Giardiasis is a chronic, intestinal protozoal infection that is seen worldwide in most domestic and wild mammals, many birds, and people. Infection is common in dogs, cats, ruminants, and pigs. Giardia spp have been reported to be found in 1–39% of fecal samples from pet and shelter dogs and cats, 1–53% in small ruminants, 9–73% in cattle, 1–38% in pigs and 0.5–20% in horses, with higher rates of infection in younger animals. Farm prevalences in production animals vary between 10% and 100%. The cumulative incidence on a farm where Giardia has been diagnosed is 100% in cattle and goats and close to 100% in sheep.
Three distinct groups or Giardia species have been described, including G duodenalis (synonyms G intestinalis, G lamblia) with a wide mammalian host range. Molecular characterization has revealed that G duodenalis is in fact a species complex, comprising 7 assemblages (A to G), some of which have distinct host preferences (eg, assemblage C/D in dogs, assemblage F in cats) or a limited host range (eg, assemblage E in hoofed livestock), while others infect a wide range of animals, including humans (assemblage A and B). There is increasing evidence that some assemblages (A and B) that infect domestic animals can infect people, although transmission patterns might be more complicated than anticipated.
Life Cycle and Transmission
Flagellate protozoa (trophozoites) of the genus Giardia inhabit the mucosal surfaces of the small intestine, where they attach to the brush border, absorb nutrients, and multiply by binary fission. Trophozoites encyst in the small or large intestine and the newly formed cysts pass in the feces. The prepatent period is generally 3–10 days. Cyst shedding may be continuous over several days and weeks but is often intermittent, especially in the chronic phase of infection. The cyst is the infective stage, and can survive for several weeks in the environment, whereas trophozoites cannot.
Transmission occurs by the fecal-oral route, either by direct contact with an infected host or through a contaminated environment. Characteristics that facilitate infection include the high excretion of cysts by infected animals and the low dose needed for infection. Furthermore, Giardia cysts are infectious immediately upon excretion and very resistant, resulting in a gradual increase in environmental infection pressure. High humidity facilitates survival of cysts in the environment, and overcrowding favors transmission.
Giardia infections cause an increase in epithelial permeability, increased numbers of intraepithelial lymphocytes, and activation of T lymphocytes. Trophozoite toxins and T-cell activation initiate a diffuse shortening of brush border microvilli and decreased activity of the small intestinal brush border enzymes, especially lipase, some proteases, and dissacharidases. The diffuse microvillus shortening leads to a decrease in overall absorptive area in the small intestine and an impaired intake of water, electrolytes, and nutrients. The combined effect of this decreased resorption and the brush border enzyme deficiencies results in malabsorptive diarrhea and lower weight gain. The reduced activity of lipase and the increased production of mucin by goblet cells may explain the steatorrhea and mucous diarrhea which has been described in Giardia-infected hosts.
Clinical Findings and Lesions
Giardia infections in dogs and cats may be inapparent or may produce weight loss and chronic diarrhea or steatorrhea, which can be continual or intermittent, particularly in puppies and kittens. Feces usually are soft, poorly formed, pale, malodorous, contain mucus, and appear fatty. Watery diarrhea is unusual in uncomplicated cases, and blood is mostly not present in feces. Occasionally vomiting occurs. Giardiasis must be differentiated from other causes of nutrient malassimilation (eg, exocrine pancreatic insufficiency [see The Exocrine Pancreas: Exocrine Pancreatic Insufficiency in Small Animals], intestinal malabsorption [see Diseases of the Stomach and Intestines in Small Animals: Malabsorption Syndromes in Small Animals]). Clinical laboratory findings usually are normal.
In calves, and to a lesser extent in other production animals, giardiasis can result in diarrhea that does not respond to antibiotic or coccidiostatic treatment. The excretion of pasty to fluid feces with a mucoid appearance may indicate giardiasis, especially when the diarrhea occurs in young animals (1–6 mo). In addition to diarrhea, there is an impact on production due to giardiasis in production animals. Experimental infection of goat kids, lambs, and calves resulted in a decreased feed efficiency and subsequently a decreased weigh gain.
Gross intestinal lesions are seldom evident, although microscopic lesions, consisting of villous atrophy and cuboidal enterocytes, may be present.
