Parasitic Diseases-Protozoa- Giardiasis in dogs
Etiology. Giardiasis is a small-intestinal disease of the dog caused by Giardia duodenalis (lamblia), a binucleate flagellate protozoan. Clinical signs. Most Giardia infections are subclinical. When dogs are clinically affected, diarrhea is the most prominent sign.
The diarrhea is a result of intestinal malabsorption and is often characterized as voluminous, light-colored, foul-smelling, and soft to watery. Weight loss has also been associated with clinical infection. Clinical illness is more often seen in young animals.
Epizootiology and transmission. Giardia has a direct life cycle. Dogs (and people) typically become infected when they consume water (or food) contaminated with Giardia cysts. The pH change from the stomach (acid) to the duodenum (neutral) causes excystation. Trophozoites migrate to the distal duodenum and proximal jejunum and attach to the villus surface.
Eventually, the trophozoites encyst and pass in the feces to perpetuate the life cycle.
Pathologic findings. Giardiasis is rarely fatal. On histopathology of duodenal or jejunal specimens, Giardia trophozoites can be seen attached to enterocytes. Mucosal inflammation and ulceration, and villous atrophy have been observed.
Pathogenesis. The exact pathogenesis of Giardia-induced illness is unknown. It is thought that tissue invasion, although occasionally observed, is unimportant for pathogenesis. It is suspected that illness is caused by physical obstruction of enteric absorption, enterotoxicity, competition for nutrients, excess mucus production, and/or secondary bacterial overgrowth.
Diagnosis and differential diagnosis. Definitive diagnosis requires observation of the organism in fecal or intestinal samples. Direct fecal smears are considered best for observing trophozoites, and zinc sulfate flotation is preferred for detection of cysts.
Commercial ELISA kits and direct immunofluorescent tests are available to detect fecal Giardia antigens, but the diagnostic specificity and/or sensitivity of these tests may not be sufficient to warrant substitution for the less expensive direct fecal examination or zinc sulfate preparation (Barr, 1998). Differential diagnoses for giardiasis include bacterial and protozoal enteritis, coccidiosis, and whipworm infestation.
Prevention. High-quality water sources will eliminate the possibility of infection developing within an animal research facility. Use of dogs with known husbandry and medical background will minimize the chances of giardiasis developing in a research colony. Control. Once giardiasis has been diagnosed in a canine population, segregation of infected animals will help to reduce further infection (provided other dogs were not preinfected at
the same source location as the signal case). Disinfection with quaternary ammonium compounds, bleach, or steam is usually successful in the eradication of Giardia cysts.
Treatment. The most common treatment for giardiasis is metronidazole (Flagyl) at 25-30 mg/kg per os twice per day for 5-10 days. Quinacrine hydrochloride (Atabrine) at 9 mg/kg
per os once per day for 6 days, furazolidone (Furoxone) at 4 mg/kg per os twice per day for 7-10 days, and the anthelmintics albendazole and fenbendazole have been proposed for use against metronidazole-resistant strains of Giardia. A1- albendazole is recommended at 25 mg/kg per os q12 hr for 2 days, and fenbendazole at 50 mg/kg per os q24 hr for 3 days.
Fenbendazole was thought to be safer for both puppies and pregnant females (nonteratogenic) (Barr, 1998).
Research complications. Typical asymptomatic infections probably have no consequence on research protocols, with the exception of intestinal physiology or immunology studies.