THE MERCK VETERINARY MANUAL
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Overview of Giardiasis

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Giardiasis is a chronic, intestinal protozoal infection 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 in 0.44%–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 0% and 100%, with the highest prevalence in younger animals. The cumulative incidence on a farm where Giardia has been diagnosed is 100% in cattle and goats and nearly 100% in sheep.

Three major morphologic groups have been described: G muris from mice, G agilis from amphibians, and a third group from various warm-blooded animals. There are at least four species in this third group, including G ardeae and G psittaci from birds,G microti from muskrats and voles, and G duodenalis (also known as G intestinalis and G lamblia), a species complex with a wide mammalian host range infecting people and domestic animals. Molecular characterization has shown that G duodenalis is in fact a species complex, comprising seven 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), whereas others infect a wide range of animals, including people (assemblage A and B). There is increasing evidence that some assemblages (A and B) that infect domestic animals can also infect people, although transmission patterns are not totally understood. Dogs have mainly assemblages C and D, cats have assemblages A1 and F, and people are infected with assemblages A2 and B; however, some studies have identified human assemblages of Giardia in canine fecal samples.

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. They usually live in the proximal portion of the small intestine. Trophozoites encyst in the small or large intestine, and the newly formed cysts pass in the feces. There are no intracellular stages. 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. Giardia cysts are infectious immediately after excretion and are 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 that has been described in Giardia-infected hosts.

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 usually not present in feces. Occasionally, vomiting occurs. Giardiasis must be differentiated from other causes of nutrient malassimilation (eg, exocrine pancreatic insufficiency [see Exocrine Pancreatic Insufficiency in Small Animals] and intestinal malabsorption [see 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 old). Experimental infection of goat kids, lambs, and calves resulted in a decreased feed efficiency and subsequently a decreased weight gain.

Gross intestinal lesions are seldom evident, although microscopic lesions, consisting of villous atrophy and cuboidal enterocytes, may be present.

The motile, piriform trophozoites (12–18 × 7–10 μm) are occasionally seen in saline smears of loose or watery feces. They should not be confused with yeast or 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 may be 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, three samples collected throughout 3–5 days). Giardia may be underdiagnosed, because the cysts are intermittently shed.

For the detection of parasite antigen, immunofluorescence assays and ELISA are commercially available. An in-house ELISA available for use in dogs and cats is a useful tool for clinical diagnosis, particularly when coupled with a centrifugal flotation examination of feces. It is best to test symptomatic animals with a combination of a direct saline smear of feces, fecal flotation with centrifugation, and a sensitive, specific ELISA optimized for use in the animal being tested (eg, ELISA for dogs and cats).

No drugs are approved for treatment of giardiasis in dogs and cats in the USA. Fenbendazole (50 mg/kg/day for 5–10 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 to treat 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 5 days. 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 species, 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 effectively decreases cyst excretion in infected dogs when administered for 3 days. 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 (extra-label at 25 mg/kg, bid for 5 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. Metronidazole benzoate is perhaps better tolerated by cats. Safety concerns limit the use of metronidazole in dogs and cats. A possible treatment strategy for dogs would be to treat first with fenbendazole for 5–10 days or to administer both fenbendazole and metronidazole together for 5 days, being sure to bathe the dogs to remove cysts. If clinical disease still persists and cyst shedding continues, the combination therapy should be extended for another 10 days.

Currently, 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). Feces should be removed as soon as possible (at least daily) and disposed of with municipal waste. Infected dogs and cats should be bathed to remove cysts from the hair coat. Prompt and 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 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 September 2013 by Sharon Patton, MS, PhD

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