosed by stool testing, using microscopy for cysts or trophozoites, antigen tests, or molecular panels. Because shedding can be intermittent, more than one sample is sometimes needed.
Most symptomatic infections resolve on their own, but some become chronic.[1] Treatment can shorten illness and reduce spread to others. Metronidazole is a standard recommended treatment.[6] Other prescription options exist, and a clinician will choose based on age, pregnancy, and other factors. Treatment failures are becoming more common worldwide, which is one reason follow-up testing matters when symptoms persist.[1]
Probiotic evidence
In a double-blind, placebo-controlled trial, 65 adults with giardiasis received metronidazole for 10 days along with either Saccharomyces boulardii (250 mg twice daily) or placebo:[6]
| Group | Still passing Giardia cysts at 2 and 4 weeks |
|---|---|
| Metronidazole + S. boulardii (n=30) | 0% |
| Metronidazole + placebo (n=35) | 17.1% (6 patients) |
This suggests Saccharomyces boulardii may be a useful add-on to standard treatment, though it was a single small trial. It is not a substitute for prescribed antiparasitic medication. See also our guide to probiotics for diarrhea.
Prevention
- Treat untreated water from lakes, rivers, and streams by boiling or using a filter rated for protozoan cysts.
- Wash hands with soap and water after using the toilet, changing diapers, and handling animals, and before preparing food.
- Avoid swallowing water while swimming, and don't swim while you have diarrhea.
- Be cautious with water and raw produce when traveling in areas with limited sanitation. See our page on travelers' diarrhea.
For another waterborne intestinal parasite, see Cryptosporidium.
When to See a Clinician
See a clinician for diarrhea lasting more than a few days, especially after drinking untreated water, traveling, or contact with someone who has giardiasis. Seek prompt care for signs of dehydration, blood in the stool, high fever, or weight loss. If symptoms persist after treatment, ask about retesting, since treatment failures occur.
This page is for educational purposes and is not medical advice. Consult a qualified healthcare provider about diagnosis and treatment of parasitic infections.