Giardia infection

Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at February 24, 2016
StartDiseasesGiardia infection

Giardia infection is caused by a protozoan parasite that thrives in unclean water and is thus a waterborne disease. The Giardia microbe is found both in nature and in contaminated drinking water, pools and wells. Giardia infection is also known as giardiasis and beaver fever. It can cause diarrhea and intestinal symptoms that last as long as six weeks, and, rarely, long-term and permanent effects as well.


Definition & Facts

Giardia infection is relatively common in people and animals around the world. It is spread both through hand-to-mouth contact and by drinking contaminated water. In a typical year, over 250 million people will experience symptoms due to giardiasis, and many more will carry the disease without symptoms.

In a developed country such as the United States, about 5% of the population carry the Giardia parasite. This number increases to 30% in less developed countries. Pets can contract giardiasis and spread it to their human owners, too. While Giardia infection is not considered fatal, it does result in a lot of illness around the world and is a major health threat.

Symptoms & Complaints

Giardiasis causes a range of intestinal symptoms in those who contract it. Some people may suffer from long-term problems, while others experience mild symptoms or none at all. Even people without symptoms can spread the disease to others, however.

Symptoms can begin as soon as a day after ingesting the parasite and include gas, intestinal cramps, loss of appetite, and severe, watery diarrhea. When prolonged, diarrhea can lead to dehydration, muscle weakness, fatigue and weight loss.

Anyone who experiences these symptoms for more than a week should seek medical attention, especially if there has been exposure to unsanitary water. If giardiasis continues or goes untreated, long-term effects are likely to occur. Permanent lactose intolerance, or the inability to digest dairy products, is one of these potential effects. Vitamin deficiency caused by malabsorption is another.


Giardia infection is acquired through the ingestion of the Giardia parasite, which is known as Giardia Lamblia. This parasite forms a protective shell, or cyst, around itself before leaving the body in feces. The cyst can survive for several months in the environment before being passed to another host. Once inside the host, the cyst dissolves and the parasite can reproduce.

Giardia parasites are most frequently found in water that has been exposed to human or animal feces. Drinking water from contaminated wells, pools, streams, lakes and swimming pools is the most common cause of giardiasis. Raw food is another common source of the Giardia parasite, so giardiasis can be considered a foodborne illness. People with giardiasis who handle food can spread the disease if they don't wash their hands. Fresh produce that is washed or irrigated with contaminated water can carry Giardia as well.

Finally, Giardia infection can be spread by fecal-oral route. This occurs most often in people who change children's diapers or engage in anal sex. Giardiasis can be considered a sexually transmitted disease for the latter reason.

Diagnosis & Tests

It can be difficult to diagnose Giardia infection. The symptoms of giardiasis are similar to those of many gastrointestinal diseases and are often mild, and as a result, most cases go undiagnosed. It is more useful to diagnose giardiasis when there's a suspected outbreak than in a single case. Even when laboratory testing takes place, however, the Giardia parasite is not easy to detect.

A single stool sample examined under a microscope will show evidence of existing Giardia only about half the time. Examining three samples taken over a period of days will reveal Giardia cysts or trophozoites—parasites in their growing stage—about 90% of the time. A more efficient way of diagnosing giardiasis is through testing for the Giardia antigens. This single test also works about 90% of the time. However, some laboratories prefer to use the microscope method, because this can aid in the diagnosis of other illnesses as well.

Examining a sample of fluid from the duodenum can also diagnose giardia infection. A sample of this fluid can be taken using a needle, but this is a painful and expensive procedure. Some doctors have a patient swallow a thread that extends into the duodenum and absorbs a sample of fluid. The string is then pulled out through the patient's mouth and the fluid squeezed out of it.

Treatment & Therapy

Many cases of giardiasis will be resolved without treatment. However, cases involving severe diarrhea, especially in children and babies, should be treated with antibiotics. The most frequently prescribed medication is metronidazole, which is both antibiotic and antiprotozoal which is highly effective when used correctly. Some doctors, however, choose other medications due to its bothersome side effects and because it must be taken three times a day. Metronidazole itself can cause diarrhea, as well as skin rash, mouth sores, fever and vomiting, dizziness and a metallic taste in the mouth.

Tinidazole is another amebicide that is sometimes used to treat Giardia infection. While is has some of the same side effects, it is taken in a single dose, which is easier for patients. As with all antibiotics, antimicrobial resistance is a concern with drugs that treat giardiasis, especially when patients take them incorrectly.

Pregnant women should take the antibiotic paromomycin, because it is thought to have less of an effect on the fetus. Paromomycin is not as absorbed out of the intestines as much as other antibiotics. Whatever antibiotic is taken, however, symptoms generally disappear within one to two weeks. Occasionally the Giardia parasite does not respond to medication right away, and the patient must be treated repeatedly.

Prevention & Prophylaxis

Since there is no vaccine for giardiasis, prevention must take the form of avoiding the parasite as much as possible. This means not drinking from untreated water sources such as rivers and lakes, which are often contaminated by the feces of wild animals. Care should be taken not to swallow this water while swimming. Water from wilderness sources can be boiled or filtered to remove or kill Giardia parasites before drinking.

In areas where the public water supply is possibly contaminated, bottled water should be used instead. Hand washing is an effective way of preventing hand-to-mouth fecal contamination. Hand sanitizers are good substitutes for soap and water if clean water isn't available. Those who engage in anal sex should use protection to avoid fecal contact.