Giardia in Dogs
by:
Dr. Murray J. Kennedy
Giardia spp. are parasitic protozoans (single celled organisms)
found in the intestines of many animals, including dogs. This microscopic
parasite clings to the surface of the intestine, or floats free in the
mucous lining the intestine. The prevalence of Giardia in dogs of
Alberta is not known. However, rates of 5 to 10 per cent have been suggested.
This level is consistent with those reported in surveys from the United
States and many other countries.
Life cycle of Giardia
Giardia occurs in two forms: a motile feeding stage that lives in
the intestine, and a non-motile cyst stage that passes in the feces. Encystment
occurs as the parasite travels to the large intestine. The cysts are fairly
resistant, and can survive for several months as long as sufficient moisture
is provided. Mature cysts are usually found in the feces of infected animals.
Animals become infected by ingesting cysts. These cysts break open in the
intestine to release the motile feeding stage (trophozoite). Giardia
increase their numbers by each organism dividing in half repeated by a
process called binary fission.
How many species of Giardia are there?
Species of Giardia are morphologically similar. It was customary
to give each Giardia a different name when it was found in a new
host. For examples, the Giardia in dogs was called Giardia canis;
in cattle, Giardia bovis, and so on. Today scientists believe that
few species occur, and each species can infect more than one host. However,
more research is needed on identifying each species and determining the
hosts they can infect.
How did my dog get Giardia?
Giardia is transmitted from host to host by ingesting cysts in contaminated
feed or drinking water. Cysts may also be found in streams or other water
sources. The trophozoite stage may also be infective to dogs. This stage
does not survive for very long after being voided in the feces. Therefore,
it probably does not contribute significantly to the transmission of Giardia.
Can I get Giardia from my dog?
Giardia can infect humans. The clinical signs are similar to those
reported for dogs. We do not know the significance companion animals serve
as a source of infection for humans. Until it has been proven otherwise,
we should assume that Giardia can be transmitted from one animal
to another, and from other animals to humans.
What harm does Giardia do to my dog?
The significance of Giardia in dogs is still not certain. Although
Giardia
has been reported from dogs with diarrhea and dysentery, it has also been
reported in apparently healthy dogs. Clinical signs usually occur in dogs
less than one year of age. But giardiasis may also occur in older dogs,
or in animals suffering from other diseases, or those undergoing chemotherapy.
Figure 2. Scanning electron micrograph of Giardia attached
to the inner surface of the intestine.
How will I know when my dog has Giardia?
Clinical signs range from none in asymptomatic carriers, to mild recurring
diarrhea consisting of soft, light-colored stools, to acute explosive diarrhea
in severe cases. Other signs associated with giardiasis are weight loss,
listlessness, mucus in the stool, and anorexia. These signs are also associated
with other diseases of the intestinal tract, and are not specific to giardiasis.
How can I be sure my dog has Giardia?
Diagnosis is confirmed by finding the cysts or motile stages in feces.
A negative report does not rule out Giardia. Because cysts are only
passed periodically, several fecal examinations may be necessary to diagnoses
this parasite. At least three fecal samples, examined over a period of
seven to ten days, should be examined.
How do I treat my dog for Giardia?
There are two antiprotozoal drugs commonly recommended (metronidazole and
quinacrine). Contact your veterinarian for the drug of choice and treatment
program. All infected animals should be treated whether or not they show
clinical signs.
