This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
If your dog has been diagnosed with Giardia, it is infected
with the one-celled protozoan parasite Giardia lamblia. These
flagellate parasites are usually contracted by drinking
contaminated water or sometimes by eating contaminated feces.
Giardiasis, the disease caused by Giardia, can range from
asymptomatic (no visible signs of distress) to extremely acute
where the dog is severely ill. Canine giardiasis should be
treated since it is potentially transmissible to humans and
Giardiasis is a malabsorptive syndrome. The parasites adhere to
the lining of the small intestine where they interfere with
absorption of nutrients. Light cases of Giardia often go
undetected and many dogs "self cure" by expelling and
developing an immunity to the parasite. In heavier infections,
Giardia can interfere with absorption of certain types of
nutrients, especially fats and certain vitamins. Fats are not
absorbed and result in excess mucus in the stools which are
very pungent and diarrhetic.
The parasites interfere with normal metabolism by forming a
physical barrier between the lumen of the intestine and the
absorptive cells. Excess mucus results from malabsorption of
fats while excess water results in the diarrhea. The intestinal
lining is not usually injured so stools should not contain
blood. The parasites feed on partially digested food in the
lumen of the intestine. They do not compete directly with the
host for food. Their metabolism is primarily anaerobic, meaning
that they do not utilize oxygen in their respiration. They lack
cellular organelles concerned with aerobic respiration such as
The active stage within the host is the trophozoite (feeding
body); this is the only pathological form. The transfer stage
of the parasite is the termed the cyst. Giardia forms cysts by
extruding cellular food particles and other vacuoles and
secreting a resistant cyst membrane around the cell. This
highly resistant cyst is then passed from the host in the
feces. Trophozoites may be passed but quickly die. Cysts that
are passed into water can survive for an extended time, up to
1-2 months under proper conditions. Survival times on land are
somewhat less. A new host becomes infected by drinking fecally
contaminated water or eating the feces of an infected animal.
While food-borne transmission is rare, it has been documented
for humans. Dogs may become infected by drinking out of
streams, lakes or ponds containing Giardia cysts. Other sources
of infection are wild animals that visit the kennel area and
deposit infected feces in an area accessible to the dog. Scats
of other dogs or wild animals are potential sources of
infection for domestic dogs. Giardia is potentially
transmissible to humans so caution is warranted.
Giardia can be difficult to detect even for professionals. It
is too small to be seen by the unaided eye. A high quality
microscope is needed for proper diagnosis; phase contrast
microscopy is helpful. A definitive negative diagnosis should
include stools collected on multiple days since cyst production
tends to be cyclic with millions produced one day and few the
following day. The cyst is the diagnostic stage of Giardia.
Cysts tend to be approximately 9-15 micrometers in length and
4-5 um in width. Cysts are identified by size, the presence of
four nuclei, axostyles and claw-hammer shaped median bodies.
The current drug of choice is metronidazole, known by the trade
name FLAGYL. Although highly effective it is a known carcinogen
and mutagen in mice. Quinacrine (ATABRINE) can also be used but
is not as effective. Treatment is usually one tablet per day
for 7-10 days, depending on the weight of the dog. Recovery is
usually uneventful but a dog may become reinfected after
treatment. Thus, it is important to try to isolate and
eliminate the source of infection.