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Compendium August 2010 (Vol 32, No 8)

Giardia Infection in Cats

by Stephanie Janeczko, DVM, MS, DABVP (Canine and Feline Practice), Brenda Griffin, DVM, MS

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    Abstract

    The protozoon Giardia duodenalis is a common gastrointestinal parasite of cats. While most Giardia-infected cats are asymptomatic, acute small bowel diarrhea, occasionally with concomitant weight loss, may occur. Giardia poses a diagnostic challenge, but newer tests, including a commercially available ELISA kit, have improved clinicians’ ability to obtain an accurate diagnosis. Several treatment options have been reported, and although none has been shown to be universally effective, most cases can be successfully managed with drug therapy, supportive measures, and environmental control. Current recommendations suggest that combination therapy with fenbendazole and metronidazole may be the safest, most effective treatment option for symptomatic cats.

    Giardia spp are intestinal protozoal parasites capable of causing clinical and subclinical disease in numerous species, including humans and cats. Prevalence estimates in cats commonly range between 1% and 10%,1,2 but prevalences as high as 50% have been reported in some catteries and animal shelters.3 Unfortunately, the zoonotic potential of Giardia spp that infect cats remains undetermined.4 Recent molecular research has demonstrated several different genotypes, known as assemblages, of Giardia duodenalis.5 Both the potentially zoonotic assemblage A and the host-adapted feline assemblage F have been detected in cats.6–10 While the presence of genotypes that do not have a narrow host specificity implies that zoonotic transmission is possible, it does not verify the occurrence or direction of transmission. Experimental observations of zoonotic transmission from cats to humans are lacking, and therefore, data on the frequency of such transmission are based on circumstantial evidence. It appears that transmission from cats to humans is rare, if it occurs at all.11

    Life Cycle, Transmission, and Pathogenesis

    G. duodenalis has a simple, direct life cycle, and transmission occurs via the fecal–oral route or through contact with contaminated fomites. The prepatent period in cats is 5 to 16 days.2,3,12 Affected cats intermittently shed small, oval to teardrop-shaped infective cysts, which contain two mitotically arrested trophozoites. Cysts are excreted in the feces along with noninfective, nonencysted trophozoites. Excreted cysts are immediately infective2 and capable of surviving outside of the gastrointestinal tract. After ingestion, the encysted trophozoites emerge in the duodenum of the new host. Cysts can become well adhered to the perianal region of cats, facilitating reinfection and complicating control efforts. They may also survive for weeks to months in cool, moist environments, but they are susceptible to drying out.2 Rapid environmental contamination is thus likely in multicat environments, especially animal shelters and catteries, and infections may become endemic.

    Clinical Signs

    Most cases of Giardia infection in cats are subclinical, but clinical disease is possible and signs may precede cyst shedding by 1 to 2 days.2,3,12 The development of clinical signs apparently relies on a combination of factors, most notably on the host’s immune status. The exact mechanisms of disease remain unknown, but enterocyte apoptosis, as well as morphologic and functional changes, including villus atrophy, diffuse shortening of microvilli, loss of epithelial barrier function, and increased permeability of enterocytes, have all been reported.11 The most common sign in cats is acute small bowel diarrhea, occasionally accompanied by weight loss. Disease may be severe or chronic in a limited number of cases, and large bowel diarrhea, vomiting,13 and failure to thrive have been described in affected cats.12

    Diagnosis

    Even though numerous testing options exist (TABLE 1), Giardia presents a diagnostic challenge and is generally considered to be underdiagnosed in cats.14,15 Diagnosis of Giardia infection requires detection of either trophozoites or cysts in the feces. Cysts are small (approximately 12 × 7 µm) and can be difficult to identify on fecal flotation even when concentrating techniques are used. The cysts are shed intermittently, can be confused with pseudoparasites such as yeast, and are easy to overlook even with careful microscopic inspection. Although identification of Giardia cysts or trophozoites confirms an infection, their absence does not exclude the diagnosis. Furthermore, the presence of Giardia cysts on a fecal examination does not confirm that clinical disease is a result of Giardia infection because cysts can be found in both healthy and diarrheic animals. Thus, the significance of finding Giardia cysts in a fecal sample is not always clear and must be interpreted in light of the clinical presentation and other findings.

    Because the intensity of cyst shedding varies substantially, the clinical significance of finding only a few cysts in a fecal sample may be no different than that of finding large numbers.14,15 Because most Giardia infections are subclinical in cats and the zoonotic risk is low, widespread screening of healthy cats is not recommended. Diagnostic efforts should be reserved for cats with compatible clinical signs or cases in which a specific concern regarding zoonotic transmission exists (such as with immunocompromised caregivers).

