Reviewed on 6/20/2022

Things to Know About Giardiasis

Giardia lamblia is a parasite that infects the small bowel and causes giardiasis.
Giardia lamblia is a parasite that infects the small bowel and causes giardiasis.
  • Giardiasis is the name of the disease caused by Giardia lamblia parasites that infect the small bowel.
  • Giardiasis is caused by Giardia lamblia parasites that damage the small intestine; Giardia lamblia forms cysts that are transmitted to humans in contaminated water or food and by person-to-person contact.
  • Symptoms of giardiasis are variable; some people have no symptoms but still pass cysts in the stool and are considered carriers of the parasite while others may develop acute or chronic diarrheal illnesses that begin to show symptoms in one to two weeks after swallowing cysts.
  • Symptoms and signs of acute giardiasis are profuse watery diarrhea that later becomes greasy and foul-smelling with occasional bloating, abdominal cramping, and passing gas (flatulence).
  • Chronic diarrheal illness symptoms include greasy, foul-smelling, yellowish diarrhea, weight loss, and abdominal pain.
  • Seek medical care if you suspect you have giardiasis; if a person develops profuse watery diarrhea with dehydration or is passing many small volume stools that contain blood or mucus, have fever greater than 100.4 F or 38 C, severe abdominal pain and/or diarrhea in individuals older than 70 years or in individuals with weakened immune systems, the person should go to an urgent care clinic or an emergency department for evaluation.
  • Giardiasis may be diagnosed by several different types of tests including examining the stool cysts, detection of antigens (substances the immune system thinks are foreign or dangerous) in the stool, a string test (patients swallow a string taped to their cheek), and for more difficult diagnoses, aspiration of small bowel contents or biopsy of the small bowel may be done.
  • Some individuals may be treated at home with fluids to prevent dehydration, bland foods, and avoiding milk for several weeks; however, this may not be the appropriate treatment for others so you should consult your health care professional before home treatment.
  • The most common treatment for giardiasis is tinidazole (Tindamax). If you are pregnant, consult your ob-gyn physician to monitor treatments.
  • Follow-up is important to reduce the spread of the disease to caregivers.
  • Giardiasis can be prevented by avoiding contaminated food and water, oral/anal sex, and by practicing excellent hand-washing techniques.
  • The prognosis for someone with giardiasis is usually very good although, in a few individuals, dehydration and malnutrition (especially young children) may lead to developmental impairment.

What Causes Giardiasis?

Giardia lamblia cysts are transmitted to humans in various ways.

  • Contaminated water supplies: Giardia lamblia is one of the most common causes of water-borne diarrhea outbreaks. Sources of contaminated water include public facilities that improperly filter and treat water, water in developing countries, or rivers and lakes used by hikers. Overseas travelers and hikers are at high risk for infection.
  • Contaminated food: Food that may have been washed in contaminated water, exposed to manure, or prepared by an infected person can transmit the disease.
  • Person-to-person contact: Infection may be caused by poor hygiene and most commonly occurs in day care centers, nursing homes, and during oral-anal sexual contact. Family members, daycare workers, and others in contact with infected stool may then themselves become infected.

What Are the Signs and Symptoms of Giardiasis?

Giardiasis can show itself in different ways. Some people can be carriers of the parasite and have no symptoms of the disease, but they pass cysts in their stool and pass the disease to others. Others may develop acute or chronic diarrheal illnesses in which the symptoms occur 1-2 weeks after swallowing the cysts.

Acute diarrheal illness may have the following signs and symptoms

  • Diarrhea: Most people with giardiasis complain of diarrhea. Stool is usually described as profuse and watery early in the disease. Later in the disease, stools become greasy, foul-smelling, and often floats. Blood, pus, and mucus are usually not present. Symptoms may last for 1 to several weeks.
  • Weight loss, loss of appetite
  • Bloating, abdominal cramping, passing excessive gas, sulfur-tasting burps
  • Occasional nausea, vomiting, fever, rash, or constipation

Chronic diarrheal illness may have the following symptoms and signs

  • Diarrhea: Stools are often greasy, foul-smelling, yellowish, and may alternate between diarrhea and constipation.
  • Abdominal pain worsens with eating
  • Occasional headaches
  • Weight loss


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When to Seek Medical Care for Giardiasis

Contact a doctor for appropriate diagnosis and treatment if your symptoms are consistent with giardiasis, if symptoms last despite treatment, or if any new symptoms develop. People should not automatically go to the hospital for these conditions.

However, individuals should go to an urgent care clinic or a hospital if the following symptoms develop.

  • Profuse watery diarrhea with dehydration
    • Signs and symptoms of dehydration in adults include being thirsty, feeling weak, decreased urination, no saliva or dry mouth, no tears, fast heart rate, and light-headedness.
    • Children with dehydration may also get irritable or sleepy.
  • Passing many small-volume stools with blood or mucus
  • Fever greater than 100.4 F (38 C)
  • More than six unformed stools in 24 hours
  • Illness longer than 48 hours
  • Diarrhea with severe abdominal pain in anyone older than 50 years
  • Diarrhea in anyone older than 70 years
  • Diarrhea in people with weakened immune systems such as those with AIDS, people receiving chemotherapy, or transplant recipients

How Do Medical Professionals Diagnose Giardiasis?

There are multiple ways to diagnose giardiasis; the following is a list of tests doctors utilize to diagnose the disease. Some individuals with acute giardiasis are relatively easy to diagnose while others may require more invasive tests.

