Giardiasis (Giardia Lamblia)

What is giardiasis?


Giardiasis (gee-ar-die-a-sis with a soft "G") is an infection of the small intestine that is caused by the parasite, Giardia intestinalis, also known as Giardia lamblia. It is the most common cause of parasitic gastrointestinal disease; it is estimated that up to 2.5 million cases of giardiasis occur each year in the U.S., and up to 20% of the world's population is chronically infected.

Giardia lamblia exists in two forms, an active form called a trophozoite, and an inactive form called a cyst. The active trophozoite attaches to the lining of the small intestine with a "sucker" and is responsible for causing the signs and symptoms of giardiasis. The trophozoite cannot live long outside of the body, therefore it cannot spread the infection to others. The inactive cyst, on the other hand, can exist for prolonged periods outside the body. When it is ingested, stomach acid activates the cyst, and the cyst develops into the disease-causing trophozoite. It takes ingestion of only ten cysts to cause infection. Trophozoites are important not only because they cause the symptoms of giardiasis, but also because they produce the cysts that exit the body in the feces and spread the infection to others.

Cysts of Giardia are present in the feces of infected persons. Thus, the infection is spread from person to person by contamination of food with feces, or by direct fecal-oral contamination. Cysts also survive in water, for example in fresh water lakes and streams. As a result, giardiasis is the most common cause of water-borne, parasitic illness in the U.S.. Domestic mammals (for example, dogs, cats, calves) and wild mammals (for example, beavers) can become infected with Giardia; however, it is not clear how often domestic or wild mammals transmit giardiasis to humans. Giardiasis also has occurred as outbreaks from recreational water sources such as swimming pools, water parks, and hot tubs, most likely because of an infected user rather than a source of water that was contaminated.

Who is at risk for giardiasis?


Giardiasis occurs where there is inadequate sanitation or inadequate treatment of drinking water. Giardiasis is one of the causes of "travelers diarrhea" that occurs duringtravel to less-developed countries , for example the Soviet Union, Mexico, Southeast Asia, and western South America. Giardiasis is a common cause of outbreaks ofdiarrhea in day-care centers because of the high probability of fecal-oral contamination from children; the children, their families, and day care center workers, all are at risk for infection. In fact, children are three times more likely to develop giardiasis than adults. Hikers exploring back-country areas who drink from contaminated fresh water lakes also are at risk for developing giardiasis. Individuals who practice anal/oral sex also may become infected.

What signs and symptoms does giardiasis cause?


The most common manifestations of giardiasis are diarrhea and abdominal pain, particularly cramping; however, diarrhea is not invariable and occurs in 60% to 90% of patients. Other common manifestations include bloating, nausea with or withoutvomitingmalaise, and fatigueFever is unusual. The severity of the symptoms may vary greatly from mild or no symptoms to severe symptoms. Stools may be foul smelling when the Giardia interferes with the absorption of fat from the intestine (malabsorption). The illness or the malabsorption may cause loss of weight.

Symptoms and signs of giardiasis do not begin for at least seven days following infection, but can occur as long as three or more weeks later. In most patients the illness is self-limiting and lasts 2-4 weeks. In many patients who are not treated, however, infection can last for several months to years with continuing symptoms. Children with chronic infection may fail to thrive. Some patients recover from their giardiasis, with or without treatment, but symptoms continue, perhaps because of a condition referred to as postinfectious irritable bowel syndrome. the cause of the continuing symptoms is not clear but may be due to bacterial overgrowth of the small intestine.

How is giardiasis diagnosed?


The best single test for diagnosing giardiasis is antigen testing of the stool. For antigen testing, a small sample of stool is tested for the presence of Giardial proteins. The antigen test will identify more than 90% of people infected with Giardia. Giardia also can be diagnosed by examination of stool under the microscope for cysts or trophozoites; however, it takes three samples of stool to diagnose 90% of cases. Despite requiring three samples of stool, microscopical examination of stool identifies other parasites in addition to Giardia that can cause diarrheal illness. Therefore, microscopical examination of stool has value beyond diagnosing giardiasis, for example, it can diagnose other parasites as the cause of a patient's illness.