The motile, piriform trophozoites (12–15 × 6–10 μm) are occasionally seen in saline smears of loose or watery feces. They should not be confused with trichomonads, which have a single rather than double nucleus, an undulating membrane, and no concave ventral surface. The oval cysts (9–15 × 7–10 μm) can be detected in feces concentrated by the centrifugation-flotation technique using zinc sulfate (specific gravity 1.18). Sodium chloride, sucrose, or sodium nitrate flotation media are too hypertonic and distort the cysts. Staining cysts with iodine aids identification. Because Giardia cysts are excreted intermittently, several fecal examinations should be performed if giardiasis is suspected; eg, 3 samples collected over 3–5 days.
For the detection of parasite antigen, immunofluorescence assays and ELISA are commercially available. In calves, both assays were found to be sensitive and specific for the diagnosis of infection, compared to microscopic examination. Rapid solid-phase qualitative immunochromatography assays are now available, enabling on-site diagnosis of giardiasis. An in-house ELISA is commercially available for use in dogs and has recently been proved to be a useful tool for clinical diagnosis. A dipstick assay is also available for diagnosis of giardiasis in calves, but the test seems to lack sensitivity. Overall, the laboratory based immunofluorescence and ELISA assays were found to be more sensitive for the clinical diagnosis of Giardia compared to the immunochromatographic assays.
Fenbendazole (50 mg/kg/day for 3 days) effectively removes Giardia cysts from the feces of dogs; no adverse effects are reported, and it is safe for pregnant and lactating animals. This dosage is approved for treating Giardia infections in dogs in Europe. Fenbendazole is not approved in cats, but may reduce clinical signs and cyst shedding at 50 mg/kg/day for 3–5 days. Oxfendazole is effective at 11.3 mg/kg for 3 days in dogs, but is not approved for treatment of giardiasis. Albendazole is effective at 25 mg/kg, bid for 4 days in dogs and for 5 days in cats, but should not be used in these animals because it has led to bone marrow suppression and is not approved for use in these species. A combination of praziquantel (5.4–7 mg/kg) pyrantel (26.8–35.2 mg/kg) and febantel (26.8–35.2 mg/kg) also decreases cyst excretion in infected dogs effectively. A synergistic effect between pyrantel and febantel was demonstrated in an animal model, suggesting that the combination product may be preferred over febantel alone.
Metronidazole (25 mg/kg, bid for 5–7 days) is ~65% effective in eliminating Giardia spp from infected dogs but may be associated with acute development of anorexia and vomiting, which may occasionally progress to pronounced generalized ataxia and vertical positional nystagmus. Metronidazole may be administered to cats at 10–25 mg/kg, bid for 5 days. Furazolidone at 4 mg/kg, PO, bid for 7 days, is also effective in cats and small dogs, although diarrhea and vomiting are possible adverse effects; it is also suspected of teratogenicity. A killed vaccine is available in the USA for dogs and cats, but preventive or curative vaccination has shown variable efficacy in reducing clinical signs and the number and duration of cysts shed into the environment.
At present no drug is licensed for the treatment of giardiasis in ruminants. Fenbendazole and albendazole (5–20 mg/kg/day for 3 days) significantly reduce the peak and duration of cyst excretion and result in a clinical benefit in treated calves. Paromomycin (50–75 mg/kg, PO, for 5 days) was found to be highly efficacious in calves.
Oral fenbendazole may be an option for treatment in some birds.
Giardia cysts are immediately infective when passed in the feces and survive in the environment. Cysts are a source of infection and reinfection for animals, particularly those in crowded conditions (eg, kennels, catteries, or intensive rearing systems for production animals). Prompt or frequent removal of feces limits environmental contamination, as does subsequent disinfection. Cysts are inactivated by most quaternary ammonium compounds, steam, and boiling water.
To increase the efficacy of disinfectants, solutions should be left for 5–20 min before being rinsed off of contaminated surfaces. Disinfection of grass yards or runs is impossible, and these areas should be considered contaminated for at least a month after infected dogs last had access. Cysts are susceptible to desiccation, and areas should be allowed to dry thoroughly after cleaning.
Last full review/revision March 2012 by Sharon Patton, MS, PhD