Etiologic agent = Giardia intestinalis/duodenalis
(formerly lamblia)
This is a flagellate protozoan that inhabits the intestinal
tract of a wide variety of domestic and wild animals species.
Life cycle of Giardia:
Giardia trophozoites are binucleate and replicate in
the small intestines by binary fission. Once they exit the jejunum,
they encyst as they begin to lose moisture content. Within their
cyst, they undergo one further division to produce a tetranucleate
cyst. This
cyst
that is passed in the feces is very resistant in the environment
and is the infective form for new hosts. When ingested, the tetranucleate
parasite emerges from the cyst in the duodenum, rapidly divides
into (2) binucleate trophozoites, and proceeds to undergo continued
binary fission.
Epidemiology of Giardia:
- Infections in humans are most commonly waterborne. Backpackers
and people pursuing other outdoor activities may be infected
from what appear to be pristine water sources because of human
or animal fecal contamination upstream.
- Beaver and muskrat have been reported to have Giardia
carriage rates of 16% and >95%, respectively. In addition,
pinnipeds in Canada have also been demonstrated to shed Giardia
cysts.
- The organism survives well in cold water and may not be inactivated
by routine chlorination tablet water purification systems. Filtration
of water is essential.
- The second most common mode of infection is person-to-person,
e.g. in day-care centers.
- Infected dogs and cats can clearly serve as a source for
contamination of the environment with the organism and must be
considered as potential zoonotic risks. The same may be true
for cattle, goats, llamas and pigs, although some isolates from
these species appear to represent strains that are restricted
only to livestock and have not been recovered from humans. Thus,
the overall zoonotic impact of giardiasis in animals remains
to be fully understood.
- It has been estimated that 1-2% of well-cared for dogs and
cats may be shedding Giardia cysts at any one time. The
rate of shedding increases to ~10% in kennel or shelter environments,
and infection and clinical disease are greatest in puppies and
kittens, with up to 50% of puppies potentially shedding the organism
at any one time.
Giardiasis clinical disease:
Following a 1-2 week incubation period, both people and dogs and
cats initially suffer an acute GI'itis with diarrhea. Fever is
much less common than with bacterial agents of gastroenteritis.
This acute phase of disease may be followed by a CHRONIC syndrome
of malabsorptive diarrhea, weight loss and abdominal pain that
waxes and wanes over a period of many months.
- The stools may initially be watery, but then typically progress
to soft, semi-formed stools with steatorrhea and a rancid, foul
odor.
- Patients may also exhibit malaise, nausea, bloating and flatulence.
- The diarrhea may continue indefinitely if not treated. In
fact, because of difficulties in diagnosis, empirical treatment
of Giardia is sometimes considered in cases of chronic,
undiagnosed malabsorptive diarrhea.
- The diarrhea appears to be due primarily to disruption of
the enterocyte brush borders and loss of disaccharidase enzymes.
- Actual invasion of enterocytes is rare, but the organism
can colonize large portions of the small intestinal tract.
- The host immune/inflammatory response may also be a contributing
factor in the pathology of Giardia infection.
Diagnosis of Giardia infections in dogs and cats:
- demonstration of trophozoites in direct fecal smears
- Multiple smears over time must be done because of the intermittent
nature of Giardia shedding.
- fecal floatation (zinc sulfate solution) for cysts
- Trophozoites will NOT be detected by floatation techniques
because the floatation solution lyses the trophozoites.
- demonstration of the organism in duodenal aspirates collected
during endoscopy
- ELISA and IFA assays for Giardia antigens in feces
- These can be highly sensitive and specific in humans, but
some studies suggest they are less so when used in domestic animals.
- response to empirical therapy
Treatment of Giardia infections in dogs and cats:
- Metronidazole (e.g., Flagyl) can be used in both dogs
and cats (not in pregnant animals).
- Albendazole (e.g., Valbazen) was recently found
to be quite effective in dogs, and may be more efficacious than
metronidazole in stopping the shedding of cysts. However, both
metronidazole and albendazole have been associated with significant
adverse reactions in dogs and cats:
- Albendazole: leukopenia +/- anemia and thrombocytopenia;
anorexia, lethargy; CNS signs; vomiting and diarrhea; salivation;
elevated hepatic and pancreatic enzyme levels; abortion and teratogenicity
- Metronidazole: vomiting; CNS signs
- Fenbendazole (e.g., Panacur® or Drontal-Plus®)
now appears to be the drug of choice. Used in dogs and cats
at 50 mg/kg for 3 (-5) days, fenbendazole has been shown to be
completely effective in eliminating experimental Giardia infections,
and with only mild vomiting/diarrhea as potential side effects.
Vaccination against Giardia in dogs:
In 1999, a killed, whole-organism vaccine (GiardiaVax, Fort
Dodge Animal Health) was approved for use in dogs in the U.S.
and has been shown by the manufacturer to reduce the shedding
of
Giardia cysts after experimental infection. (9/20 vaccinates
shed cysts on day 7 after infection and 0/20 vaccinates shed cysts
on day 42 after infection, compared to 10/10 placebo controls
on both days.)