Stool exam

  • The most common way to diagnose giardiasis is by the visualization of the Giardia lamblia cysts by experienced laboratory professionals. The cysts are detected more than half the time in the first stool specimen examined. Most of the time, the cysts are detected after three stool specimens have been examined. So more than one specimen may be required.
  • Another method of diagnosis that is commonly used as a screening tool in outbreaks or in day care centers is antigen assay of stool. This method detects a certain protein found in the wall of Giardia lamblia. A stool sample is mixed with a solution that detects the cysts in the stool.

String test

  • The string test involves swallowing a fuzzy string enclosed in a gelatin capsule. The free end of the string is taped to the person's cheek. Once swallowed, the string collects secretions and mucus from the small bowel. Four hours later the string is pulled back out and examined for organisms.

Aspiration and biopsy

  • This is the most invasive method of diagnosis. After the passage of a small tube (endoscope) through the person's mouth and stomach into the small bowel, the doctor removes a small amount of tissue for examination. This method is reserved for difficult cases in which the cause of diarrhea cannot be determined using other methods. It allows for visualization of all abnormalities of the small intestine, which may cause diarrhea including giardiasis.

Home Remedies for Giardiasis

  • Drink fluids such as sports drinks, diluted fruit juices, flat soda (such as 7-Up or ginger ale, none with caffeine), broth, soups, or preparations such as Pedialyte for children. Fluids should be taken in small amounts frequently throughout the day. Avoid fluids containing caffeine.
  • Suck on ice chips to keep from becoming dehydrated if you cannot keep fluids down.
  • After 12 hours, the diet can be advanced to bland foods such as potatoes, noodles, rice, toast, cereal, crackers, and boiled vegetables. Avoid spicy, greasy, and fried foods.
  • After stools become formed, return to a regular diet. Avoid milk for several weeks.

What Are the Medications and Treatments for Giardiasis?

Medications are used effectively to treat giardiasis.

Metronidazole (Flagyl)

  • This is the most common and most often recommended drug to treat giardiasis.
  • Treatment is highly effective with these pills, given three times a day for 5 days.
  • Side effects may include nausea, vomiting, headache, dry mouth, or a metallic taste in the mouth. Urine may turn dark or reddish-brown. Metronidazole brings on nausea and vomiting when alcohol is consumed while on the medication. Avoid alcohol 24 hours before starting the drug and for 48 hours after the last dose.


  • Treatment is highly effective with these pills, given over 5 days. Side effects may include nausea, vomiting, yellow skin and eyes, dark urine, and a rash.

Nitazoxanide (Alinia)

  • Nitazoxanide is available in liquid form and must be taken with food. Side effects may include stomach pain, diarrhea, vomiting, or headache.

Other treatments

  • Tinidazole (Tindamax) is an oral single-dose antiprotozoal agent used in children

Pregnant women with giardiasis are treated somewhat differently because of the possible risk to the fetus by some of the medications.

  • If the disease is mild and dehydration can be avoided, treatment may be postponed until after delivery.
  • If treatment is necessary, paromomycin may be used but is less effective. The effects of metronidazole on the fetus appear to be minimal and occur mostly in the first trimester.
  • If therapy is necessary, metronidazole is probably safe to use in the second and third trimesters, but women should see their ob-gyn doctor before any treatment is started.


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What Is the Follow-up for Giardiasis?

The American Academy of Pediatrics recommends for day care outbreaks that all employees and children improve their hand washing techniques and sanitation. All children, workers, and family members with symptoms of giardiasis should be treated. Children and staff members should be kept away from the center until they no longer have symptoms.

How Can You Prevent Giardiasis?

  • Avoid contaminated water
    • Travelers to developing countries and hikers in the wilderness should consider all water sources contaminated. For travel to high-risk areas (Africa, Latin America, southern Asia), where medical care may not be readily available, talk to your doctor about taking prescription medication with you. Over 80% of diarrhea in travelers is caused by bacteria and is usually treated with a quinolone antibiotic. Diarrhea that does not go away with a quinolone antibiotic may be caused by a parasite such as Giardia lamblia and can be treated with metronidazole. Any diarrheal illness that does not include fever or bloody diarrhea can be relieved with Imodium or Pepto-Bismol as directed, as long as the antibiotic is also taken.
    • All water must be boiled, filtered, or treated with halogenated tablets or solutions (chlorine-type treatment to purify).
    • Avoid foods washed in contaminated water or that cannot be cooked or peeled. Travelers to foreign countries should be especially careful to avoid drinking water in foreign countries (including ice cubes in drinks). Drink bottled water. Avoid fresh fruits and vegetables that may have been washed in contaminated water. Eat only items that can be peeled.
    • In the United States, many reported cases of Giardia lamblia occur in the summer months. This may be due to the use of community swimming areas by young diaper-aged children (such as lakes, pools, and water parks.)
  • Avoid risky sexual behaviors.
  • Wash hands thoroughly with soap and warm water after using the bathroom and before eating.
  • Wash hands after changing a baby's diaper. If you are a day care worker, it is especially important to wash after changing each child.

What Is the Prognosis for Giardiasis?

Cure rates are high with appropriate medication in an otherwise healthy person. Treatment failure may be due to poor compliance with medication, reinfection, resistant strain, or underlying difficulty with the immune system. Resistant strains may require a second course of the same medication or a change to a different medication.

  • Giardiasis may lead to lactose intolerance during the infection and up to several weeks after treatment. Symptoms of lactose intolerance, which include bloating, cramping, and watery diarrhea after ingestion of milk products, may be misinterpreted as reinfection.
  • In someone with a weakened immune system, giardiasis may lead to chronic infection. Chronic infection is associated with difficulty absorbing some important vitamins, minerals, and protein that may lead to malnutrition. Malnutrition in infants and young children may lead to impairment of growth and development.

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Reviewed on 6/20/2022
Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease

Nazer, Hisham. "Giardiasis." Medscape. Oct. 1, 2018. <>.