An Overview of Giardia

A Common Parasite Found Worldwide

Table of Contents
View All
Table of Contents

Giardia lamblia, Giardia duodenalis, or simply Giardia is a common parasite found all over the world world. People living in developing countries are most at risk for infection with Giardia (giardiasis). The infection is usually contracted by drinking water that has been contaminated with the parasite.

Compared to the prevalence in developing countries, Giardia infection is much less common in the United States. Giardiasis is most likely to be diagnosed in someone with diarrhea who has recently traveled to a developing country.

As it is in the developing world, drinking water from environmental and industrial sources that are prone to contamination is also a risk factor. According to annual surveillance reports from the Centers for Disease Control and Prevention (CDC), contaminated water sources (such as those found in agricultural or occupational settings) are the most frequent cause of giardiasis outbreaks in the U.S.

The primary symptom of giardiasis is diarrhea, though it can also cause gas and an upset stomach. While the infection can be challenging to diagnose, giardiasis is usually treatable with medications.

Giardiasis / Getty Images


Not everyone who has giardiasis has symptoms. Those who do typically start to feel sick within one to two weeks after infection, with the average being seven days.

Symptoms of giardiasis include:

  • Diarrhea
  • Foul-smelling gas and bloating
  • Watery or greasy stools that float or are difficult to flush
  • Fatigue
  • Headache
  • Low-grade fever
  • Upset stomach or nausea
  • Abdominal cramps
  • Lack of appetite
  • Weight loss

These symptoms typically last one to three weeks. Sometimes, the symptoms will get better only to come back again. In cases of chronic giardiasis, a person's symptoms might continue even after the infection is gone.

Some people develop lactose intolerance after having giardiasis, which can be temporary or long-lasting.


Giardiasis is rarely life-threatening in the United States. Certain complications such as dehydration and malabsorption can severely damage the body, and the effects can be particularly serious for infants and children, who may experience stunted growth or other developmental problems.

  • Dehydration. Diarrhea from giardiasis can lead to dehydration. Any diarrheal illness can disrupt the fluid balance in the body. Left untreated, dehydration can be dangerous. Infants and children, people who are pregnant, and older adults tend to be more vulnerable to dehydration and its effects.
  • Malabsorption. Malabsorption occurs when the intestines can't adequately absorb vital nutrients, such as fats and vitamins, from food. Many diarrheal diseases can lead to malabsorption, including Giardia infection. Infants and children need proper nourishment to support growth and development, so they are especially at risk for longterm problems if they contract giardiasis.


Giardiasis is caused by a microscopic parasite called Giardia. People and animals infected with the parasite shed cysts (or eggs) in their stools. It takes very few cysts to infect a person with Giardia. A single stool of someone with giardiasis can contain as many as 10 billion cysts, but someone only needs to ingest around 10 cysts to become infected.

In the absence of proper sanitation and hygiene practices, infected feces can contaminate hands, objects, and water sources. Giardia can then be passed to other people via the fecal-oral route.

The small cysts are tough and can survive a wide range of environments, including being submerged in cold water for months. They can also lurk in undercooked food or raw food that has been handled by someone with the parasite.

When you eat or drink something contaminated with Giardia cysts, the parasite makes itself at home in your small intestine.

Anyone can get giardiasis—especially if they do not practice proper hand hygiene. However, some people are more likely to become infected than others. People at increased risk for giardiasis include:

  • Diaper-aged infants, toddlers, and children (as well as those who care for them)
  • People who care for or have contact with infected animals, including pets (including dogs, cats, rodents, and exotic animals)
  • Campers, hikers, and travelers who drink water or use ice from water sources that could be contaminated (rivers, streams, lakes)
  • Swimmers or people wading/playing in recreational bodies of water (ponds and lakes) that could be contaminated
  • International travelers and those living and working in areas without access to safe drinking water or sanitation facilities
  • People who are exposed to feces such as childcare workers, nurses, and people who come into contact with feces during sexual activity

Animals can also be infected with Giardia and may pass the infection on to humans, though this means of transmission is less common. Animals commonly kept as pets, such as cats and dogs, are not usually infected with the type of Giardia parasite that can make humans sick.

