vvEPA
United States
Environmental Protection
Agency
Health Effects Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S1-82-013 Jan. 1983
Project Summary
Cross Transmission of
Giardia
R. B. Davies, K. Kukutaki, and C. P. Hibler
Giardia cysts isolated from fecal
samples obtained from humans (Homo
sapiens), beaver (Castor canadensis),
dogs (Cam's familiaris). cats (Felis
domesticus), bighorn x moufIon sheep
(Ovis canadenis x O. musimon),
guinea pig (Cavis porcellus), muskrat
(Ondatra ziethica), and mule deer
(Odocoileus hemionus) were given to
a variety of experimental animals.
Human source Giardia cysts estab-
lished infections in dogs, cats, beaver,
rats (Rattus norvequicus). gerbils
(Gerbillus gerbillus). guinea pig,
raccoon (Procyon lotor), bighorn x
mouflon sheep, and pronghorn ante-
lope (Antilocapra americana). Giardia
cysts from naturally occurring beaver
successfully infected dogs. A dog was
infected with Giardia cysts from a
bighorn x mouflon sheep which had
been infected with human source
Giardia. Human source Giardia cysts
were used to infect cats and cysts
from these cats were used successfully
to infect dogs.
Evidence exists that once dogs are
treated with metronidazole and then
reexposed to Giardia cysts they be-
come infected yet do not shed cysts.
This most likely occurs in natural cases
of giardiasis in dogs where the animal
stops shedding cysts yet has a latent
infection. Female dogs and cats may
start shedding Giardia cysts 3-4
weeks after parturition.
This Project Summary was developed
by EPA's Health Effects Research
Laboratory. Research Triangle Park,
NC, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Giardia (Protozoa: Hexamitidae) has
been known as a parasite of humans
since Leeuwenhoek found trophozoites
of the protozoan in his own feces A
recent report indicates that Giardia
lamblia is the most common parasite in
stool specimens submitted for examina-
tion in the United States, with preva-
lences ranging from 2 to 20% with the
average at 3.8%. Within Colorado,
giardiasis exists in two forms: endemic
and epidemic. Previous parasitological
state surveys showed that Giardia is the
most common parasite identified, with
prevalence ra:es of 5% and 5 6%. A third
survey in Colorado reported 3% of the
people examined were infected with
Giardia. Infected persons had diarrhea
which lasted an average of 3.8 weeks A
correlation between seasonal distribu-
tion of cases and fecal contamination of
mountain streams indicates drinking
untreated water is an important cause
of endemic giardiasis
Numerous epidemics of giardiasis
have occurred in Colorado. The precise
source of the Giardia in these outbreaks
is not known, but Giardia cysts have
been recovered from samples of public
water supplies in various other localities.
Cross connections between water and
sewage lines were determined to be the
cause of one Colorado outbreak, others
were associated with incompletely
treated surface water. Beaver infected
with Giardia were found below the
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water inlets for the water system in a
Washington State 01 tbreak.
Sylvatic giardiasis has been described
in Colorado with beaver (18%), cattle
(10%), domestic cat (25%), and dogs
(1 3%), being positive iorGiardia. Two of
34 coyotes (Cam's /atoansjfrom northern
New Mexico were also positive for
Giardia.
Early parasitologists, describing species
of Giardia from various hosts, named
species after the host in which they
were found irrespective of morphologic
similarities between Giardia in the dif-
ferent hosts In the absence of cross-
transmission experiments to determine
the validity of speciations, this probably
was the safest approach. However, as
early as 1952 investigators could not
find any morphologic differences be-
tween species of Giardia described from
the laboratory rat and a number of wild
rodents. A review of the literature deter-
mined that most experimental cross-
transmission studies were question-
able. This prompted the proposal of two
species, G. muris in the mouse, rat, and
hamster, and G. duodena/is in the rab-
bit, man, dog, cat, cattle, and various
rodents.
Although this proposed speciation
was based on morphology, the implica-
tion that other animals could serve as
reservoirs for man was extremely
important. Although the author of this
proposed speciation did not accept the
success obtained by two other investi-
gators in infecting laboratory rats with
Giardia from man, these e;irly investi-
gators were aware that the various
Giardia might not be host-specific.