Other tests that can be used for diagnosing giardiasis are collection and examination of fluid from the duodenum or biopsy of the small intestine, but these require tests that involve expense and discomfort. The string test is a more comfortable method for obtaining a sample of duodenal fluid. For the string test, a gelatin capsule that contains a loosely-woven string is swallowed. One end of the string protrudes from the capsule and is taped to the patient's outer cheek. Over several hours, the gelatin capsule dissolves in the stomach, and the string uncoils, with the last 12 inches or so passing into the duodenum. In the duodenum the string absorbs a small amount of duodenal fluid. The string then is untapped from the cheek and is removed. The collected duodenal fluid is expressed from the string and is examined under the microscope. Although more comfortable than some of the other tests, it is not clear how sensitive the string test is, for example, does it diagnose 60% (not very good) or 90% (very good) of cases of giardiasis.


How is giardiasis treated?


The most common treatment for giardiasis ismetronidazole (Flagyl) for 5-10 days. It eradicates the Giardia more than 85% of the time, but it often causes gastrointestinal side effects such as nausea and a metallic taste as well as dizziness and headache. Despite its effectiveness, metronidazole is not approved by the FDA in the U.S. for treatment of giardiasis.

The only drug approved for treating giardiasis in the U.S. is furazolidone (Furoxone) for 7-10 days. It is approximately as effective as metronidazole. Tinidazole is available outside the U.S. and is highly effective at treating giardiasis(>90%). It also can be given as a single dose and is well tolerated. Quinacrine is very effective for treating giardiasis but is no longer available in the U.S. Paromomycin and albendazole are less effective than other treatments.

Occasionally, treatment fails to eradicate Giardia. In such cases, the drug may be changed or a longer duration or higher dose may be used. Combination therapy also may be effective (e.g., quinacrine and metronidazole).

How can giardiasis be prevented?


  • Drink only water that has been treated in established treatment facilities. 
  • If there is any doubt about the adequacy of the treatment of water, boil the water or filter it through a filter with a pore size of <1 micrometer, which will exclude trophozoites and cysts. 
  • Do not drink from fresh water streams or lakes without boiling or filtering the water. 
  • Do not use ice or drink beverages made from tap water that may be contaminated. 
  • Do not brush teeth with tap water that may be contaminated. 
  • Do not eat uncooked or unpeeled fruits or vegetables grown in conditions in which contamination with Giardia might occur. 
  • Wash hands before eating food, after changing diapers, using the bathroom, or touching pets. 
  • Avoid oral/anal sex.


I have just been diagnosed with Giardia and have been ill for nearly four weeks. I've had constant diarrhea, tiredness and no feelings of hunger whatsoever. It's been great for weight loss, though I wouldn't recommend this to anyone! The first tests didn't show anything abnormal. The doctor recommended starving for two to three days, which I did. Obviously, it didn't help. Fortunately, the second test revealed Giardia, which was a slight relief as I thought I had become intolerant to some food. I am just about to pick up a prescription of a three-day course of antibiotics that will, I pray, cure it completely. I believe I may have picked it up following eating something horrid at a carnival.


I had a cyclic occurrence. About every 5 to 10 days I would become sicker with all the symptoms except the fatigue, which would persist all the time. Stool samples were tested and were negative each time. Then the symptoms would stop and I'd feel mostly better except for the fatigue so I thought that whatever I had had cured itself. A week or so it would start again. I'd send a sample for testing and get a negative result again. Finally I got a positive result from a sample which showed the presence of Giardia and the doctor said "good news it is easy to treat." The doctor gave me these large tablets of Flagyl. They were hard to take as I couldn't take them with any food and only some water and they were revolting. They worked fast. The next day I felt better and two days later I felt like superman (this is the best way to describe the feeling of improvement after the weeks of feeling tired and lazy). It was the most amazing recovery I have every experienced. I have had Giardiasis at least twice since.


 have come in contact with giardia lamblia several times in my life and it is a horrible experience. Usually during camping trips, but most recently at a popular water park. The first time I ever got it I had no idea what was wrong and I was so very tired and had no energy and just had to lay down so often. I had weight loss, loss of appetite, fatigue, diarrhea, abdominal pain, dizziness, bloating, sometimes gas and foul order to the stools. It was very uncomfortable. Sometimes doctors just blow it off as something you ate and that it will pass. If you know you went camping or to a water park do not ignore these signs and insist on the stool test or insist on the giardia treatment because I had suffered for a few weeks before the doctor finally diagnosed me the first time. Now I know the signs all to well and go immediately to the doctor for treatment. The treatment is very effective and you feel better pretty quickly.



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