Giardia felis Hegner, 1925

(Figures 1-27 through 1-29)

ETYMOLOGY:Giardia for Dr. Giard (1846-1908) and felis for the feline host

SYNONYMS:Giardia lamblia Stiles, 1915, Giardia duodenalis (Daviane, 1875), Giardia cati Deschiens, 1925

HISTORY: The organism was discovered by Leeuwenhoek in his own stools. The parasite was first described by Lambl who gave it the name intestinalis. Stiles (1915) created a new binomial for this species, Giardia lamblia, due to some confusion as to whether intestinalis was actually available as a name for this parasite. In 1925, Hegner described Giardia felis from domestic cats in the United States, while Deschiens (1925) described Giardia cati from cats in France. In 1952 Filice examined specimens of many different species of rodent Giardia and decided that there were two species in mammals, Giardia duodenalis (Daviane, 1875) originally described from rabbits and Giardia muris (Grassi, 1879) from mice. If one accepts the belief of Filice that most mammals are infected with different manifestations of Giardia duodenalis, then, this name has precedence. If there are different species of Giardia present in different mammalian hosts, then Giardia felis would be the binomial used to describe the species typically found in the cat.

GEOGRAPHICAL DISTRIBUTION:Giardia felis is found in cats throughout the world. Infections with Giardia are not uncommon in cats in the United States (Kirkpatrick, 1986) Hitchcock (1953) found Giardia in 57% of 14 cats in Michigan; Bemrick (1961) found Giardia in 3% of stools from 291 cats in Minnesota.. Surveys have reported infections in 14% of 226 cats in Australia (Swan and Thompson, 1986) and 6.7% of 120 cats in New Zealand (Tonks et al., 1991). In Europe, cats have been identified as hosts of Giardia on numerous occasions: 5.2% of 94 cats in Switzerland (Seiler et al., 1983); 35% of 20 cats in the United Kingdom (Winsland et al., 1989), 10.8% of 500 cats in Belgium (Vanparijs and Thienpoint, 1973), 2.3% of 910 cats in Germany (Bauer and Stoye, 1984), 5.6% of 90 cats in Italy (Carneri and Catellino, 1963), 1.2% of cats in Czechoslovakia (Kucharova, 1989), 4% of cats in Turkey (Burgu et al., 1985). 7.2% of 97 cats in Iran have been identified as infected with Giardia (Anwar, 1974). In Chile, 2% of 50 cats were found infected (Franjola and Matzner, 1982).

LOCATION IN HOST: Hitchcock and Malewitz (1956) stated that they found trophozoites of Giardia throughout the small intestine below the duodenum and in the cecum and large intestine. Kirkpatrick and Farrell (1984) reported that the trophozoites were concentrated in the lower portions of the jejunum, whereas lower numbers were observed in the upper portions of the jejunum and in the ileum; these authors did not find trophozoites in the large intestine. It has been shown (Tsuchiya, 1931) that the location of Giardia trophozoites within the intestinal tract of the dog is affected by diet with diets rich in carbohydrates causing the majority of trophozoites to be found nearer the stomach while they were more posterior in animals fed a diet high in protein; also fewer organisms were present in the intestines of animals fed the high protein diet.

PARASITE IDENTIFICATION: Within cats, Giardia occurs as a binucleate, flagellate trophozoite (Figs. 1-27 and 1-28). The trophozoite is small (10.5 to 17.5 m long by 5.25 to 8.75 m in maximum width), and has eight trailing flagella. The two nuclei are just posterior to midbody; these large nuclei are about 3 μm long and 1.5 μm wide. The ventral surface of the trophozoite bears a sucking disc that occupies about one-third to one-half of the anterior surface. The trailing flagella both propel the organism and create a vacuum for the sucking disc by pumping fluid out from under it. The trophozoite moves along the surface of the intestinal epithelium.

The cyst stage is found in the large bowel as the trophozoite prepares to enter the external environment (Fig. 1-29). Cysts are about 7.4 μm wide and 10.5 μm long, and have a length-to-width ration of about 1.4. In response to an undefined stimulus, trophozoites produce the resistant cyst wall as they pass from the small to the large intestine. The trophozoite then divides, resulting in two trophozoites within the mature cyst. The cyst stage is passed in the feces.

