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Prevalence of Giardia lamblia in Illinois Beavers
Lance McNew, Stan McTaggart and Tom Nelson
Department of Biological Sciences, Eastern Illinois University.
BACKGROUND
Giardia lamblia is a microscopic protozoan parasite that can infect
warm-blooded animals, including humans. Ingestion of Giardia cysts can
cause giardiasis, an intestinal disorder characterized by nausea, fever & severe
diarrhea. Giardia is the second leading cause of waterborne disease in
the U.S. accounting for 25% of all cases of gastro-intestinal illness. Most
cases of giardiasis occur when humans are exposed to contaminated water.
Contamination with Giardia can occur either by the activity of animals or
by the introduction of untreated human sewage into the watershed. The animals
most likely to contaminate watershed are beavers, hence the common name "beaver
fever." Whether the primary cause of contamination is Giardia cysts
excreted by beavers or humans is hotly debated. Regardless, beavers play an
important role in disease transmission, acting as "amplifying hosts" capable of
shedding millions of cysts for every one ingested.
OBJECTIVES
The objectives of our study were to:
1. Survey the prevalence of G. lamblia cysts in
beavers in eastern Illinois
2. Test whether the prevalence of Giardia differs
between sexes and/or age-classes, and
3. Survey spatial patterns of Giardia in the
watersheds of Illinois.
METHODS
Beaver were collected by trapping at lodges and dens in
seven watersheds in central & southern Illinois. Fecal samples were
collected from the upper small intestine of each beaver & preserved in
formalin. A thin smear of each sample was dried on a microscope slide,
then treated with fluoroscein-labeled mouse monoclonal antibodies made
against cysts of G. lamblia. Treated slides were incubated for 40
min. in a humid chamber to allow the antibody-antigen reaction to occur,
then rinsed and counterstained. Samples were then viewed at 100x & 400x
magnification with a fluorescence microscope with blue excitation light
and recorded as positive or negative for Giardia. Cysts appeared as
bright green against a red background (Figure 2). C2-tests were used to
test for differences in prevalence between sexes, age classes, and
watersheds.
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Figure 2. Giardia cysts
(green) with fluorescence microscopy |
RESULTS
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Figure 3. Distribution of
Giardia positive and negative Beavers. |
Of
107 beavers trapped and tested (Figure 3), 33 (31%) carried Giardia cysts. We did not find
differences in prevalence
between males and females (X2 = 1.62; 1 df; P=0.203).
Thirteen of 52 (25%) females were infected and 20 of 55
males (36%) were infected. Prevalence did not differ between juvenile and adult
beavers (X2 = 0.51; 1 df; P=0.479). One-third of the 66 juveniles tested
positive for Giardia whereas 27% of the adults tested positive.
We did not find significant differences in prevalence
among beavers in the 6 watersheds tested
(X2 = 4.42; 5 df; P=0.491). Prevalence
was highest (42%) in the Big Muddy
watershed in southern Illinois. Most of the beavers in this watershed were
trapped in Rend Lake in Jefferson County.
Prevalence was also high in the Kaskaskia watershed,
particularly in beavers inhabiting Lake Shelbyville.
CONCLUSIONS
Giardia cysts were common in the intestines of
beavers in 4 of 6 watersheds surveyed in eastern
Illinois. We found no differences in the prevalence of
infection in any sex-age class probably because beavers
live in close-knit colonies, sharing dens & food supplies.
Prevalence did not vary among watersheds, but within
watersheds tended to be higher among beavers
inhabiting lakes & ponds, particularly near parks and
campgrounds.
ACKNOWLEDGEMENTS
Funding for this study was provided by the Illinois
Department of Natural Resources and Eastern Illinois
University.
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