    Direct Fecal Smears

    Microscopic evaluation of direct fecal smears to identify trophozoites is a simple, inexpensive technique that can be quickly performed by trained personnel. A drop of freshly collected, still warm feces is mixed with a drop of normal saline on a slide and examined under 40× magnification for the presence of trophozoites. Alternatively, feces can be mixed with a drop of Lugol’s iodine, which kills and stains the trophozoites, making detection easier. Giardia trophozoites have a ventral sucking disk and flagella that give them a characteristic appearance (FIGURE 1), but they must be distinguished from Tritrichomonas foetus trophozoites, which do not have a ventral disk. T. foetus trophozoites move in a rolling or jerky forward motion compared with the “falling leaf” movement of Giardia trophozoites.a

    Fecal Flotation

    Historically, the zinc sulfate centrifugation–flotation test (ZNCT) has been considered the gold standard for the diagnosis of Giardia when used by trained personnel.2,14,15 Fecal samples that have been freshly collected or refrigerated at 4°C for up to 48 hours are centrifuged with zinc sulfate solution to concentrate the cysts (FIGURE 2). Unlike sucrose solutions, which cause significant distortion, the zinc sulfate solution preserves the integrity of the cysts. Because cyst shedding is intermittent, a single ZNCT will identify approximately 70% of infected cats, while two or three tests will identify approximately 93% and >95% of infected cats, respectively.2 For this reason, at least three ZNCTs performed on consecutive fecal samples must be negative to rule out Giardia infection. While the ZNCT increases recovery and maintains the integrity of cysts, it does not eliminate problems associated with cyst identification and low numbers of organisms and is impractical in many settings because multiple samples are required for accurate results.

    Antigen-Based Assays

    The use and acceptance of ELISAs and direct immunofluorescence tests in the diagnosis of Giardia infection are increasing.2,16 These tests are generally highly sensitive and specific and can be used on fresh or formalin-fixed feces. They can alleviate problems associated with detection and identification of Giardia cysts but, with the exception of one commercially available in-house test kit (SNAP Giardia Test, IDEXX Laboratories), generally need to be performed by diagnostic laboratories.

    Because they detect Giardia antigens, these tests are less affected by low numbers of organisms than the ZNCT. By coupling the antigen–antibody binding to a secondary reaction, ELISAs produce a color change and thus are easier to interpret than fecal flotation (FIGURE 3). The SNAP Giardia test targets a continuously shed trophozoite antigen, thereby theoretically avoiding the problem of intermittent cyst shedding (FIGURE 4). Direct immunofluorescence tests must be run by a diagnostic laboratory because they require a fluorescent microscope (FIGURE 5). Although this limits their practical utility, immunofluorescence tests have been shown to be more sensitive than the ZNCT when few cysts are present, and they may be most useful to rule out a diagnosis of Giardia if previous diagnostics have been unrewarding.17

    Debate continues over which testing protocol is the most accurate, but most experts agree that some combination of multiple tests on multiple samples is necessary to minimize false-negative results.16 In our opinion, the ease of use of antigen-based tests, combined with the difficulties in identification of Giardia cysts, intermittent cyst shedding, and difficulty in obtaining multiple fecal samples, makes antigen-based tests such as the ELISA superior to the ZNCT in many clinical settings.

    Treatment Considerations

    Cats with clinical disease should be treated for Giardia; however, the necessity of treating asymptomatic Giardia-positive cats has been debated. Current professional guidelines published by the Companion Animal Parasite Council16 (http://www.capcvet.org) state that cats should not be treated solely for the purpose of preventing zoonotic transmission and that repeated courses of treatment are not indicated in dogs or cats without clinical signs. Based on the current understanding of the epidemiology and pathogenesis of disease, we recommend appropriately counseling clients and reserving treatment for cats exhibiting clinical signs consistent with Giardia infection. Cats that test positive for Giardia in the absence of consistent clinical signs should not be routinely treated unless there is a compelling clinical reason to administer specific antigiardial drug therapy. Such reasons may include client wishes, ownership by an immunocompromised individual, eradication of infection from a group housing setting (e.g., cattery, animal shelter, multicat household), or prevention of introduction to naïve animals. In addition to specific drug therapies, it is imperative that steps be taken to improve an infected cat’s overall health and immune system, including treatment of concurrent diseases and other parasitic infections, reduction of stress, and improved nutrition.