If a cat has diarrhea and an accompanying infection with Giardia, a confirmatory diagnosis will often require that a direct saline examination of a small quantity of fresh feces be performed. This will allow the identification of the trophozoite stage. Examination of diarrheic feces for cysts is often inconclusive because only trophozoites are found in stools of this type. In human medicine, fixed fecal smears are often prepared and stained with iron hematoxylin or by using a trichrome staining method for the examination of protozoa. The major reason for the examination of fixed smears of human feces is the many kinds of protozoa that must be differentiated. Few fecal protozoans are present in cats, and it is typically not necessary to perform such procedures. Fixed fecal samples do have the advantage, however, of producing a permanent slide that can be examined at a later date.

Another method now used routinely is the detection of Giardia in fecal smears using fluorescein-conjugated antibodies to Giardia, and subsequent examination under a fluorescence microscope. This method increases the likelihood of finding the trophozoites and cysts, because they will fluoresce when examined, but the equipment is costly, and other procedures are still be required to concentrate the organisms in many samples.

A method that eliminates some of these problems is the detection of Giardia antigens within fecal samples. antigen detection tests developed for use with human feces are capable of detecting infections in diarrheic fecal samples in which, due to the degeneration of non-viable trophozoites, there may be no detectable organisms. Such tests work by detecting antigens of the Giardia organism that are shed in the feces (Barr et al., 1992). These tests, such as ProSpecT®/Giardia™ (a product of Alexon Inc., Mountain View, CA), have not yet been approved for use in cats, but there are indications that they are likely to work very well. Cats should not be examined for giardiasis using the duodenal aspirate method used for dogs, because the organisms may live further posteriad in the intestine in the cat than they do in the dog.

Cysts can usually be detected by using a centrifugal flotation procedure. The zinc-sulfate method is preferred. Because cyst excretion has been shown to be sporadic (Kirkpatrick and Farrell, 1984), it may be best to examine the samples by both flotation and antigen detection. Cysts can be observed in sugar flotations of fecal matter, but the cysts will rapidly collapse. Collapsed cysts look like small crescent moons rather than the ovoid forms that are observed in direct smears or zinc-sulfate preparations soon after they are made. Antigen detection assays will also detect cysts.

LIFE CYCLE: The cyst is passed in the feces of the host and infects a new host by direct fecal-oral contamination. Cysts may also be transmitted through contaminated drinking water to a next host. When a host ingests a cyst, the trophozoites leave the cyst within the small intestine and take up residence on the intestinal mucosa. They divide repeatedly until they have populated the intestines of the new host with trophozoites. Some trophozoites periodically are carried in the fecal stream towards the anus, encysting on the way. Within five to sixteen days after a cat has ingested a cyst, the cat is capable of excreting thousands of cysts in the environment.

CLINICAL PRESENTATION AND PATHOGENESIS: Cats can and do develop clinical signs from infections with Giardia felis. The typical sign is diarrhea (Barr and Bowman, 1994). Cats may undergo weight loss, and kittens may fail to gain weight. Cats with signs of diarrhea will usually be observed to maintain normal appetites and clinical values. Cats infected with Giardia will typically not undergo periods of vomiting. The diarrhea that is observed tend to be due to problems with malabsorption and steatorrhea, and for this reason, the feces tends to be soft and pale in color, and to have increased levels of neutral fats. It has been shown in experimentally infected lambs, that infection with Giardia lamblia was associated with decreased weight gain and feed efficiency when compared to uninfected controls (Olson et al., 1995)

TREATMENT: Several formulations of benzimidazoles have activity against Giardia infections (Barr and Bowman, 1994). Albendazole cleared eighteen of twenty dogs that had all been shedding cysts at the beginning of therapy. A similar treatment (25 mg/kg body weight orally twice daily for two days) did not work in cats. However, increasing the number of treatments (25 mg/kg body weight twice daily for five days) successfully cleared five cats of the cysts they were shedding in their feces. This compound is currently not approved for use in cats or dogs, although it is used as an anthelminthic in cattle. Further, albendazole has been associated with bone marrow aplasia in one cat when used to treat giardia infection (Stokol T, Randolph JF, Nachbar S, Rodi C, Barr SC. 1997). Fenbendazole has been shown to stop beagles from shedding cysts in their feces at the dosage routinely applied for anthelminthic therapy (50 mg/kg body weight orally once a day for three days) (Barr et al., 1994). This compound is a routinely-used and approved anthelminthic in dogs, but is not approved for use in cats. Fenbendazole has been administered to cats as an anthelminthic without any apparent detrimental effects (Roberson and Burke, 1980). Febantel is approved for use in cats, but has not been examined for its efficacy against Giardia in this species, although a compound containing febantel has been found to be effective in treating giardiasis in dogs (Barr SC, Bowman DD, Frongello MM, Joseph S. 1998.)

