Management of Parasitic and Bacterial Causes of Infectious Diarrhea in Dogs and Cats
World Small Animal Veterinary Association Congress Proceedings, 2018
M. Lappin
Center for Companion Animal Studies, Colorado State University, Fort Collins, CO, USA


The primary objectives of this session are to review the known parasitic and bacterial causes of diarrhea followed by a discussion of optimal diagnostic tests, treatments and preventions.

Gastrointestinal (GI) signs can be the result of primary diseases of the GI system or secondary GI diseases. The secondary GI diseases are generally those of the kidneys, liver, pancreas (pancreatitis or exocrine pancreatic insufficiency [mainly dogs]), endocrine system (hypoadrenocorticism; diabetic ketoacidosis; hyperthyroidism [mainly cats]), or central nervous system.

Differential diagnoses for primary GI diseases are often grouped into obstruction (masses, foreign body, and intussusception), dietary intolerance, drugs/toxins (garbage gut), inflammatory gastric and bowel diseases, neoplasia, infectious diseases, and parasites.

The primary bacteria associated with gastrointestinal tract disease in dogs and cats include Salmonella spp., Campylobacter spp., Clostridium perfringens, Helicobacter spp., bacterial overgrowth syndrome, bacterial peritonitis, and bacterial cholangiohepatitis. The primary nematodes are Ancylostoma/Uncinaria, Trichuris vulpis (dogs), Strongyloides, Dirofilaria immitis (vomiting in cats), Toxocara spp., Toxascaris leonina, Ollulanus tricuspis (cats), and Physaloptera spp. Common enteric protozoans include Giardia spp., Cystoisospora spp., Cryptosporidium spp., and Tritrichomonas foetus (blagburni).

Occasionally, otherwise healthy dogs or cats with acute diarrhea and normal physical examination findings can be handled conservatively by withholding food for 24 hours followed by introduction of a bland food for several days. For all animals with diarrhea with no apparent cause on physical examination, I will perform a fecal flotation, fecal wet mount examination, complete blood cell count (CBC), and rectal cytology if diarrhea is present. While the CBC generally does not lead to a specific diagnosis, the presence of eosinophilia makes inflammatory bowel diseases and parasitism more likely.

I perform acid-fast staining of a fecal smear or immunofluorescence antibody staining (Merifluor Giardia/Cryptosporidium, Meridian Diagnostics) on all animals with diarrhea to assess for the presence of Cryptosporidium spp. oocysts. A wet mount examination may aid in identifying trophozoites of Tritrichomonas and Giardia. If neutrophils or spirochetes are evident on rectal cytology I recommend fecal culture (or PCR) for Salmonella spp. and Campylobacter spp. If spore-forming rods consistent with Clostridium perfringens are present in large numbers, fecal enterotoxin assays and PCR assays can be performed to help confirm the diagnosis. However, these tests can be positive in healthy animals as well and so have less than 100% predictive value.

There are multiple drugs used in the treatment of gastrointestinal parasitic infections. For all puppies and kittens, the strategic deworming recommendations for the control of hookworm and roundworm infections from the Centers for Disease Control and the American Association of Veterinary Parasitologists should be followed by veterinary practitioners (; (VIN editor: link modified on 1/31/19).

If owners are interested in more in-depth information, a good website is available ( (VIN editor: link modified on 1/31/19).

Giardia infections often respond clinically to the administration of metronidazole but infection is usually not eliminated. Administration of metronidazole benzoate at 25 mg/kg, q12h, PO, for 7 days was effective in suppressing cyst shedding to below detectable limits in 26 cats. This is the maximum dose of metronidazole that should be used; CNS toxicity can be induced by overdosing or as a cumulative neurotoxin. I personally use fenbendazole at 50 mg/kg, PO, daily for at least 5 days in dogs or cats with giardiasis. Metronidazole and fenbendazole can be combined in resistant cases. Febantel-containing products have been used successfully in dogs and cats and this drug is approved for the treatment of giardiasis in some countries. The empirical dog dose is the deworming dose, daily for at least 3 days. Paromomycin, ronidazole, and nitazoxanide are alternate drugs that could be tried in cases with resistant giardiasis. However, in my experience, dogs or cats with Giardia that fails to respond to metronidazole and fenbendazole have another underlying problem. Tinidazole at 30 mg/kg, PO, daily for 7–14 days may be effective in some dogs and cats for the treatment of giardiasis. Secnidazole at 30 mg/kg, PO, once was reported for treatment of cats with Giardia in Brazil. Additional information is needed before this protocol can be widely recommended.

