Update on Infectious Causes of Diarrhea in Cats
WSAVA 2002 Congress
Michael R. Lappin, DVM, PhD
Department of Clinical Sciences, Colorado State University
Fort Collins, Colorado, USA


There are many infectious agents associated with vomiting or diarrhea in cats. Primary clinical signs associated with infectious causes of gastrointestinal tract diseases are abbreviated as follows: S = small bowel; M = mixed bowel; L = large bowel; V = vomiting. Any of the agents that induce diarrhea potentially can induce vomiting as well. The primary bacteria associated with gastrointestinal tract disease in cats include Salmonella spp. (S,M,L), Campylobacter jejuni (M,L), Clostridium perfringens (L,rare), Helicobacter spp.(V), bacterial overgrowth syndrome (S), bacterial peritonitis (S), and bacterial cholangiohepatitis (S). The primary viral agents include feline coronaviruses (S), feline leukemia virus (FeLV; V,S,M,L), feline immunodeficiency virus (FIV; V,S,M,L), and feline panleukopenia virus (V only frequently, S). The primary helminths are Ancylostoma/Uncinaria (S,M), Strongyloides cati (S,M, rare), Dirofilaria immitis (V), Toxocara cati (V), Toxascaris leonina (V), Ollulanus tricuspis (V), and Physaloptera spp. (V). Enteric protozoans include Giardia spp. (S,M), Cystoisospora spp. (Isospora; M,L), Cryptosporidium spp. (S,M), Entamoeba histolytica (L, rare), and Tritrichomonas foetus (L, rare). The cestodes Taenia, Dipylidium, and Echinococcus generally cause subclinical infection.


Direct smear. Fresh diarrhea samples should be microscopically examined immediately for the presence of the motile protozoal trophozoites, including those of Giardia spp., and Tritrichomonas foetus. The amount of feces required to cover the head of a match is mixed thoroughly with one drop of 0.9% NaCl. Following application of a coverslip, the smear is evaluated for motile organism by examining it under 100X magnification.

Stained smear. A thin smear of feces should be made from all cats with diarrhea. A cotton swab is gently introduced 3-4 cm through the anus into the terminal rectum, directed to the wall of the rectum, and gently rotated several times. Placing a drop of 0.9% NaCl on the cotton swab will facilitate passage through the anus. Roll the swab on a slide to give areas with varying smear thickness, air dry, and stain. White blood cells and bacteria morphologically consistent with Campylobacter jejuni or Clostridium perfringens can be observed after staining with Diff-Quick or Wright's-Giemsa stains. Histoplasma capsulatum or Prototheca may be observed in the cytoplasm of mononuclear cells. Methylene blue in acetate buffer (pH 3.6) stains trophozoites of the enteric protozoans. Acid-fast or monoclonal antibody staining of a fecal smear should be performed in cats with diarrhea to aid in the diagnosis of cryptosporidiosis. Cryptosporidium parvum is the only enteric organism of approximately 4 to 6 microns that will stain pink with acid-fast stain. Neutrophils on rectal cytology can suggest inflammation induced by Salmonella spp., Campylobacter jejuni, or Clostridium perfringens; fecal culture is indicated in these cases.

Fecal flotation. Most ova, oocysts, and cysts are easily identified after zinc sulfate centrifugal flotation. This procedure is considered by many to be optimal for the demonstration of protozoan cysts, in particular, Giardia spp. and so is a good choice for a routine flotation technique. Sugar centrifugation can be used for routine parasite evaluation and may be superior to many techniques for the demonstration of Toxoplasma and Cryptosporidium oocysts. Giardia cysts are distorted by sugar centrifugation but can still be easily identified. Fecal sedimentation will recover most cysts and ova, but will also contain debris.

Culture. Culture of feces for Salmonella spp. or Campylobacter spp. is occasionally indicated. Approximately 2-3 grams of fresh feces should be submitted to the laboratory immediately for optimal results, however, Salmonella and Campylobacter are usually viable in refrigerated fecal specimens for 3-7 days. The laboratory should be notified of the suspected pathogen so appropriate culture media can be used. Viral culture can be performed at some laboratories.

Immunologic techniques. Parvovirus, Cryptosporidium parvum, and Giardia spp. antigen detection procedures are available for use with feces. Canine parvovirus antigen assays appear to detect feline parvovirus antigen. Minimal sensitivity and specificity results for the C. parvum and Giardia spp. ELISA when used with feces from small animals are currently available. When used, results of these assays should be interpreted in conjunction with results from fecal examination techniques. IFA for detection of C. parvum oocysts have been validated for use with feline feces; this assay is commonly available at human hospitals. Polymerase chain reaction can be used for the detection of some organisms but is not routinely available. Clostridium perfringens enterotoxin assay results do not discriminate between healthy cats and those with diarrhea.

Electron microscopy. Electron microscopy can be used to detect viral particles in feces of cats with gastrointestinal signs of disease. Approximately 1-3 grams of feces without fixative should be transported to the laboratory by overnight mail on cold packs.

