Giardia is the genus of a protozoan parasite
that is infectious to both humans and pets all over the world. Giardia
consists of flagellates, which mean they move by means of several
whip-like structures called “flagella.” They live as a form called a “trophozoite,”
or “troph” for short, in the intestine where it causes diarrhea. In
fresh fecal samples, trophozoites can sometimes be captured. They swim
around in a jerky fashion characteristic of flagellates and appear as a
funny face (the two nuclei form the eyes and median bodies form the
mouth).
After a short period of time outside the host’s intestine, the
trophozoites round up and form cysts that enable them to survive
environmental conditions without a host to protect them. The cyst can be
dried out to decontaminate the environment, but if it is cold and wet
the cyst can live for many months with two incompletely formed
trophozoites inside, ready to infect a new host. Contaminated water is
the classical source of a Giardia infection.
After having been swallowed, the cyst shell is digested away, freeing
the two trophozoites who go and attach to the intestinal lining. The
troph has a structure called a “ventral disc,” which is sort of like a
suction cup, and this is used to stay attached to the intestine. If the
troph wants to move to another spot, it lifts itself up and swims to a
new spot via its flagella (trophs tend to live in different intestinal
areas in different host species, depending on the host’s diet). If the
host has diarrhea, trophs are shed in the diarrhea, but Giardia may also
form cysts within the host in preparation to be shed. Either form can be
found in fresh stool.
After infection, it takes 5 to 12 days in dogs or 5 to 16 days in cats
for Giardia to be found in the host’s stool. Diarrhea can precede the
shedding of the Giardia. Infection is more common in kennel situations
where animals are housed in groups.
How Does Giardia Cause Diarrhea?
No one is completely sure but infection seems to cause problems with
normal intestinal absorption of vitamins and other nutrients. Diarrhea
is generally not bloody. Immune suppressive medications such as
corticosteroids can re-activate an old Giardia infection.
Diagnosis
In the past, diagnosis was difficult. The stool sample being examined
needed to be fresh, plus Giardia rarely show up on the usual fecal
flotation testing methods used to detect other parasites. Traditionally,
a fecal sample is mixed in a salt or sugar solution such that any
parasite eggs present will float to the top within 10 to15 minutes.
What has made Giardia testing infinitely easier is the development of a
commercial ELISA test kit (similar in format to home pregnancy test
kits). A fecal sample is tested immunologically for Giardia proteins.
This method has dramatically improved the ability to detect Giardia
infections and the test can be completed in just a few minutes while the
owner waits.
Giardia shed organisms intermittently and may be difficult to detect.
Sometimes pets must be retested in order to find an infection.
Treatment
A broad spectrum dewormer called fenbendazole (Panacur®) seems to be the
most reliable treatment at this time. Metronidazole (Flagyl®) in
relatively high doses has been a classical treatment for Giardia but
studies show it to only be effective in 67% of cases. The high doses
required to treat Giardia also may result in temporary neurologic side
effects or upset stomach. Often both medications are used concurrently
to enhance efficacy and allow for lower dosages of metronidazole. The
ELISA test for Giardia should go negative within 2 weeks of treatment
indicating success.
Because cysts can stick to the fur of the infected patient and be a
source for re-infection, the positive animal should receive a bath at
least every other day to help reduce the chance of reinfection. .
Not all patients with Giardia actually have diarrhea but because Giardia
is the most common intestinal parasite affecting humans in North
America, treatment is generally recommended for the pet testing positive
even if no symptoms are being shown.
Treatment generally lasts 1 month and involves two bouts of five day
courses of fenbendazole separated by 2 weeks, and a single 8 day
treatment with metronidazole. .
Pets should be retested by the elisa test two weeks after completion of
the last dose of medication.
Vaccine?
A Giardia vaccine made by Fort Dodge Animal Health is on the market but
it is not intended to prevent infection in the vaccinated animal.
Instead the vaccine is licensed as an adjunct to treatment and is used
to reduce the shedding of cysts by the vaccinated patient. This would be
helpful in a kennel situation that is trying to reduce environmental
contamination during an outbreak or where an animal keeps getting
reinfected, but it is not helpful to the average dog whose owner wants
to simply prevent infection.
The 2006 American Animal Hospital Association Guidelines list this
vaccine as “not recommended.”
Environmental Decontamination
The most readily available effective disinfectant is probably bleach
diluted 1:32 in water, which in one study required less than one minute
of contact to kill Giardia cysts. Organic matter such as dirt or stool
is protective to the cyst, so on a concrete surface basic cleaning
should be effected prior to disinfection. Animals should be thoroughly
bathed before being reintroduced into a “clean” area. A properly
chlorinated swimming pool should not be able to become contaminated. As
for areas with lawn or plants, decontamination will not be possible
without killing the plants and allowing the area to dry out in direct
sunlight.
Transmission to Humans:
Giardia can infect humans, cats and dogs (among other animals). The
cysts present in the pet’s feces or on their coat, could potentially
infect a person if that person were to put their hands in their mouths
after coming in contact with the cysts. Small children, who have a
tendency to put their hands in their mouths often are at greater risk of
infection. In addition, people with compromised immune systems are more
likely to get infected as well (HIV positive, people on immune
suppressive drugs).
There are 6 different subtypes of Giardia in the US. Recent studies have
shown that the Giardia which infect dogs and cats is not very likely to
infect people, however, if you have an infected pet, you should be sure
to wash your hands after handling the pet until the treatment is
completed and the testing shows there are no more cysts.
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