Giardia lamblia or Giardia intestinalis are facultative anaerobic protists without any mitochondria, or peroxisomes.
Giardia lamblia trophozoites live in the small intestine of the host. Cysts, which are resistant to adverse environmental conditions, are passed in the feces of an infected host, and the next host is infected when it ingests cysts in food or water contaminated with feces. Giardiasis is diagnosed by finding cysts or trophozoites in the feces, and both life cycle stages have a characteristic appearance. The trophozoites average about 15 µm in length, have a distinct "tear-drop" shape and two nuclei at the anterior end. The characteristic shape of the trophozoite is particularly interesting when they are viewed with an scanning electron microscope.
Giardia lamblia trophozoites, as they appear with the scanning electron microscope. Original image by Arturo Gonzalez, CINVESTAV, Mexico. Used with permission of Javier Ambrosio, UNAM, Mexico.
People who see G. lamblia under the microscope often say that it appears that the trophozoites are "staring back at them." The trophozoites also contain a dark transverse rod, the axostyle, which seems to be a supportive element. The cysts average about 13 µm in length, are oval, and contain two nuclei and remnants of the axostyle. Because of these unique characteristics, G. lamblia is one of the easiest intestinal protozoans of humans to diagnose.
Unlike Entamoeba histolytica which can invade the tissues of the large intestine, G. lamblia does not invade the tissues of the small intestine. However, the trophozoites do adhere closely to the lining of the small intestine, and in heavy infections much of the lining of the small intestine can be covered with trophozoites. The symptoms associated with giardiasis range from none (in light infections) to severe, chronic diarrhea (in heavy infections), but not dysentery.
One person can pass millions of G. lamblia cysts each day, and most infections probably result from water or food contaminated with human sewage. Open sewers in city streets and contamination of drinking water with this sewage undoubtedly results in many infections. However, in some countries the use of human fecal material ("night soil") as a fertilizer is also an important source of infection. Many cases of "traveler's diarrhea" are caused by Giardia. Even in developed countries potable water can be contaminated with small amounts of sewage, especially when septic systems are build too close to wells. Thus, it is not surprising that G. lamblia is found throughout the world.
Every year many people return from camping trips to find that they are suffering from giardiasis, but the source of these infections remains uncertain. Some authorities believe that Giardia infects a number animals other than humans, particularly beavers, and that campers contract giardiasis from drinking stream water contaminated with cysts from beavers. Other authorities, however, believe that these cases result from streams contaminated with human feces. Although mountain streams may appear to be "sparkling clean," some camper upstream may be using your drinking water as a toilet! Thus, no matter what it looks like, stream water should be treated before drinking. Boiling will kill Giardia cysts, and there are commercially available filters that will remove the cysts from water.
Giardia lamblia trophozoites in the human stool. Microscopic examination of the stool for the presence of Giardia trophozoites allows the diagnosis of the disease.
The parasite is also transmitted through anal sexual intercourse by homosexuals males.