Microbiological Drinking Water Parameters - The Drinking Water - page 120

well documented. 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. Wells that pose the greatest risk of fecal
contamination are poorly constructed or improperly located ones. A few out-
breaks 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 filtration. Occasional outbreaks
have also occurred because of accidental cross-connections between water
and sewage systems. From these data we can conclude that two major ingre-
dients are necessary for waterborne outbreaks. Giardia cysts must be present
in untreated source water, and 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 is not likely that they are responsible for introducing the parasite
into new areas. Far more likely is 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 may originate from cabin toilets that drain
directly into streams and rivers. Backpackers, campers, and sports enthusi-
asts may also deposit Giardia -contaminated feces in the environment which
are subsequently washed into streams by rain. In support of this concept is a
growing amount of data indicating a higher Giardia infection rate in beavers
living downstream from U.S. National Forest campgrounds when compared
with a near-zero rate of infection in beavers living in more remote areas.
Although beavers may be unwitting victims of 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 def-
ecate 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%) (Frost et al.,
1984). 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) (Frost et al., 1984).
Scientific knowledge about what is required to kill or remove Giardia cysts
from a contaminated water supply has increased considerably. We know, for
example, that cysts can survive in cold water (4°C) for at least 2 months and