Endoscopy or exploratory laparotomy. Ollulanus and Physaloptera rarely pass ova in feces and so frequently are diagnosed only by endoscopy. Gastric biopsies should be placed on urea slants to assess for urease which is found in the cell wall of Helicobacter spp.. The combination of inflammation, exclusion of other causes of inflammation, presence of gastric spiral bacteria, and positive urease testing can be used as a presumptive diagnosis of gastric helicobacteriosis. There is no benefit to performing duodenal aspirates for quantitative bacterial cultures or Giardia trophozoite evaluations in cats; the normal bacterial count range is very broad in cats and Giardia is found in the distal small intestine. Regional enteritis due to feline infectious peritonitis can be confirmed by documenting the organism in tissue after immunohistochemical staining.


There are multiple drugs used in the treatment of gastrointestinal tract infections. Pyrantel pamoate or fenbendazole are usually effective treatments for helminths causing gastrointestinal tract disease; Physaloptera may response better to pyrantel. Cestodes usually are eliminated by praziquantel; fenbendazole is effective for Taenia spp.. Since Echinococcus multilocularis can be a significant zoonosis, hunting cats in endemic areas should be treated frequently.

Some isolates of Giardia, and Cryptosporidium are zoonotic; cats are of minimal risk to pass Entamoeba to people because they are unlikely to form cysts. Giardia and Tritrichomonas generally respond to metronidazole. Fenbendazole was recently shown to be ineffective for the treatment of Giardia. It is unknown if albendazole is effective and it causes hematologic toxicity. A Giardia spp. vaccine has been introduced for use in cats. When given twice, the vaccine lessens numbers of cysts shed and lessens clinical disease on challenge. Unfortunately, the vaccine is adjuvanted and given subcutaneously and may ultimately be proven to be associated with fibrosarcomas. Since the disease is usually not life-threatening and has response to therapy of at least 90%, routine use in client-owned cats seems unnecessary.

Sequential administration of clindamycin followed by tylosin blocked oocyst shedding and resolved diarrhea in one cat with chronic, clinical cryptosporidiosis. Tylosin was apparently successful in blocking oocyst shedding in multiple other cats with diarrhea. Unfortunately, tylosin is very bitter and usually has to be given to cats in capsules; I am currently using it twice daily in cats. Paromomycin is effective for treating cryptosporidiosis in cats and also is an alternate anti-Giardia drug. However, this orally administered aminoglycoside may cross the diseased intestinal wall to induce renal insufficiency. The Toxoplasma gondii oocyst shedding period can be shortened by administration of clindamycin. Isospora spp. generally respond to the administration of sulfadimethoxine or other sulfa drugs.

Clostridium perfringens and bacterial overgrowth generally respond to treatment with tylosin, metronidazole, ampicillin, amoxicillin, or tetracyclines. The drug of choice for campylobacteriosis is erythromycin; an alternative drug is enrofloxacin. Salmonellosis should only be treated parenterally due to rapid resistance that occurs following oral administration of antibiotics. Appropriate antibiotics for the treatment of salmonellosis include chloramphenicol, trimethoprim-sulfa, amoxicillin; quinolones are also effective. Helicobacter spp. infections are usually treated with the combination of metronidazole and tetracycline or amoxicillin and metronidazole in dogs. Clarithromycin is a logical choice in cats since the species is often difficult to treat with multiple drugs. An antacid like famotidine should also be used.

Cats with apparent bacteremia due to enteric bacteria should be treated with parenteral antibiotics with a spectrum against anaerobic and gram negative organisms. The combination of enrofloxacin with a penicillin or first generation cephalosporin is generally effective. Second generation cephalosporins or imipenem are also appropriate choices.

Cats that have hepatic infections and signs of bacteremia should be treated with antibiotics that kill gram positive, gram negative and anaerobic bacteria as discussed before. Non-septic hepatic infections generally respond to amoxicillin, first-generation cephalosporins, or chloramphenicol. Decreasing numbers of enteric flora by oral administration of penicillins, metronidazole, or neomycin can lessen the clinical signs of hepatic encephalopathy.

Panleukopenia virus, feline leukemia virus, feline immunodeficiency virus, and coronaviruses are the most common viral causes of gastrointestinal tract disease in cats. Viral diseases are managed by supportive treatment. Make sure to maintain hydration, correct hypoglycemia, and maintain normal potassium concentrations. Use of jugular catheters is superior to leg veins since blood samples can be drawn and CVP can be measured. Administration of plasma or serum (1 ml/kg) from your hyperimmune blood donor cat may be helpful due to passive transfer of immunity. Antibiotics effective against gram negative and anaerobic bacteria are commonly indicated.


References are available on request.

Speaker Information
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Michael R. Lappin, DVM, PhD
Department of Clinical Sciences
Colorado State University
Fort Collins, Colorado, USA

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