Other drugs used to treat Giardia infections in cats have included quinacrine, metronidazole, and furazolidone (Kirkpatrick, 1986). Quinacrine was given orally for 12 days (2.3 mg/kg body weight once a day) to 5 cats, but four continued to pass cysts. Oral metronidazole at 10 or 25 mg/ kg body weight given twice a day for 5 days was noted to cure cats of their infections with Giardia. Furazolidone given orally at 4 mg/kg body weight twice a day has also been shown to be effective.

EPIZOOTIOLOGY: Cats are probably infected from the ingestion of the cyst in direct fecal-oral transmission or through contaminated water or fomites. The trophozoite stage passed in diarrheic feces is not considered infectious.

Due to improved sanitation, humans are no longer routinely becoming infected during childhood with constant reinfection as adults, and it has been suggested that one reason that Giardia is now considered a cause of diarrhea in humans when it was once thought to be a commensal is that in the past, the immune status of humans was enhanced by constant re exposure (Beaver et al, 1984). It would seem that as cats are prevented access to cysts through improved hygiene, they, like humans, will assume a greater risk of developing disease assignable to Giardia if they become infected.

HAZARD TO OTHER ANIMALS:Giardia from cats has been transmitted experimentally to gerbils (Swabby et al., 1988). It is not known to what extent cross-transmission to other animals will occur, but precautions need to be taken when animals of different species are housed in the same facilities.

HAZARDS TO HUMANS: It is still unclear as to the relationship between infections in cats and in associated humans. It has been reported that cats and humans have been found to harbor genetically-identical forms of Giardia, and suggested that the cat is a likely reservoir of human infection (Thompson et al., 1988). A study in the United Kingdom of the potential of zoonotic transmission of Giardia from pets to their owners showed that cats and dogs, but especially feral dogs, were correlated with infection of humans with Giardia (Warburton et al., 1994). A similar study in Czechoslovakia found no correlation between infection in pets and in humans (Pospisilova and Svobodova, 1992). It has however been shown that cats can be infected with human and canine isolates of Giardia (Svovodova et al., 1990). Thus, at this time, every effort should be taken to prevent the potential transmission of cysts from cats to humans.

CONTROL/PREVENTION: In catteries, infections with Giardia can become a chronic problem, causing periodic bouts of diarrhea in groups of animals. Typically, older cats tend to become refractory to infection and some cats spontaneously stop shedding cysts; these manifestations have been interpreted to mean that cats may develop some form of acquired resistance to giardiasis. When Giardia is identified in catteries, all cats housed in contiguous cages must be treated, and particular attention given to increasing the levels of sanitation and fecal control.

The task of simultaneously clearing many animals of their infections can make disease control in a cattery very difficult. Important facts to remember in infection control are that (1) the cysts stage of the parasite tends to be waterborne; (2) the cats will not stop shedding cysts in their feces immediately after treatment, and not all will stop at the same time; and (3) if the cysts are not removed from the environment, the condition may persist through reinfection. Thus, control involves the treatment of all cats simultaneously, followed by moving the cats to clean cages. If cages cannot be steam cleaned, they should be washed with hot soapy water, rinsed, and dried thoroughly. If it is possible to dry the cages in the heat of the sun, this would be an excellent means of disinfection. If conditions are such that the cages cannot be dried completely, a disinfectant (e.g., ammonia, Lysol™, or bleach) should be added to the water. The day after the cats have received their initial treatment, they should be moved to clean cages. Following the second treatment, the cats should be moved again. This movement can be repeated a third time. If the cats cannot be moved this often, it will reduce the chances of successfully breaking the cycle of transmission.

Fecal samples to verify that the cats have been cleared of cysts should be taken one week after the first treatment. Because of the short prepatent period following infection, delay of more than a week after treatment in collection of fecal samples should be avoided. If such a delay occurs, a negative examination for Giardia will indicate clearance, but a positive result may indicate either failure to clear, or reinfection, or both.


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FIGURE 1-27.Giardialamblia trophozoite in human feces. This trophozotie was fixed then stained with iron hematoxylin and photographed using phase microscopy.

Figure 1-28.Giadialamblia trophozoite in culture. This living organism is being viewed form the bottom where it is attached to a glass coverslip in culture medium. The view gives a very clear image of the sucking disk and the trailing flagella.

FIGURE 1-29.Giardiacanis cyst in the fresh feces of a dog photographed using phase microscopy.