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Date Posted: 16:47:00 10/17/99 Sun
Author: clover
Subject: Re: purifying water...page 5
In reply to: clover 's message, "purifying water" on 18:14:46 10/15/99 Fri

Parasite Biology

To understand the finer aspects of Giardia transmission and the strategies for control, one must become familiar with several aspects of the parasite's biology. Two forms of the parasite exist: a trophozoite and a cyst, both of which are much larger than bacteria.
Trophozoites live in the upper small intestine where they attach to the intestinal wall by means of a disc-shaped suction pad on their ventral surface. Trophozoites actively feed and reproduce at this location. At some time during
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, which is about 6 t o 9 micrometers in diameter x 8 to 12 micrometers (1/100 millimeter) 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 for 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 swallowed, 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
theoretically possible, the minimum number of cysts shown to infect a
human under experimental conditions is ten (18). Trophozoites divide by
binary fission about every 12 hours. What this
means in practical terms 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
understood, but is not necessarily related to the number of organisms
present. Nearly all of the symptoms, however, are related to dysfunction
of the gastrointestinal 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.

Transmission

Data reported to the CDC indicate that Giardia is the most frequently
identified cause of diarrheal outbreaks associated with drinking water in
the United States. The remainder of this article will be devoted to
waterborne transmission of Giardia. Waterborne epidemics of giardiasis
are a relatively frequent occurrence. In 1983, for example, Giardia was
identified as the cause of diarrhea in 68% of waterborne outbreaks in
which the causal agent was identified (19). From 1965 to 1982, more than
50 waterborne outbreaks were reported (20). In 1984, about 250,000 people
in Pennsylvania were advised to boil drinking water for 6 months because
of Giardia-contaminated water. Many of the municipal waterborne outbreaks
of Giardia have been subjected 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 is obtained
from clear running streams, is chlorinated but is 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 well documented. It is
worth emphasizing that water obtained from deep wells is an unlikely
source of Giardia because of the natural filtration of water as it
percolates through the soil to reach underground cisterns. Well-water
sources that pose the greatest risk of
fecal contamination are those that are poorly constructed or improperly
located. A few outbreaks have occurred in towns that included filtration
in the water treatment process, but the filtration was not effective in
removing Giardia cysts because of defects in filter construction, poor
maintenance of the filter media, or inadequate pretreatment of the water
before it was filtered. Occasional outbreaks have also occurred because of
accidental cross-connections between water and sewerage systems.

One can conclude from these data that two major ingredients are necessary
for waterborne outbreak. First, there must be Giardia cysts in untreated
source water and, second, the water purification process must either fail
to kill or fail to remove Giardia cysts from the water.

Although beavers are often blamed for contaminating water with Giardia
cysts, it seems unlikely that they are responsible for introducing the
parasite into new areas. It is far more likely that they are also victims:
Giardia cysts may be carried in untreated human sewage discharged into
the water by small-town sewage disposal plants or originate from cabin
toilets that drain directly into streams and rivers. Backpackers, campers,
and sports enthusiasts may also deposit Giardia-contaminated feces in the
environment that are subsequently washed into streams by rain. In support
of this concept is a growing amount of data that indicate a higher Giardia
infection rate in beavers living downstream from U.S. National Forest
campgrounds compared with a near zero rate of infection in beavers living
in more remote areas.

Although beavers may be unwitting victims in the Giardia story, they still
play an important part in the transmission scheme, because they can (and
probably do) serve as amplifying hosts. An amplifying host is one that is
easy to infect, serves as a good habitat for the parasite to reproduce,
and, in the case of Giardia, returns millions of cysts to the water for
every one ingested. Beavers are especially important in this regard
because they tend to defecate in or very near the water, which ensures
that most of the Giardia cysts excreted are returned to the water

The contribution of other animals to waterborne outbreaks of Giardia is
less clear. Muskrats (another semiaquatic animal) have been found in
several parts of the United States to have high infection rates (30 to
40%) (2l). Recent studies have shown that muskrats can be infected with
Giardia cysts obtained from humans and beavers. Occasional Giardia
infections have been reported in coyotes, deer, elk, cattle, dogs, and
cats, but not in horses and sheep, encountered in mountainous regions of
the United States. Naturally occurring Giardia infections have not been
found in most other wild animals (bear, nutria, rabbit, squirrel, badger,
marmot, skunk, ferret, porcupine, mink, raccoon, river otter, bobcat,
lynx, moose, bighorn sheep) (22).

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Re: purifying water...page 6...IF YOU SKIP THE REST ,READ THIS!... -- clover, 16:52:48 10/17/99 Sun