Environmental Engineering Reference
In-Depth Information
Trophozoites actively feed and reproduce at this location. At some time dur-
ing the trophozoite's life, it releases its hold on the bowel wall and floats in the
fecal stream through the intestine. As it makes this journey, it undergoes a
morphologic transformation into an egglike structure called a
cyst
. The cyst,
about 6 to 9 µm in diameter by 8 to 12 µm in length, has a thick exterior wall
that protects the parasite against the harsh elements that it will encounter
outside the body. This cyst form of the parasite is infectious to other people
or animals. Most people become infected either directly (by hand-to-mouth
transfer of cysts from the feces of an infected individual) or indirectly (by
drinking feces-contaminated) water. Less common modes of transmission
included ingestion of fecally contaminated food and hand-to-mouth transfer
of cysts after touching a fecally contaminated surface. After the cyst is swal-
lowed, the trophozoite is liberated through the action of stomach acid and
digestive enzymes and becomes established in the small intestine.
Although infection after the ingestion of only one
Giardia
cyst is theo-
retically possible, the minimum number of cysts shown to infect a human
under experimental conditions is 10 (Rendtorff, 1954). Trophozoites divide
by binary fission about every 12 hours. What this means in practical terms is
that, if a person swallowed only a single cyst, reproduction at this rate would
result in more than 1 million parasites 10 days later and 1 billion parasites
by day 15.
The exact mechanism by which
Giardia
causes illness is not yet well under-
stood but is not necessarily related to the number of organisms present.
Nearly all of the symptoms, however, are related to dysfunction of the gas-
trointestinal tract. The parasite rarely invades other parts of the body, such
as the gall bladder or pancreatic ducts. Intestinal infection does not result in
permanent damage.
Data reported by the CDC indicate that
Giardia
is the most frequently iden-
tified cause of diarrheal outbreaks associated with drinking water in the
United States. The remainder of this section is devoted to waterborne trans-
missions of
Giardia
. Waterborne epidemics of giardiasis are a relatively fre-
quent occurrence. In 1983, for example,
Giardia
was identified as the cause of
diarrhea in 68% of waterborne outbreaks in which the causal agent was iden-
tified (CDC, 1984). From 1965 to 1982, more than 50 waterborne outbreaks
were reported (Craun, 1984). In 1984, about 250,000 people in Pennsylvania
were advised to boil their drinking water for 6 months because of
Giardia
-
contaminated water.
Many of the municipal waterborne outbreaks of
Giardia
have been sub-
jected to intense study to determine their cause. Several general conclusions
can be made from data obtained in those studies. Waterborne transmission
of
Giardia
in the United States usually occurs in mountainous regions where
community drinking water obtained from clear running streams is chlo-
rinated but not filtered before distribution. Although mountain streams
appear to be clean, fecal contamination upstream by human residents or
visitors, as well as by
Giardia
-infected animals such as beavers, has been
