Once considered to be a rare infestation
in dogs, advancements in testing tell us this intestinal
parasite found in water is more prevalent than previously
estimated. In fact, between five and 10 per cent of dogs
carry the organism. In puppies, infestations can run as high
as 50 per cent, while in kennel settings, an entire
population can have giardia.
Giardia infects many mammals, even
birds. Older reference books described a giardia species for
every animal. For example, Giardia bovis infested cattle
while Giardia canis infested dogs. Now researchers believe
there are fewer species of giardia and each is capable of
infesting more than one type of animal.
Once a host consumes a giardia cyst, it
invades the intestine, hatches into a motile form, feeds,
then multiplies. As the cyst progresses through its life
cycle, it generates inflammation in the intestine. The
motile form eventually passes into the large intestine where
it changes into a non-motile cyst. This is shed in the host
feces, serving as the source of infestation for the next
animal.
Giardia cysts are hardy. They can remain
viable for months, particularly in moist and cool
environments. They thrive in clear, clean water. Stagnant,
contaminated water harbours bacteria that compete with the
cysts, decreasing their survival time.
The giardia organisms settle on the
intestinal villi – microscopic projections in the lining of
the gut. As they damage cells, they shorten the villi,
reducing the surface area of the intestine. This alters
digestion of food as well as nutrient and water absorption.
The result is diarrhea – a mushy, cow-pie stool that is
often malodourous.
Risk and diagnosis
The effect giardia has on the host
depends on the dog’s age as well as its immune status.
Puppies with poor immunity suffer from more-severe clinical
symptoms. When roundworm or coccidial infestations combine
with giardia, the poor pup gets severe fluid diarrhea that
results in dehydration. In mature dogs, giardia has a
minimal effect, often causing just soft stool. However,
older dogs with failing immunity or those on cancer
chemotherapy are also at risk.
Giardia should be suspected when a dog
suffers from recurring diarrhea consisting of soft, light-coloured
stools. It can also be a reason for acute explosive
diarrhea. Weight loss as well as listlessness, anorexia and
mucus in the stool may also occur with a
giardia infestation.
Clinical signs generated by giardia can
mimic other diseases, so it’s important to make a specific
diagnosis. Unfortunately, detecting giardia is a challenge.
Veterinarians have traditionally relied on analyses of feces
– a small stool sample is mixed with a concentrated salt
solution and the cysts float to the top. The cysts are then
viewed with a microscope.
Regrettably, fecal tests are not very
accurate because grass pollens and yeasts can be mistaken
for giardia cysts, creating a false positive. As well,
because dogs shed cysts only intermittently, it’s easy to
miss an infestation – a false negative. To get around this
problem, veterinarians recommend doing at least three fecal
examinations before declaring a dog free of giardia.
A newer solution to this dilemma is a
test that detects giardia protein in a fecal sample. This is
a good screening test because it can detect minute
quantities of giardia, unveiling infestations that would
normally be missed.
Treatment
A confirmed case of giardia is treated
with medications such as metronidazole and albendazole.
Metronidazole is the most economical, but the least
effective. Only two-thirds of patients clear the parasite.
Albendazole is 90-per-cent effective following a two-day
course of medication.
Even though treatment may resolve a
dog’s clinical symptoms, he can still be a carrier, shedding
cysts with his feces. Follow-up fecal examinations are
needed to confirm whether the organism has been cleared from
the intestine.
A giardia vaccine, containing chemically
inactivated motile stages of the organism, is available for
dogs. Studies show that vaccines mean less chance of
diarrhea occurring, and it if does, it lasts for a shorter
time. Vaccination also helps reduce the amount of
cyst shedding.
The vaccine has also been touted for
dogs with existing giardia infections. Theoretically, it
boosts the dog’s immune response so the organism can be
fought off more effectively. Studies on the benefit of this
vaccine under these circumstances offer mixed results.
We are probably missing many cases of
giardia in dogs because, especially in adult animals, it
causes only mild symptoms. It’s also challenging to
diagnose. Thankfully, new testing can help us find cases
that have previously run under the radar, but we have to
keep it in mind and test for it.
A multi-published writer, Jeff Grognet, D.V.M., B.S.c.(Agr.),
runs a veterinary practice in Qualicum Beach, B.C.,
along with his wife, Louise Janes, D.V.M.
Originally
published in the August 2007 issue of
Dogs in Canada.