However, there are many illnesses that can cause diarrhea in pets, some of which pose risk to humans. Use caution when handling soiled material and caring for your sick pet, and be sure to consult with your vet.


Giardia infection can be challenging to diagnose; Health care providers rely on a stool analysis to confirm the presence of the parasite.

Stool Analysis

Checking your stool for the parasite is the primary way doctors diagnose a Giardia infection. The test can be done in one of two ways, both of which will require you to provide a stool sample.

  • Stool Ova and Parasites Exam. For this exam, a small amount of stool is smeared on a slide. The sample is examined with a microscope for signs of cysts or adult parasites.
  • Antigen Test. An antigen test doesn't look for the whole parasite—rather, it looks for a protein made by Giardia when it’s in the human body. These proteins are what the immune system responds to when it’s trying to defend itself from the infection.

Giardia parasites can be hard to spot and don't always show up in a stool sample from someone who is infected. If your doctor suspects you have giardiasis, they might order both tests at the same time. Or, if the first test comes back negative, they might have you give additional stool samples—especially if your symptoms match up with giardiasis.

String Test

If stool tests don't confirm the diagnosis but your doctor still suspects giardiasis, they might use another tool called the string test or Entero-test.

For the test, you swallow a string with a weighted gelatin capsule attached to one end. As the string moves through the digestive tract, it collects samples from the upper part of the small intestine. About four hours later, your doctor removes the string and examins the fluids it gathered under a microscope for the parasite.

The string test is not typically the first test your doctor will use if they think you have giardiasis. Some researchers believe the string test is better at spotting Giardia than stool tests, but other studies claim it isn't effective.

Since the research has been inconsistent and the test can be uncomfortable for patients, it's typically only used when stool tests are negative but giardiasis is highly suspected.


Not everyone who is infected with Giardia needs treatment. Infections sometimes clear up on their own within a few weeks. If you don’t have symptoms or your symptoms are mild, your doctor might tell you that treatment isn’t necessary.

If you become infected with Giardia while you are pregnant, you'll need to discuss your treatment options with your doctor. Some drugs used to treat parasites might not be safe or effective if you are pregnant or breastfeeding.

Your doctor's recommendation might also depend on which trimester of pregnancy you are in. If your symptoms are mild, your doctor might have you wait until after you deliver to start treatment.

If you have severe or persistent symptoms, medications can be used to treat the infection. Drug treatment may also be necessary if you're at risk for spreading the parasite to others, such as if you work at a nursing home or childcare center.

Both antibiotics and anti-parasite drugs can be used to treat a Giardia infection, though not all medications are available in the U.S.


Even though Giardia is a parasite, antibiotics are frequently used to treat the infection. Common antibiotics prescribed for giardiasis include:

Furazolidone, an antimicrobial, is also considered an anti-parasite medication and can be used with antibiotics to treat giardiasis.


Anti-parasitic drugs can also be effective against Giardia, including:

  • Nitazoxanide
  • Quinacrine

Research has suggested that another anti-parasite drug called albendazole could also be a viable option for treating Giardia infection, especially for cases that haven’t responded to other drugs. A 2012 Cochrane Review found that albendazole might be just as effective as metronidazole, but with fewer side effects and a simpler regimen.

Your doctor will consider your health history and the state of your immune system when deciding which drug to prescribe. They will also tell you about the possible side effects. For example, you might experience a metallic taste in your mouth, react poorly to alcohol, or become nauseated while taking medications to treat giardiasis.


Giardia infection is almost entirely preventable if you take precautions. The following are a few strategies for preventing giardiasis you can take at home as well as when traveling.

