walled and oval in shape, about 8–12 micrometers
long and 5–15 micrometers wide. They are hardy and
can survive for several months in coldwater. They
usually enter the human body through the mouth.
The cysts may be transferred to the mouth directly
from unwashed hands that have touched fecal matter
containing cysts, or through having oral sex with an
infected person. They may also enter the mouth
through eating food or swallowing liquids contami-
nated by fecal matter containingG. lambliacysts.G.
lambliais not, however, transmitted through blood.
Once inside the body, the cysts pass through the
digestive tract until they reach the small intestine.
Each cyst then opens—often within 5 minutes after
arrival—and releases two trophozoites, which are the
active feeding stage of the parasite. The trophozoites
multiply rapidly, reproducing every 9 to 12 hours.
They may remain free within the central cavity
(lumen) of the small intestine or attach themselves to
the mucous tissue lining the intestine by a sucking disk
located on their ventral surface. It is the trophozoites
that cause the violent diarrhea, nausea, intestinal gas,
and cramping associated with giardiasis. As of 2007,
however, researchers do not know the exact reason for
the symptoms; some think that the parasites compete
with the host for nutrients, while others think that they
affect the host’s immune system, cause damage to the
tissues lining the intestine, or block the functioning of
the intestinal mucosa by their sheer numbers.
As the trophozoites are carried toward the colon,
they begin to secrete proteins to form the walls of a
new cyst. Within the next 24 hours, the trophozoiteKEY TERMS
Barberry—A shrub native to southern Europe and
western Asia that produces oblong red berries that
have a sour taste. Barberry has been used as a natural
treatment for giardiasis.
Beaver fever—An informal name for giardiasis, so
called because beavers are a common animal reser-
voir of the parasite that causes giardiasis.
Carrier—A person who bears or carries a disease
agent in or on their body and can transmit the disease
to others, but is immune to the disease or has no
symptoms of it. Some people can be carriers of the
organism that causes giardiasis without developing
diarrhea.
Cyst—The protective shell formed byG. lambliathat
keeps the organism alive after it has been expelled
from the host’s body.
Endoscope—A special tube-shaped instrument that
allows a doctor to examine the interior of or perform
surgery inside the stomach or intestines.
Flatulence—The passage of intestinal gas. Flatu-
lence is a common symptom of giardiasis.
Gastroenterologist—A doctor who specializes in the
diagnosis and treatment of diseases of the stomach
and intestines.
Lactose—A sugar found in milk and milk products
that requires an enzyme called lactase to digest it.
Some people with giardiasis must avoid foods con-
taining lactose for several weeks or months after they
are treated for the disease because their bodies aretemporarily unable to produce enough lactase to
digest the milk sugar.
Malabsorption syndrome—A condition character-
ized by indigestion, bloating, diarrhea, loss of appe-
tite, and weakness, caused by poor absorption of
nutrients from food as a result of giardiasis, other
bowel disorders, or certain surgical procedures
involving the digestive tract.
Protozoan—Any member of a phylum of one-celled
eukaryotes (organisms with nuclei) that are able to
move but are not animals in the strict sense. The
organism that causes giardiasis is a protozoan.
Reservoir—A term used for animals that can carry
parasites that cause disease in humans without fall-
ing ill themselves. Beavers, dogs, cats, cattle, and
horses are common reservoirs ofG. lamblia.
Steatorrhea—The passage of large amounts of fat or
grease in the stool, caused by failure to absorb it
during digestion. Steatorrhea is often associated
with chronic giardiasis.
Traveler’s diarrhea (TD)—A nonspecific term for a
form of diarrhea that frequently affects tourists abroad.
TD is the most common illness affecting visitors to other
countries. Some cases of TD are caused byG. lamblia,
but others result from infection with various bacteria,
rotaviruses, and other intestinal parasites.
Trophozoite—The active feeding stage in the life
cycle ofG. lamblia. It is the trophozoites that multi-
ply within the small intestine and cause the diarrhea
and other symptoms of giardiasis.Giardiasis