Medically Reviewed by Traci C. Johnson, MD on August 12, 2022
3 min read

Giardiasis, also known as a giardia infection, is an intestinal disease marked by diarrhea, cramps, nausea, and bloating. A tiny parasite called Giardia intestinalis causes the infection. This bug lives all around the world in places that don’t have clean drinking water. It’s a common cause of waterborne illness in the U.S.

Symptoms usually start 1-3 weeks after you’re exposed. They’ll probably last for 2-6 weeks. The most common include:

  • Diarrhea
  • Gas or flatulence
  • Greasy stool that floats
  • Stomach or abdominal cramps
  • Upset stomach or nausea
  • Dehydration
  • Weight loss

When should I call a doctor?

If you have any of these symptoms for more than a week, call your doctor to see if you should be tested for giardiasis. It’s possible for a giardia infection to get better on its own, but you may need to have treatment.

You usually pick up a giardia infection from the stool of an infected person or animal. Before they’re pooped out, the parasites grow a hard shell called a cyst. It helps them live for months outside of a body, either in water or on a surface. The infection comes after you come in contact with the parasite or a cyst. You might:

  • Come into close contact with someone who has giardiasis
  • Touch contaminated surfaces like bathroom handles, changing tables, diaper pails, or toys, then eat without washing your hands
  • Drink water or use ice made from an untreated water source -- like a lake, stream, or well -- that’s home to giardia
  • Swallow the parasite while you swim or play in water
  • Eat uncooked food that contains giardia
  • Travel to countries where it’s common

While anyone can catch giardiasis, some people are more likely to come in contact with the parasite:

  • Parents and child care workers who change diapers
  • Children in child care centers
  • People who live in the same household as someone with giardiasis
  • Those who drink water or use ice made from untreated water
  • Backpackers, hikers, and campers who drink unsafe water or who don’t practice good hygiene (like proper handwashing) on the trail
  • International travelers
  • People who have anal sex

To find out if you have giardiasis, you’ll need to give your doctor a stool sample. They’ll send it to a lab for testing. For the best results, you may need to give samples for testing for several days.

If you do have giardiasis, you’ll need to have your stool checked again to see if the parasites have cleared.

Many people get better without treatment. But your doctor may want to treat you with medication. Metronidazole (Flagyl), nitazoxanide (Alinia), and tinidazole (Tindamax) are among the drugs used to treat giardia infections.

Giardiasis can cause problems even after the infection has gone away. These can be especially serious in infants and children.

  • Dehydration. This is when your body doesn’t have enough water to work normally. It’s often the result of severe diarrhea.
  • Failure to thrive. Long-term diarrhea can affect a child’s mental and physical growth.
  • Lactose intolerance. Many people who have a giardia infection find they can’t digest milk sugar (lactose) the way they used to. This can make it hard to eat milk, cheese, and other dairy foods.

There isn’t a vaccine to prevent it, but there are steps you can take:

  • Wash your hands after you use the toilet, after you change diapers, and before you eat or prepare food.
  • Filter or boil water if you’re outdoors.
  • Try not to swallow water when you swim in a pool, lake, or stream.
  • Drink bottled water when you travel to places with unsafe water. And don’t use ice cubes.
  • Use a condom if you have anal sex.