Giardia duodenalis and G. felis are a cosmopolitan protozoan intestinal parasite that causes diarrhoea of cats. Co-infection with Tritrichomonas fetus is common. A recent survey has shown that random testing of adults cats shows a prevalence of Giardia spp in 1% of normal cats, and approx 30% in cats with diarrhoea.
Similar to Isospora spp, and Cryptosporidium spp, Giardia spp have been reported to be found in a significant number of cats in multi-cat households and cat shelters, with a higher rate of infection in younger animals.
The infective Giardia trophozoites 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 pass in the faeces. The cyst is the infective stage, and transmission occurs by the faecal-oral route. Cyst shedding may be continuous over several days and weeks but is often intermittent. Although occasionally passed in the feces, trophozoites are not infective. Incubation and prepatent periods are generally 5-14 days. Cysts can survive in the environment, but trophozoites cannot. Overcrowding and high humidity favour survival of cysts and transmission. Earlier classifications have assigned different species names to the Giardia of various hosts; it is generally agreed that all species infecting mammals (except some rodents) are structurally similar.
Infections with giardia are often subclinical or may present with mild, non-refractory diarrhoea. Faeces usually are soft, poorly formed, pale, malodorous, contain mucus, and appear fatty. Watery diarrhoea is unusual in uncomplicated cases, and blood is not present in faeces; typical small-bowel diarrhoea is more common. Occasionally vomiting occurs. Giardiasis must be differentiated from other causes of coccidiosis, nutrient malassimilation (eg, exocrine pancreatic insufficiency, intestinal malabsorption).
Clinical laboratory findings usually are normal. Pathogenesis of Giardia spp infections is poorly understood. Coinfection with Tritrichomonas spp and/or Isospora spp or Cryptosporidium spp may result in more severe clinical signs that make it more difficult to treat.
Gross intestinal lesions are seldom evident, although microscopic lesions, consisting of villous atrophy and cuboidal enterocytes, may be present. Laboratory studies have demonstrated malabsorption of nutrients, decreased quantities of intestinal disaccharides, increased enterocyte turnover, lymphocytic infiltration, and villous atrophy.
There no single test which is definitive for diagnosis of this parasite, although visualisation of trophozoites from fecal samples of sick cats is suspicious of the disease. The motile, piriform trophozoites (10-20 × 7-10 µm) are occasionally seen in saline smears of loose or watery faeces. 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) are best detected in faeces concentrated by the zinc sulphate (specific gravity 1.18) flotation technique. Sodium chloride, sucrose, or sodium nitrate flotation media are too hypertonic and severely distort the cysts. Staining cysts with iodine aids identification. Because Giardia cysts are excreted intermittently, several faecal examinations should be performed if giardiasis is suspected; eg, 3 samples collected and examined over 3-5 days. About 70% of infected dogs can be identified with a single zinc sulphate flotation; 93% can be identified with 2. In dogs, duodenal aspiration for trophozoite detection is useful; however, in cats, Giardia spp are more prevalent in the mid to lower small intestine.
ELISA assays are readily available as well as PCR testing for detection of fecal Giardia antigen.
No drugs are approved for treating giardiasis in animals.
- Fenbendazole (50 mg/kg/day) effectively removes Giardia cysts from the faeces of dogs; no side effects are reported, and it is safe for pregnant and lactating animals. This dosage is approved for controlling and removing Toxocara canis , Trichuris vulpis , and Ancylostoma caninum in dogs. Recently, a combination product of praziquantel, pyrantel pamoate, and fenbendazole decreased cyst excretion in infected dogs. Fenbendazole is not approved in cats, but may reduce clinical signs and cyst shedding at 50 mg/kg/day, PO, for 3-5 days. Albendazole is effective at 25 mg/kg, PO, bid for 2 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. Giardia-infected calves may be treated with albendazole or fenbendazole. Oral fenbendazole may also be an option in large animals and some birds.
- Metronidazole (25 mg/kg, PO, 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, PO, bid for 5 days.
- Furazolidone at 4 mg/kg, PO, bid for 7 days, is also effective in cats and small dogs, although diarrhoea and vomiting are possible side effects; it is also suspected of teratogenicity.
- A killed vaccine, available for dogs and cats, reportedly reduces clinical signs and the number and duration of cysts shed into the environment.
- Secnidazole has recently been trialed in cats and appears relatively efficacious as a single dose treatment for G. duodenalis giardiasis. Blood parameter changes were noted, but no clinical signs of toxicity were reported.
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