What is Giardia?

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Giardia is a protozoan flagellate and the most common pathogen causing intestinal disease in humans worldwide. It is more common in developing countries but also found in developed countries. An estimated 280 million people worldwide have been infected with Giardia. G. intestinalis was first discovered in 1681 by Antoni van Leeuwenhoek, who examined his own diarrheal feces under a rudimentary microscope. G. intestinalis is also known as G. lamblia and G. duodenalis.
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Giardia, a genus of protozoan flagellates, is the most common intestinal-disease-causing pathogen in humans worldwide. An estimated 33 percent of all people in developing countries have giardiasis, a protozoan infection caused by Giardia lamblia. Though the disease is less common in developed countries, it is still present, affecting an estimated 2 percent of adults and 6–8 percent of children. G. lamblia was first discovered in 1681 by Antoni van Leeuwenhoek, who examined his own diarrheal feces under a rudimentary microscope. This species is also sometimes known as G. intestinalis or G. duodenalis.

Natural Habitat and Features

Giardia's natural environment is in rivers, lakes, and streams. People who hike or backpack may develop giardiasis by drinking water from these sources. In its active disease-causing stage, Giardia resembles a pear that has been cut in half lengthwise. Once consumed, the parasite rapidly activates.

Giardia also can be transmitted in undercooked or raw foods, although waterborne contamination is believed far more common. When transmitted in food, it is most often transmitted through contaminated water used to prepare food or by an infected food handler. Giardia has also been reported in day-care centers where infant diapers are changed and proper sanitation has not been practiced. Animal contamination of food may cause infection in humans, and some cross-species reports of G. lamblia have been reported. Giardia also infects amphibians, rodents, birds, voles, and muskrats.

There are two key aspects to the life cycle of Giardia, including encystation and active infection. In the cyst stage, Giardia protects itself by creating a hard cyst, in which it can survive for long periods in damp, cool environments. The cyst comprises 40 percent protein and 60 percent carbohydrate; some studies have shown that the cyst is up to 0.5 micrometers (µm) in thickness. The cysts are oval-shaped and have outer and inner layers that form two protective membranes.

Ingestion of as few as ten cysts can trigger an active infection in a host; an average bowel movement of a person infected with giardiasis could contain three hundred million cysts. Once ingested, the parasite is activated by the gastric acid of the host, which triggers excystation (the end of the cyst stage) and the beginning of the second phase of the life cycle, in which the cyst is deactivated and activation of Giardia occurs. Between two and four trophozoites (the active stage of the organism) may be excysted from each cyst. Each trophozoite reproduces itself at five- to ten-minute intervals. The reproduction stage is not completely understood, and experts disagree on whether the trophozoites reproduce via cell division or sexual reproduction (which has never been observed).

Excystation occurs in about fifteen minutes, starting with the flagella breaking through the cyst. The parasite uses its sucking disk like a suction cup to attach to the intestinal wall of the host. The trophozoite of G. lamblia is up to 9 µm wide and 15 µm long. G. lamblia consumes some of the nutrients eaten by the host before they can be used by the host. People who are most susceptible to infection are those whose immune systems are compromised.

There are two primary genetic groups of G. lamblia that affect humans: assemblage A and assemblage B. Assemblage B appears to be more common in humans overall, though assemblage A predominates in some parts of the world.

G. lamblia can survive a moderate level of chlorine and, according to the Centers for Disease Control and Prevention (CDC), can remain alive for up to one hour in chlorinated pools. Children and adults can become infected in pools that house the germ by swallowing the pool water.

G. lamblia never enters the bloodstream of the infected host. Its actions are not entirely understood by experts, but it appears that the pathogen may release substances such as lectins or proteinases that can damage the epithelium of the host. Giardia may also trigger proteinase-activated receptors by the host, although Giardia itself never invades the skin. Some research also indicates that Giardia can cause a hypersecretion of chloride in infected humans and the malabsorption of sodium, glucose, and water, which together may be responsible for fluid accumulation of the host during infection.

According to the CDC, 16,868 cases of giardiasis were reported in the United States in 2011, and 15,223 cases were reported in 2012.

Pathogenicity and Clinical Significance

Up to three stool specimens are needed to identify Giardia, and only sensitive immunoassays should be used to identify this pathogen. The enzyme-linked immunosorbent assay (ELISA) is one means of testing for Giardia, as is direct fluorescence testing (DFA). Cysts can be readily seen with the fluorescence method.

Drug Susceptibility

The first-line treatment for G. lamblia is metronidazole, given in a dosage of 250 milligrams three times per day for up to ten days. Resistance to metronidazole has been reported in about 20 percent of cases. If such a resistance occurs, tinidazole may be given three times per day for seven days. If the pathogen is resistant to both metronidazole and tinidazole, then other drugs may be prescribed, including albendazole, furazolidone, and quinacrine.


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