Microbiology and Immunology

Giardia and giardiasis WORLD OF MICROBIOLOGY AND IMMUNOLOGY

248

•

History of microbiology; History of public health; History of

the development of antibiotics; Infection and resistance;

Laboratory techniques in microbiology

GGiardia and giardiasisIARDIAAND GIARDIASIS

Giardiais 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 giardia-

sis. The intestinal upset has also been dubbed “beaver fever.”

The natural habitat of Giardiais the intestinal tract of

warm-blooded animals. In the wild, warm-blooded creatures

such as beavers and bears are natural reservoirs of the proto-

zoan. Also, domestic dogs and cats can harbor the microbe.

Typically, Giardiais passed onto humans by the fecal con-

taminationof drinking water by these animals. The ingestion

of only a few cysts is sufficient to establish an infection

Giardiahas two distinct morphologies. In the environ-

ment, such as in water, Giardiais 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 function-

ally 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, Giardiareverts to

an actively growing and dividing form that is termed a tropho-

zoite. 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 microscopethat 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 treat-

ment 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 shar-

ing 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 Giardiadoes

not produce symptoms, many people experience prolonged

diarrhea. Indeed, giardiasis is a major cause of intestinal upset

Scanning electron micrograph of Giardia.

womi_G 5/6/03 3:18 PM Page 248