Giardiasis: Introduction - The Merck Veterinary Manual

Giardiasis: Introduction
( Giardosis, Lambliasis, Lambliosis)

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 and cats, occasional in ruminants, and rare in horses and pigs. The number of different species and the zoonotic potential of Giardia spp are controversial. There is circumstantial evidence that Giardia spp that infect domestic animals can infect people. It appears that some Giardia spp isolates are infective to a variety of mammals, while others are more species specific. Wild animals may also be reservoirs. Giardia spp have been reported to be found in 1-39% of fecal samples from pet and shelter dogs and cats, with a higher rate of infection in younger animals.

Etiology 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 pass in the feces. The cyst is the infective stage, and transmission occurs by the fecal-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 favor 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.

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. Clinical disease is also reported in calves. Feces usually are soft, poorly formed, pale, malodorous, contain mucus, and appear fatty. Watery diarrhea is unusual in uncomplicated cases, and blood is not present in feces; typical small-bowel diarrhea is more common. Occasionally vomiting occurs. Giardiasis must be differentiated from other causes of nutrient malassimilation (eg, exocrine pancreatic insufficiency [ Exocrine Pancreatic Insufficiency], intestinal malabsorption [ Malabsorption Syndromes]). Clinical laboratory findings usually are normal. Pathogenesis of Giardia spp infections is poorly understood. 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.

Diagnosis:

The motile, piriform trophozoites (10-20 × 7-10 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) are best detected in feces concentrated by the zinc sulfate (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 fecal 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 sulfate 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. An ELISA that detects Giardia antigen in the feces of dogs and cats is available, but field data on sensitivity and specificity are lacking.

Giardia spp cysts

Treatment:

No drugs are approved for treating giardiasis in animals. Fenbendazole (50 mg/kg/day) effectively removes Giardia cysts from the feces 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 febantel 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 diarrhea 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.

Control:

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 and catteries). Prompt removal of feces from cages, runs, and yards limits environmental contamination. Cysts are inactivated by most quaternary ammonium compounds, household bleach (1:32 or 1:16 dilution), steam, and boiling water.

To increase the efficacy of disinfectants, solutions should be left for 5-20 min before being rinsed off kennel or run surfaces. Disinfection of grass yards or runs is impossible. 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. Cysts contaminating the hair of dogs and cats may be a source of reinfection. Shampooing and rinsing the animals well can help remove cysts from hair. The killed vaccines that are available for dogs and cats aid in disease prevention by decreasing or preventing cyst shedding.