  • Drink only treated, filtered, or boiled water. Avoid swallowing any water from potentially unsafe recreational sources like lakes and hot tubs. If you're traveling and not sure the water is safe to drink, stick to bottled water.
  • Cook foods thoroughly. Do not eat raw or undercooked food, especially when traveling to areas that don't have clean water. For raw food, like fresh produce, be sure to wash with safe water and peel before eating.
  • Wash your hands. Practice proper hand hygiene by frequently washing your hands with soap and water, especially before eating and after using the bathroom.
  • Use condoms. Since the infection can be spread through fecal matter, use condoms during anal sex and avoid contact with feces during sexual activity.

A Word From Verywell

Giardiasis can be hard to diagnose and may cause unpleasant symptoms, but it can be treated. Sometimes, giardiasis doesn't cause any symptoms and goes away on its own without treatment. You can prevent exposure to Giardia parasites by using good hygiene, practicing safe sex, and avoiding unsafe drinking water.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Haque R. Human intestinal parasites. J Health Popul Nutr. 2007;25(4):387–391.

  2. Division of Global Migration and Quarantine, National Center for Emerging and Zoonotic Infectious Disease, CDC, USA. Surveillance for travel-related disease — GeoSentinel Surveillance System, United States, 1997–2011. Morbidity and Mortality Weekly Report (MMWR). Published July 2013.

  3. McClung RP, Roth DM, Vigar M, et al. Waterborne disease outbreaks associated with environmental and undetermined exposures to water — United States, 2013–2014. MMWR Morb Mortal Wkly Rep. 66(44):1222-1225. doi:10.15585/mmwr.mm6644a4

  4. Centers for Disease Control and Prevention. Illness & Symptoms | Giardia | Parasites | CDC. Updated July 1, 2015.

  5. Hanevik K, Dizdar V, Langeland N, Hausken T. Development of functional gastrointestinal disorders after Giardia lamblia infection. BMC Gastroenterol. 2009;9(1). doi:10.1186/1471-230X-9-27

  6. Halliez MC, Buret AG. Extra-intestinal and long term consequences of Giardia duodenalis infectionsWorld J Gastroenterol. 2013;19(47):8974–8985. doi:10.3748/wjg.v19.i47.8974

  7. Robertson LJ, Hanevik K, Escobedo AA, Mørch K, Langeland N. Giardiasis – why do the symptoms sometimes never stop? Trends in Parasitology. 2010;26(2):75-82. doi:10.1016/

  8. Centers for Disease Control and Prevention (CDC). Pathogen & Environment | Giardia | Parasites. Updated February 2017.

  9. CDC. Giardiasis surveillance – United States, 2006-2008. MMWR Morb Mortal Wkly Rep. 2010;59(SS06):15-25.

  10. Centers for Disease Control and Prevention. Giardia & Pets | Giardia | Parasites | CDC. Updated July 21, 2015.

  11. The University of Michigan. Antigen Tests For Giardiasis | Michigan Medicine. Updated July 30, 2018.

  12. Hooshyar H, Rostamkhani P, Arbabi M, Delavari M. Giardia lamblia infection: review of current diagnostic strategiesGastroenterol Hepatol Bed Bench. 2019;12(1):3–12.

  13. Center for Disease Control and Prevention. General Information | Giardia | Parasites | CDC. Updated July 1, 2015.

  14. Ryan ET. Hunter’s Tropical Medicine and Emerging Infectious Diseases E-Book. 10th ed. Elsevier Health Sciences; 2019:705-711.doi:10.1016/C2009-0-51934-4

  15. Centers for Disease Control and Prevention. Treatment | Giardia | Parasites | CDC. Published July 1, 2015. 

  16. Hess, MD E, McMillion, MD B, Niere, MD J. What is the best treatment for Giardia? Evidence-Based Practice:. 2017;20(6):10. doi:10.1097/01.EBP.0000541735.14090.2b

  17. Granados CE, Reveiz L, Cuervo LG, Uribe LG, Criollo CP. Drugs for treating giardiasis. Granados CE, ed. Cochrane Database of Systematic Reviews. Version published: 2012. doi:10.1002/14651858.cd007787

  18. Weir CB, Le JK. Metronidazole.StatPearls - NCBI Bookshelf. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Updated May 17, 2019.