• Giardia lamblia that causes giardiasis


Giardiasis is a gastrointestinal infection that causes diarrhea. It is caused by the parasite Giardia intestinalis (formerly known as Giardia lamblia).

What is going on in the body?

Giardiasis occurs all over the world. The parasite is found in the stool of an infected person or animal and is frequently excreted into the environment. This organism has a protective shell that allows it to live for long periods of time outside the body. Thus, it can be found in drinking water and in recreational bodies of water, such as lakes. Giardiasis is the most common waterborne disease in the United States.


What are the causes and risks of the infection?

Giardiasis is spread by contact with the parasite that causes the infection. The parasite can be found in food, water, soil, and other surfaces contaminated by stool from an infected person. A person can be infected in several ways:
  • accidentally swallowing stool from surfaces, such as bathroom fixtures and toys
  • eating contaminated food, especially raw fruits and vegetables
  • ingesting stool from dirty diapers, diaper pails, or changing tables
  • ingesting the parasite during oral-anal or oral-genital sex
  • swallowing infected water from swimming pools, lakes, hot tubs, fountains, or streams
  • using a contaminated drinking water source
  • using poor hygiene after handling feces of an infected person


What can be done to prevent the infection?

Many cases of giardiasis can be prevented by taking the following steps:
  • Avoid contaminated recreational water.
  • Avoid oral-anal or oral-genital sex.
  • Avoid untreated water sources.
  • Follow tips to avoid traveler's diarrhea.
  • Put pool diapers on infants and toddlers to keep feces out of recreational water.
  • Use good handwashing techniques after using the bathroom.
  • Use good hygiene when diapering infants.


How is the infection diagnosed?

Diagnosis of giardiasis begins with a medical history and physical exam. A specimen of the person's stool may be checked in the lab for signs of the parasite.

Long Term Effects

What are the long-term effects of the infection?

In severe cases, there may be weight loss or stunted growth in children.

Other Risks

What are the risks to others?

Anyone who has the organism in the feces can spread it to others, especially if they have poor hygiene practices. Infected people should use good handwashing techniques. They should not swim in pools or other bodies of water until the infection is gone.


What are the treatments for the infection?

Metronidazole (i.e., Flagyl) is the medicine of choice for most people with giardiasis. Other treatments include tinidazole (i.e., Tindamax) and paromomycin (i.e., Humatin, especially for pregnant women).

Side Effects

What are the side effects of the treatments?

People taking metronidazole should avoid alcohol. Some side effects are as follows:
  • abdominal cramps
  • diarrhea
  • headache
  • loss of appetite
  • nausea and vomiting

After Treatment

What happens after treatment for the infection?

Giardiasis usually responds readily to the medicine. If symptoms continue, an ongoing source of infection (such as a contaminated water supply) should be suspected.


How is the infection monitored?

The healthcare professional may request a stool sample to be sure the parasite has been eliminated. Any new or worsening symptoms should be reported to the healthcare professional.

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