Edited October 31, 2015
The Giardiasis in cats guidelines that the present article is updating were published in J Feline Med Surg 2013; 15: 650-652 by Tim Gruffydd-Jones; this update has been compiled by Corine Boucraut-Baralon.
Giardia is a protozoal parasite that infects the small intestine of cats and can cause diarrhoea. The biotypes that affect cats do not appear to infect humans. Infection is most common in young cats particularly from multicat backgrounds. Infected cats that develop clinical signs show small intestinal diarrhoea and there may be associated weight loss. Diagnosis of infection is usually based on an in practice ELISA for faecal antigen or zinc sulphate flotation of several pooled faecal samples. PCR tests are available but not used so widely. Infection can be detected in clinically healthy cats so interpretation of a positive result in cats with diarrhoea requires care. Fenbendazole or metronidazole are regarded as the treatments of choice. Secondary gut changes may take some some time to resolve so diarrhoea may continue for some time after infection has been eliminated.
A number of names have been used for the coccidian flagellate protozoan parasite giardia – G..duodenalis, G. lamblia and G. intestinalis.. Giardia can infect a number of hosts including man. Seven different molecular subtypes have been identified designated A-G. F is the subgroup seen in cats whereas A and B are the main subgroups in man.1 This is therefore not considered to be a zoonotic infection.2,3
However, recent European studies demonstrated the presence of subgroup A in cats,14,15,16 either alone or as a dual infection (A and F).15 Genotype B has also been identified in cats, but A is most prevalent, according to a Canadian study.17
The parasite has a direct life cycle. It lives in the lower small intestine of the cat in its trophozoite form, adherent to the intestinal wall. It replicates by binary fission to produce the encysted form, which is passed in the faeces in addition to the trophozoites..
Giardia is transmitted by the faecal-oral route. Although trophozoites are excreted in the faeces, these do not survive well in the environment and are unlikely to cause infection. In contrast, cysts are highly infectious and successful transmission requires only a small number to be ingested. The cysts can survive in the environment for up to several months in ideal conditions and indirect transmission via faecal contamination can occur.
Epidemiological studies in different countries, and sampling different cat populations have shown a variable prevalence. It depended upon the diagnostic screening test used, but generally a prevalence of 1-20% has been reported.14,15,16 In some studies the prevalence has not been notably different in cats with diarrhoea as compared to healthy cats.
The parasite can cause damage to and loss of the epithelial cells of the lower small intestine provoking an inflammatory response. There may be blunting of the intestinal villi leading to malabsorption.
Young cats are more susceptible to both infection and associated disease, with most clinical infections occurring in cats under one year of age. Many infections are not followed by overt disease, and the importance of Giardia as a diarrhoeal pathogen in cats is not clear. Experimental infections have induced clinical signs, but not in all cases. The mechanism by which diarrhoea is induced is also not clear, but thought to be related to malabsorption, which may be accompanied by weight loss, which is a prominent feature in some cases. The diarrhoea is typically of a small intestinal nature with passage of liquid or semi-liquid faeces but may sometimes show large intestinal features containing mucus/blood. The clinical course may last for weeks.
The immune response to giardia infection is poorly understood in cats. Based on information from infection in other species it is presumed that cellular immunity and an IgA response are key to providing protective immunity.
The infection is diagnosed using direct examination of faecal smears (wet mount examination), faecal flotation methods, faecal ELISA antigen assays, direct immunofluorescence on faecal smears and PCR.
Trophozoites can be identified in fresh faecal smears. They are motile with a rolling action. A small amount of freshly passed faeces or mucus is mixed with a drop of saline solution on a microscope slide, covered with a coverslip and immediately examined under a microscope at a magnification of x100. Further examination at x400 allows definitive identification. It is also possible to use microscopic examination of duodenal aspirates collected during endoscopic small intestinal intubation for trophozoites. However, Giardia resides further down the small intestine of cats beyond the reach of endoscopic intubation.17
A zinc sulphate flotation method is recommended for faecal screening. Excretion of cysts is erratic and therefore several (usually three) faecal samples collected on consecutive or alternative days should be screened. Routine saturated salt or sucrose methods are unsatisfactory since they lead to distortion of the cysts.
It is also possible to use a direct fluorescent antibody technique to detect cysts in faecal smears, a test not widely used in Europe.
ELISA techniques for detecting antigen in faeces are available, including an in-practice SNAP test (IDEXX Ltd.) but do not appear to be more sensitive than careful faecal screening.8 Recent studies have shown that ELISA detection of antigen correlates well with direct fluorescent antibody screening results.9
PCR tests are available but not widely used. They have the advantage of being able to identify the subtype present. However PCR-based studies have shown a high proportion of positives (up to 80%) which has raised concerns that they may detect infections that are not clinically relevant.10
The faecal flotation method has been the standard test used in the past, but the in-practice faecal antigen test appears to be equally sensitive and specific, and is convenient to perform. Examination of faecal smears is cheap and has the advantage of identifying other potential parasites – but it is not popular in practice and less sensitive.11
A pragmatic approach often used by practitioners as an alternative to testing is to assess the response to treatment.
The standard treatment of Giardia infection has generally been an imidazole, usually fenbendazole (Panacur) given at 50 mg/kg for 5-7 days.12,13 However, metronidazole is an alternative, and the original recommendation was to use it at a dosage of 50 mg/kg for five days. This dosage carries an increased risk of side effects – central nervous toxicity causing weakness, ataxia, disorientation and seizures. Recently it has been suggested that a daily dosage of 25 mg/kg is effective, which is unlikely to induce side effects.
A vaccine based on inactivated trophozoites (Giardia Vax – Fort Dodge/Pfizer) has been used in the USA but not in Europe and is no longer available. It was used for treatment as well as for prevention.
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13 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; 64:1027-1029.
15 Dado D, Montoya A, Blanco MA, Miró G, Saugar JM, Bailo B, et al. Prevalence and genotypes of Giardia duodenalis from dogs in Spain: possible zoonotic transmission and public health importance. Parasitol Res 2012; 111: 2419-2422.
17 McDowall RM, Peregrine AS, Leonard EK, Lacombe C, Lake M, Rebelo AR, et al. Evaluation of the zoonotic potential of Giardia duodenalis in fecal samples from dogs and cats in Ontario. Can Vet J 2011; 12: 1329-1333.