    Environmental Control Measures

    Successful clearance of Giardia infection from individual cats can be difficult, and elimination of endemic disease in feline populations is particularly challenging. Even when initial treatment is successful at stopping cyst shedding, reinfection from a contaminated facility, other cats, or an infected cat’s own haircoat while grooming is common.18 For these reasons, increased sanitation and disinfection are essential factors in eliminating infection. Giardia cysts are destroyed by many disinfectants used in animal holding facilities, including 5% sodium hypochlorite (e.g., household bleach) at a 1:30 dilution and quaternary ammonium–containing products used in accordance with manufacturers’ recommendations.19 Cysts are also susceptible to desiccation, and ensuring a dry environment is helpful in breaking the cycle of transmission. The importance of bathing cats in order to remove infective cysts from their haircoats should not be overlooked.2 After bathing, drying the haircoat (especially in the perineal area) using warm air may help ensure thorough decontamination. Litterboxes and litter scoops should be replaced. Disposable litterboxes can be used while the cat is being treated; new boxes and scoops should be purchased after successful elimination of infection. Other cats in the household should be tested and treated if infected, or all cats can be empirically treated if there are no contraindications to treatment. Although such procedures are usually recommended only for populations of cats, they may be particularly helpful in eliminating difficult infections.

    Specific Drug Therapy

    No FDA-approved drugs exist for the treatment of giardiasis in cats. Although designed and FDA approved as a preventive rather than a treatment, a commercially available vaccine (GiardiaVax, Fort Dodge Animal Health) has been studied as a possible treatment for experimental Giardia infection but has not been demonstrated to be effective for this purpose.20 Available but unapproved drugs (e.g., extralabel use) include metronidazole, albendazole, fenbendazole, and a combination product of praziquantel, pyrantel pamoate, and febantel (Drontal Plus, Bayer HealthCare; TABLE 2 ). Little information exists on the effectiveness of these products in eliminating cyst shedding in cats, with reported efficacies as low as 20% in some clinical trials.21–23 Furthermore, it is unclear whether drug therapy eliminates Giardia infections or merely suppresses cyst shedding, and efficacy may be further reduced in animals with diarrhea because rapid gastrointestinal transit time may result in decreased drug contact with the trophozoites.24

    Fenbendazole is highly effective against Giardia and many other gastrointestinal parasites, seldom produces adverse effects, and can be used in pregnant animals. Once-daily dosing makes this a convenient choice for cats that may be difficult to medicate. Metronidazole may be useful for its effects against other gastrointestinal organisms but should be used with caution, as high doses (>30 mg/kg) can cause numerous adverse effects, including vomiting, anorexia, ataxia, and nystagmus.25 The use of albendazole should be avoided in cats because of the potential for myelosuppression.26 Some authors suggest concurrent treatment with metronidazole and fenbendazole; although no controlled studies have been performed to evaluate such a protocol,16 these drugs act on two different targets and may have synergistic effects. In our experience, such therapy is more effective than single-drug protocols, and in our opinion, this is the treatment of choice in cats, particularly when timely elimination of infection is a high priority.

    Evaluation of Treatment Efficacy

    Ideally, all cats undergoing treatment for Giardia infection should be reevaluated 1 to 3 days after completion of treatment and then again 2 to 3 weeks later to assess their response. Cats that are shedding cysts at the end of treatment or immediately thereafter have not successfully cleared the infection, while those cats that are initially negative after treatment but are shedding cysts a few weeks later have become reinfected; this distinction is critical to refining the treatment strategy. Most apparent treatment failures are the result of rapid reinfection rather than failure of the drug to eliminate cyst shedding. However, it is important to note that SNAP Giardia test results can remain positive for several weeks after treatment even in the absence of detectable cyst shedding.b It is unknown whether this truly represents a false-positive result or if such results are indicative of the presence of trophozoites with no, or low numbers of, Giardia cysts. Because the cyst is the infective form of the organism, we recommend that treatment success or failure be defined by the presence of cysts detected in the feces with ZNCTs or immunofluorescent tests and that the SNAP test not be used as the sole indicator of treatment efficacy.In general, cats that continue to have detectable cysts in their feces but are not showing clinical signs should not receive multiple courses of treatment unless such treatment is prescribed in conjunction with treatment of multiple cats or of a facility.16 Symptomatic cats that continue to have detectable cysts in their feces should be reevaluated. Further treatment may be indicated, but the clinician must also consider whether Giardia infection is causing the clinical signs noted. In some cases, empirical therapy for other common gastrointestinal parasites and infections or dietary manipulation may yield an adequate clinical response. However, further diagnostics are often indicated and may include fecal flotation for other gastrointestinal parasites, fecal and rectal cytology, fecal cultures for Salmonella and Campylobacter spp using the dedicated media for these agents, appropriate diagnostics for T. foetus, blood tests (complete blood count, chemistry panel, total T4, and cobalamin, folate, and feline trypsin-like immunoreactivity levels), abdominal imaging, endoscopy, and biopsy.

