G iardia and giardiasis

Giardia is a protozoan parasite that can be transmitted to humans via drinking water that is contaminated with feces. The prototypical species is Giardia lamblia. The protozoan causes an intestinal malady, typified by diarrhea that is called giardiasis. The intestinal upset has also been dubbed "beaver fever."

The natural habitat of Giardia is the intestinal tract of warm-blooded animals. In the wild, warm-blooded creatures such as beavers and bears are natural reservoirs of the protozoan. Also, domestic dogs and cats can harbor the microbe. Typically, Giardia is passed onto humans by the fecal contamination of drinking water by these animals. The ingestion of only a few cysts is sufficient to establish an infection

Giardia has two distinct morphologies. In the environment, such as in water, Giardia is in the form of what is termed a cyst. An individual cyst is egg-shaped and contains four eye-like appearing nuclear bodies. This form is functionally analogous to a bacterial spore. It is a dormant form of the organism that is designed to allow preservation of the genetic material in a hostile environment. Cysts can remain capable of growth for months in water.

In the more hospitable intestinal tract, Giardia reverts to an actively growing and dividing form that is termed a trophozoite. A trophozoite has a distinctive "tear-drop" shape and flagella protruding from five regions on the surface. Two nuclei present the appearance of eyes and a darker central body looks somewhat like a mouth. The effect is to produce an image in the light microscope that is reminiscent of a face.

When excreted from an animal into water, the cyst form is particularly insidious because of the small size, which can allow the cyst to elude filtering steps in a drinking water treatment plant. Also, the cyst is resistant to chlorine, which is the most common means of disinfecting drinking water. Other documented routes of fecal to oral transmission are the sharing of toys in day care facilities (where hands are soiled) and via oral/anal sexual acts. Food borne transmission can occur, but is rare.

While in some people an infection with Giardia does not produce symptoms, many people experience prolonged diarrhea. Indeed, giardiasis is a major cause of intestinal upset in the world. In North America, giardiasis is the leading cause of non-bacterial diarrhea. The diarrhea typically persists for a few weeks to a few months, although in extreme cases the infection can persist for years. Infection produces a general malaise and considerable weight loss. The diarrhea tends to be mushy but, in contrast to the diarrhea produced in bacterial and amoebic dysentery , it is not bloody. Other symptoms of giardiasis include flatulence, sore abdomen, foul-smelling breath and, particularly in infants, the disruption of normal body growth. Research on animal models has also demonstrated that the infection disrupts the ability of the intestinal epithelial cells to absorb nutrients from the intestinal contents. The decreased absorption of compounds such as vitamin B₁₂ and lactose can have deleterious effects on overall health.

The molecular basis of the infection is still not fully resolved. However, the trophozoite form of the protozoan is required, as is associated with the surface of the intestinal epithelial cells. In contrast to another intestinal disease causing microbe, Entamoeba histolytica, Giardia lamblia does not invade the host tissue. Studies with animal models have indicated that the symptoms of giardiasis may be due to a physical barrier to the absorption of nutrients, the disruption of intestinal structures called microvilli by the adherent protozoa , and production of a toxin that damages the epithelial cells.

During an active infection, trophozoites undergo division to new daughter trophozoites. Also, formation of cysts occurs and the cysts are excreted with the feces in huge numbers. These can be passed onto someone else to establish a new infection.

Treatment for giardiasis can include the use of antibiotics . Often, however, the malady is self-limiting without intervention. Prevention of giardiasis is a more realistic option, and involves proper treatment of drinking water and good hygienic practices, especially handwashing.

Currently, the detection of Giardia is based on the microscopic detection of either form of the protozoan, although animal models of the infection are being researched. The infection has been produced in gerbils. The lack of a routine detection method is problematic for water treatment. The need for rapid testing of drinking water for Giardia is pressing as the incidence of infection is increasing, with the encroachment of human habitation on previously pristine areas.

See also Amebic dysentery; Parasites; Water quality