Researchers gave human-source Gi-
ardia to dogs and reported establishing
infections with the prepatent period
ranging from 3 to40 days. However, this
experiment was not well controlled.
Another experiment, not adequately
controlled, infected six dogs with
human-source Giardia cysts and found
the prepatent period was 6-9 days. The
dogs used in this experiment were
examined for Giardia for two weeks,
without positive findings, prior to
inoculation. All of these results strongly
suggest that the premise of only a few
species of Giardia was probably correct.
In another cross-transmission study it
was reported that G. muris from
laboratory mice, G. simonifrom labora-
tory rats and G. peromysci from deer
mice were very host-specific, while G.
microti and G. mesocricetus were not
host-specific
The present cross-transmission studies
were stimulated by the increasing
number of unexplained epidemics of
giardiasis in humans, all apparently
waterborne, but not readily traceable to
human contamination of the water
supplies. They pointed to another
possible source of infection, a wild or
domestic mammal This, of course,
necessitated more extensive cross-
transmission studies involving a multi-
tude of wild animal hosts to determine if,
indeed, a wild animal species was
responsible for the epidemics.
Results
Animals exposed to Giardia cysts
from clinically-ill humans produced
data which varied both within and
among experimental groups. Hamsters,
domestic rabbits, laboratory mice,
wapiti, mule deer, white-tailed deer,
black bear, domestic sheep, and domestic
cattle were not infected successfully
with human source Giardia cysts.
Animals which did become infected
were laboratory rats, gerbils, guinea
pigs, beaver, dog, raccoon, bighorn x
moufIon sheep and pronghorn antelope.
Cysts from all animals, with the exception
of some of those from rats, ranged in
size from 9.5 to 11.0/L/m x 8.0 to 9.5/ym.
After intubated animals became patent,
the m-group control often started shedd-
ing cysts 8-20 days after exposed
animals became patent. This indicated
transmission of Giardia from the exposed
animals to the m-group controls. None
of the control groups, held in the same
facilities as the exposed animals and the
m-group control, became positive for
Giardia.
Giardia cysts were recovered from
the only composite fecal sample from
rats on days 22, 25, and40postexposure
(PE) Cysts were of two sizes, 5 /um long
and 10/ym long, but were identical in all
other respects. All other experiments
were performed with a ni ma Ism individ-
ual cages. In another experiment using
rats, cysts were shed for one day at 34
PE. Infected feces from this group were
fed to an SPF dog which began shed-
ding Giardia cysts eight days PE Con-
trol dogs remained negative
Gerbils exposed to human source
cysts began shedding cysts 8, 13, and
18 days PE. In one test, the in-group
control was positive 33 days PE The
cysts were shed in varying numbers and
not consistently in all samples Some
exposed animals remained negative un-
til the experiment was terminated in 42
days PE. One exposed guinea pig
became positive for Giardia two days PE
and continued shedding for 31 days. All
other animals remained negative
Beaver exposed to human Giardia
cysts from one human source were
negative for 40 days preexposure and
remained negative for 40 days PE. The
control and one exposed beaver were
inoculated with human Giardia from
another source. These beaver started
shedding Giardia cysts 25 days PE and
continued shedding for 22 days, after
which they shed cysts intermittently.
The beaver used as a control for the
second exposure remained negative.
The Giardia from the first human source
apparently were not infective, whereas
those from the second source were
infective.
SPF beagle puppies exposed to
human source Giardia cysts began
shedding cysts six to eight days PE and
the m-group controls began shedding
cysts 13 to 15 days after the exposed
dogs were inoculated.
A young raccoon in a group of ten
shed cysts for one day at eight days PE.
A five-month old black bear cub re-
mained negative for cyst shedding.
Bighorn x mouflon sheep exposed to
human Giardia began shedding cysts
nine days PE and shed cysts for four
days. Sheep isolated cysts were
inoculated to SPF beagles. A pup
inoculated with cysts from one sheep
started shedding cysts ten days PE,
whereas a pup inoculated with cysts
from another sheep remained negative
for cyst shedding.