    Summary

    Diagnosis of Giardia is challenging, but by combining the available tests to evaluate multiple fecal samples from cats with compatible clinical signs, clinicians can greatly improve the chance of obtaining a correct diagnosis. Because of their relatively high sensitivity compared with other in-house tests, commercially available ELISA kits are a practical way to improve diagnostic yield, especially if collection of multiple fecal samples is not feasible. Because most infections are subclinical and there are no conclusive data that Giardia-infected cats pose a human health risk, treatment may not be indicated for every cat found to be shedding cysts. When treatment is elected, a multimodal approach using pharmacologic therapy, supportive care, and environmental control is thought to be the most efficacious at eliminating cyst shedding.

    Downloadable PDF

    Dr. Janeczko discloses that she has received financial support from IDEXX Laboratories.

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    2. Barr SC. Enteric protozoal infections. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 3rd ed. Philadelphia: WB Saunders; 2006:736-742.

    3. Kirkpatrick CE. Giardiasis in a cattery. JAVMA 1985;187(2):161-162.

    4. Bowman DD, Lucio-Forster A. Cryptosporidiosis and giardiasis in dogs and cats: veterinary and public health importance. Exp Parasitol 2010;124:121-127.

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    9. Souza SL, Gennari SM, Richtzenhain LJ, et al. Molecular identification of Giardia duodenalis isolates from humans, dogs, cats, and cattle from the state of Sao Paulo, Brazil, by sequence analysis of fragments of glutamate dehydrogenase (gdh) coding gene. Vet Parasitol 2007;149(3-4):258-264.

    10. Vasilopulos RJ, Rickard LG, Mackin AJ, et al. Genotypic analysis of Giardia duodenalis in domestic cats. J Vet Intern Med 2007;27:352-355.

    11. Thompson RCA, Palmer CS, O’Handley R. The public health and clinical significance of Giardia and Cryptosporidium in domestic animals. Vet J 2008;177:18-25.

    12. Belosevic M, Faubert GM, Guy R, MacLean JD. Observations on natural and experimental infections with Giardia isolated from cats. Can J Comp Med 1984;48:241-244.

    13. Vasilopulos RJ, Mackin AJ, Rickard LG, et al. Prevalence and factors associated with fecal shedding of Giardia spp. in domestic cats. JAAHA 2006;42:424-429.

    14. Dryden MW, Payne PA, Smith V. Accurate diagnosis of Giardia spp. and proper fecal examination procedures. Vet Ther 2006;7(1):4-14.

    15. Dryden MW, Payne PA, Ridley RK, Smith VE. Gastrointestinal parasites: the practice guide to accurate diagnosis and treatment. Compend Contin Educ Pract Vet 2006;28(7A):3-13.

    16. Companion Animal Parasite Council. Recommendations: giardiasis. Accessed June 2009 at http://capcvet.org/recommendations/Giardia.html.

    17. Deng MQ, Cliver DO. Rapid DNA extraction methods and new primers for randomly amplified polymorphic DNA analysis of Giardia duodenalis. J Microbiol Methods 1999;37(2):193-200.

    18. Ryan-Gullahorn J. Giardia and effective cattery management for the veterinarian. Feline Pract 2000;28(2):8-9.

    19. Zimmer JF, Miller JJ, Lindmark DG. Evaluation of the efficacy of selected commercial disinfectants in inactivating Giardia muris cysts. JAAHA 1988;24:379-385.

    20. Stein JE, Radecki SV, Lappin MR. Efficacy of Giardia vaccination in the treatment of giardiasis in cats. JAVMA 2003;222(11):1548-1551.

    21. Keith CL, Radecki SV, Lappin MR. Evaluation of fenbendazole for treatment of Giardia infection in cats concurrently infected with Cryptosporidium parvum. Am J Vet Res 2003;64(8):1027-1029.

    22. Scorza AV, Lappin MR. Metronidazole for the treatment of feline giardiasis. J Feline Med Surg 2004;6:157-160.

    23. Scorza AV, Radecki SV, Lappin MR. Efficacy of a combination of febantel, pyrantel, and praziquantel for the treatment of kittens experimentally infected with Giardia species. J Feline Med Surg 2006;8:7-13.

    24. Carlin EP, Bowman DD, Scarlett JM. Prevalence of Giardia in symptomatic dogs and cats in the United States. Compend Contin Educ Pract Vet 2006;28(11A):2-12.

    25. Caylor KB, Cassimatis MK. Metronidazole neurotoxicosis in two kittens. JAAHA 2001;37:258-262.

    26. Stokol T, Randolph JF, Nachbar S, et al. Development of bone marrow toxicosis after albendazole administration in a dog and cat. JAVMA 1997;210:1753-1756.

    aA videomicrograph of trophozoite movement can be seen at http://www.ncsu.edu/project/cvm_gookin/Tfoetusvideo.mov.
    bPersonal communication. Allison Hamlyn, IDEXX Technical Support; June 23, 2009.

    References »

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