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The Facts

Giardiasis is an intestinal infection caused by a microscopic, single-celled parasite known as Giardia lamblia. A parasite is an organism that lives on or inside another organism called the host. Typically found in lakes, streams, or ponds that have been contaminated by human, muskrat, dog, or beaver feces, giardiasis is also known as "beaver fever."

Giardia lamblia is one of the most common human parasitic infections in Canada. Higher numbers of infections are seen in the late summer months and even a few deaths have been reported.

Travellers to regions of Africa, Asia, and Latin America where clean water supplies are low are at increased risk of contracting the infection.

Some healthy people do not get sick from Giardia lamblia; however, they can still pass the infection on to others. Children, the elderly, and people with long-term illnesses may be especially prone to contracting the illness, as the risk of transmission is higher in daycare centres, chronic care facilities, and seniors' residences.


The parasite that causes giardiasis lives in the intestines of infected humans and animals. It enters the soil, water, food, or other surfaces after bowel movements. The most frequent method of infection is by drinking contaminated water. However, people may also become infected through hand-to-mouth transmission. This involves eating contaminated food or touching contaminated surfaces and unknowingly swallowing the parasite.

The parasites produce cysts (resistant forms of the parasite), which are swallowed. The cysts then reproduce in the intestines causing the signs and symptoms of giardiasis. The parasites then form new cysts that are passed in the stool, continuing the life cycle of the parasite. Ingestion of as little as 10 cysts is enough to cause illness.

Symptoms and Complications

The signs and symptoms of giardiasis usually occur within 7 to 14 days of exposure to the parasite, although symptoms may appear as early as 3 days or as late as 25 days.

They frequently include diarrhea, pale greasy stools, stomach cramps, gas, nausea, vomiting, bloating, weight loss, and weakness. Some people may experience explosive, foul smelling diarrhea.

Fever, rash, and joint pain are less common. The symptoms usually last for 1 to 2 weeks, but may last as long as 6 weeks.

Individuals who have other illnesses may experience longer-lasting symptoms resulting in complications such as prolonged diarrhea leading to dehydration, further weight loss, and malnutrition.

Other complications include arthritis and damage to the cells that line the intestine.

Making the Diagnosis

Since the signs and symptoms of giardiasis are similar to many other illnesses, your doctor will ask you for a stool sample. The presence or absence of cysts (the resistant form of the parasite found in stool) or parasite antigen helps determine whether or not you have the condition. This test may be repeated several times over several days to confirm the presence of the parasite.

Treatment and Prevention

In some cases, giardiasis goes away on its own in about one month. Other people need antibiotics (e.g., metronidazole*) to shorten the duration of the infection and to kill the parasite.

Because the disease can spread quickly, your doctor may suggest that the whole family be treated at the same time. Your doctor may also suggest that you take your medication for a longer time or change your medication depending on the severity of your illness. It is very important that you let your doctor know if you are pregnant because some medications used to treat this condition can harm the fetus.

Finally, it is very important that you drink enough water and electrolyte-rich drinks (solutions containing sugar and salts) because your body will be losing water due to diarrhea. Signs of dehydration are extreme tiredness; dry skin, mouth, and tongue; sunken eyes; and very little production of urine or tears.

Children are at a higher risk of dehydration than adults due to their small body size, so parents or caregivers should watch for signs of dehydration and ensure that the child drinks plenty of rehydrating solution. Oral rehydration solutions, which are available at pharmacies as liquids or powder packets to mix with water, are an excellent way to keep a child hydrated. If you are mixing electrolyte powder with water, make sure the water is clean to avoid reinfection.

There are several effective ways to avoid getting or spreading this infection. Keep these tips in mind:

  • Do not drink or brush your teeth and wash food or dishes with untreated water from streams, rivers, or lakes, even if they look sparkling clean. Make sure you boil water from these sources for 1 to 2 minutes (or 3 minutes if you are at a high altitude) before use.
  • Wash your hands thoroughly with soap and water before and after eating, preparing food, changing diapers, and using the toilet.
  • Do not send a child who is infected and cannot control his or her bowel movements to daycare or school.
  • Avoid swallowing water when swimming in public pools or lakes. Chlorine commonly used in swimming pools will not kill the cysts.
  • Try to eat well-cooked hot foods and always peel raw vegetables and fruit.

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