A young pronghorn antelope exposed
to human Giardia cysts started shedding
cysts 16-18 days PE and shed cysts for
three days.
Wapiti, mule deer, white-tailed deer,
and domestic sheep and cattle did not
become infected with human Giardia
cysts However, naturally-infected mule
deer and cattle have been reported in
the literature. Infection of muskrats was
confounded since all animals including
the control were found to be positive 18
hours after being exposed to human
source Giardia cysts
Puppies and kittens exposed to
human source Giardia cysts exhibited
prepatent periods of 6-10 days and 6-27
days, respectively. Cysts isolated from
one of the exposed kittens were inocu-
lated into puppies and kittens. Most of
these inoculated puppies and kittens
began shedding cysts. All control
animals remained negative throughout
these experiments
Giardia cysts recovered from the
feces of naturally-infected, free-ranging
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beaver were given to mice, rats,
hamsters, guinea pigs, and puppies
Only the exposed puppies began shedd-
ing cysts eight days PE and the m-group
control puppy began shedding cysts 17
days PE of the inoculated animals.
All dogs, including the m-group
controls in one group, began shedding
cysts at four days PE when exposed to
Giardia isolated from muskrats. It is
assumed that all of these dogs were
exposed to Giardia before the experi-
ment was begun A second group of
dogs exposed to Giardia cysts from
muskrats remained negative for 63 days.
Giardia cysts from a naturally-infected
mule deer were given to SPF beagle
puppies which remained negative for 28
days PE.
Reinfection with human Giardia cysts
was attempted on positive dogs treated
with metronidazole. Following six days
of treatment, seven of the puppies were
negative for Giardia shedding Seven
days after treatment five of the six
treated puppies were exposed to human
Giardia cysts None of the puppies shed
cysts for 40 days PE. The untreated
puppy continued to shed; however,
cysts were not observed in feces from
the puppy which was negative when all
of the dogs originally were obtained
After 40 days PE, intestinal scrapings
indicated Giardia trophozoites in three
of the five exposed dogs The dog that
was treated but not exposed to Giardia
did not exhibit trophozoites in intestinal
scrapings.
G/ard/a-iree dogs and cats were
difficult to obtain. Therefore, pregnant
dogs and cats were obtained and held in
clean rooms until they gave birth and
the offspring were weaned The female
dogs began shedding Giardia cysts two
to four weeks after parturition and all of
the offspring were positive seven days
after the female started shedding cysts
These female dogs were examined daily
and criteria for selection included being
Giard/a-iree for seven days prior to
acceptance. None of these dogs shed
cysts until two or four weeks after
parturition. To solve this problem the
adult animals were treated with metro-
nidazole for five days and mthis manner
G/arc//a-free offspring, were produced
and the adults did not shed cysts after
parturition
Conclusions
1. Giardia cysts obtained from
human, dog, cat, and beaver sources are
not host specific
2 Giardia from human sources will
readily infect dogs, cats, and beaver
3 Giardia from beaver sources will
readily infect dogs
4 Giardia from dog or cat sources
cross-transmit between these species.
5 Therefore, it must be assumed that
Giardia from dog, cat, and beaver
sources will infect humans
Recommendations
The results of this cross-transmission
study showed that the Giardia found in
humans, dogs, cats, and beaver sources
are not host-specific and will readily
establish in other animal species All
three species are important as potential
sources of Giardia for epidemics of
waterborne giardiasis, but the beaver
probably plays the most important role
because of its closer association with
water used by communities as their
source of domestic supply Therefore, a
study of the host-parasite relationship
between beaver and Giardia, together
with a study of the factors predisposing
toward an epidemic of waterborne
giardiasis (water pH, hardness, tem-
perature, etc.) is the next logical step
necessary to understand waterbrone
giardiasis
P. B. Da vies, K. Kukutaki. and C. P. Hibler are with Colorado State University,
Ft. Co/tins, CO 80523.
T. H. Erich sen is the EPA Project Officer (see below).
The complete report, entitled "Cross Transmission of Giardia," (Order No. PB
83-117 747; Cost: $8.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Health Effects Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
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U S GOVERNMENT PRINTING OFFICE 1983 659-O17/C
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