An Overview of Giardia

A Common Parasite Found Worldwide

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Giardia is a common parasite worldwide, including in the United States. One in three people in developing countries have been infected by the parasite at least once, and it’s the most common human intestinal parasite found in the United States.

The primary symptom of a Giardia infection (or giardiasis) is diarrhea, though it can also cause awful gas and an upset stomach. While it’s sometimes tough to diagnose, giardiasis is easy to treat with the help of medications.


Not everyone who has giardiasis has symptoms. Those who do typically start to feel sick about one to three weeks after getting infected. Symptoms of giardiasis include:

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

These symptoms typically last anywhere from two to six weeks. In cases where someone has had Giardia for a long time, the symptoms might continue even after they’re no longer infected.


Giardiasis isn’t often life-threatening, but it can be. Certain complications—like dehydration and malabsorption—can cause fatal or life-long damage to your health or development.

  • Dehydration. Because giardiasis can lead to diarrhea, some people infected with the parasite can lose too much fluid and become dehydrated. Dehydration is fairly common among diarrheal diseases, but it can be dangerous if left untreated, especially for certain populations like pregnant women and infants.
  • Malabsorption. Malabsorption is when the body has a hard time capturing nutrients from food, such as vitamins, proteins, or fats. It’s a possible complication of many diarrheal diseases, and it can be especially damaging in children who need the nutrients to grow and develop normally.


Giardiasis is caused by a microscopic parasite called Giardia. People and animals infected with the parasite shed cysts (or eggs) in their stools. Without proper sanitation like bathrooms and hand-washing, the infected feces end up on hands, objects, and in water sources, where they can be passed onto other people.

The small cysts are tough—they can survive a wide range of environments, including months in cold water and in undercooked foods.

You get infected when you eat or drink something contaminated with the cysts, and the parasite is able to make its home in the small intestine. 

Anyone can get giardiasis. It’s found in every region of the world, including all over the United States. However, some people are more likely to become infected than others, including:

  • diaper-aged kids and those who care for them
  • those who drink or use ice from water sources that might be contaminated with the parasite, like rivers or lakes
  • campers or hikers, especially those who don’t practice good hygiene like washing their hands with soap and water
  • swimmers or those playing in recreational water infected with the parasite, such as ponds or lakes
  • international travelers and those living and working in areas without access to safe drinking water or sanitation facilities
  • those who come in contact with feces during sexual activity


A Giardia infection can sometimes be challenging to diagnose, especially if you don’t have any symptoms. Doctors and other health care providers typically rely on a stool analysis to confirm the infection.

Stool Analysis

Checking your stool for the parasite is the primary way doctors and other health care providers diagnose a Giardia infection. This is typically done in one of two ways, both of which require a stool sample.

  • Stool Ova and Parasites Exam. For this exam, a small amount of stool is smeared on a slide and looked at through the microscope to spot the cysts or the adult parasites.
  • Antigen Test. An antigen test isn’t looking 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. 

Giardia can be tricky to spot and doesn’t always show up in stool samples when a person is infected. Doctors might order both tests at the same time or have you submit additional stool samples if your results come back negative—especially if your symptoms match up with giardiasis.

String Test

Though significantly less common, another tool doctors use to diagnose a Giardia infection is the string test. To do this test, you’re asked to swallow a string with a weighted gelatin capsule attached to the end. The string works its way through the digestive tract to collect samples from the upper part of the small intestine.

The string is removed four hours later, and then any fluids gathered by the string are looked at under a microscope for signs of the parasite. The whole process can be really uncomfortable, and it’s typically only done after a stool analysis comes back negative. 


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

Likewise, most pregnant women infected with Giardia should postpone getting treated until after delivery because the drugs could be harmful to the unborn baby.

Medications, however, are available to treat those who have severe or persistent symptoms or who are at risk of spreading the disease like those in childcare or nursing facilities. Both antibiotic and anti-parasite drugs are available to treat a Giardia infection, though not all medications are available in the United States.


While Giardia is a parasite, antibiotics are frequently used to treat the infection. These include:

  • Tinidazole
  • Metronidazole
  • Paromomycin
  • Furazolidone (also considered an anti-parasite medication)


Anti-parasitic drugs are also sometimes prescribed to cure giardiasis, including:

  • Nitazoxanide
  • Quinacrine

In addition to the medications mentioned above, the drug albendazole might also be a viable option, though more research is needed before it is routinely used to treat giardiasis. A Cochrane Review of drugs used to treat Giardia infections found that the anti-parasite might be just as effective as metronidazole, but with fewer side effects and a simpler regimen.

The drug you are prescribed to treat giardiasis might depend on a wide range of things, including your health history and the condition of your immune system. The side effects for some of the medications can also be pretty uncomfortable. Some people experience a metallic taste in the mouth, react poorly to alcohol, or become nauseated while taking them.


Because Giardia is spread through contaminated food and water, it is almost entirely preventable, so long as you take some precautions. To prevent giardiasis:

  • Drink only treated, filtered, or boiled water. This includes being careful not to swallow any water from unsafe recreational sources like lakes or hot tubs.
  • Cook foods thoroughly. Be sure to wash fresh produce before eating it.
  • Wash your hands. Wash using soap and water frequently, especially before eating and after using the bathroom.
  • Use condoms. Be sure to use condoms if having anal sex, and avoid contact with feces during sexual activity.

A Word From Verywell

Giardiasis is unpleasant but not often fatal, and it can be easily prevented by practicing good hygiene and sticking to safe water sources.

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