Diet changes and probiotics can be tried for the management of parasitic diarrhea. Multiple drugs have been evaluated for the treatment of cats with T. foetus infections; until recently, no drug eliminated infection and diarrhea rarely resolves during the treatment period. Recently ronidazole at 30 mg/kg, PO, q24h, for 14 days eliminated clinical signs of disease and trophozoites from cats infected with one strain of the organism. In another small study, administration of metronidazole and enrofloxacin lessened diarrhea in kittens but it is unknown if the organism infecting those cats was T. foetus. Tinidazole may control the diarrhea but was less likely to eliminate the infection compared to ronidazole. Some puppies have recently been shown to be infected by T. foetus.

Sequential administration of clindamycin followed by tylosin blocked oocyst shedding and resolved diarrhea in one cat with chronic, clinical cryptosporidiosis. Tylosin (10–15 mg/kg, PO, twice daily) has been apparently successful in lessening diarrhea and oocyst shedding in multiple other cats and dogs with diarrhea that were Cryptosporidium positive. However, infection is not eliminated. Unfortunately, tylosin is very bitter and usually has to be given to cats in capsules. Treatment duration may need to be weeks. In cats with naturally occurring cryptosporidiosis, response to azithromycin has been variable (Lappin MR, unpublished data). If tried, use 10 mg/kg, PO, weekly for at least 10 days. If responding, continue treatment for at least 1 week past clinical resolution. Nitazoxanide is a new drug being studied for the treatment of Cryptosporidium and Giardia. Little information is available concerning dosages, but in one dog study seemed safe and effective at 75 mg/kg, PO, on days 0 and 14. The drug Alinia® is available and is labeled for both organisms in humans. The primary side effect to date has been vomiting and so it should be given with food if used.

Cystoisospora spp. generally respond to the administration of sulfadimethoxine, other sulfa-containing drugs, macrolides, or ponazuril (or toltrazuril). Ponazuril is superior to other drugs and should be administered at 50 mg/kg, daily for 3 days. If there are multiple puppies or kittens with diarrhea, treatment of all in contact animals should be considered.

Since many of the gastrointestinal parasites that infect dogs and cats are transmitted by carnivorism, they should not be allowed to hunt or be fed raw meats. Additionally, infection by many parasites results from ingestion of contaminated water. Clinical disease in some parasitized animals can be lessened by eliminating stress and providing a quality diet and clean environment.

Unless signs of bacteremia are present or signs are persistent, most bacterial enteritis cases are now treated by diet change and probiotics. If antibiotics are needed, Clostridium perfringens and bacterial overgrowth generally respond to treatment with tylosin, metronidazole, ampicillin, amoxicillin, or tetracyclines. The drug of choice for campylobacteriosis is erythromycin; however, oral administration of tylosin or quinolones is often less likely to potentiate vomiting. Salmonellosis should only be treated parenterally due to rapid resistance that occurs following oral administration of antibiotics. Tylosin can be administered at either 5 mg/kg or 15 mg/kg per dose; 25 mg/kg was shown to not be needed in one study. Boxer colitis is due to E. coli and should be treated with enrofloxacin at 5 mg/kg, PO, daily for 6–8 weeks. Appropriate antibiotics for the empirical treatment of salmonellosis while awaiting susceptibility testing results include ampicillin or trimethoprim-sulfa; quinolones are also effective. Some animals with infectious diarrhea will respond to the administration of a probiotic. We recently showed administration of Enterococcus faecium SF-68 (FortiFlora, Nestle Purina PetCare) lessened diarrhea in shelter cats and improved metronidazole responses in non-specific diarrhea in shelter dogs. The probiotic is not resistant to metronidazole and so both can be administered simultaneously.

Helicobacter spp. infections are usually treated with the combination of metronidazole, amoxicillin, and bismuth subsalicylate in dogs. Clarithromycin or azithromycin may be logical choices in cats since the species is often difficult to treat with multiple drugs. Whether to concurrently administer an antacid like famotidine is controversial but seems to lessen vomiting in some cats.


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Speaker Information
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M. Lappin
Center for Companion Animal Studies
Colorado State University
Fort Collins, CO, USA

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