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.)

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