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The following tables summarise the causes of acute and chronic small and large bowel diarrhoea (Tables 4.2–4.4).

Table 4.2 Causes of acute small bowel diarrhoea in dogs and cats.

Cause	Examples	Comments
Diet related	Overeating (especially puppies)Dietary changeSpoiled foodDietary indiscretion	Including change to food that causes allergy/hypersensitivity.
Parasites/protozoa	ParasitesMost commonly ascarids (Toxocara and Toxascaris spp., also hookworms Ancylostoma and Uncinaria spp.) Protozoa Giardia spp.Coccidia, for example Cystisospora spp. (formerly called Isospora)Cryptosporidium spp.	Zinc sulphate flotation is a sensitive test for Giardia cysts, provided three faecal samples collected over 5 days are examined (∼95% sensitive).The ELISA test can identify Giardia antigen in faeces. The test is reported to be about 90% sensitive and is probably more sensitive than performing zinc sulphate flotation examination on a single faecal sample.Faecal IFA and PCR are also available.A negative result does not necessarily exclude Giardia infection, and some clinicians will treat with metronidazole or fenbendazole regardless and proceed with further investigations only if the diarrhoea persists.
Infection (bacterial/viral)	Viral enteritisParvovirus/panleukopeniaCoronavirusDistemperFeline immunodeficiency virus (FIV)/Feline leukaemia virus (FeLV)Other viruses (e.g. adenovirus, norovirus, torovirus)Bacterial enteritisCampylobacter spp.Salmonella spp.E. coliClostridium spp.	Microbial culture of faeces is often unrewarding due to the abundant normal flora in the gut and the predominance of anaerobes.E. coli is frequently, and Salmonella is sometimes, isolated from faeces of normal animals, and therefore their presence does not necessarily imply that they are the cause of the diarrhoea. Campylobacter spp. and Clostridium spp. are also found in animals with and without diarrhoea, which makes interpretation of results quite difficult.Clostridium perfringens and Clostridium difficile have equal prevalence in dogs with and without diarrhoea. However, there is a correlation between the presence of diarrhoea and the detection of toxins that are produced by these bacteria, although enterotoxin can also be found in healthy dogs.Identification of an overgrowth of Clostridia or Clostridial spores on faecal smears means nothing diagnostically and will occur in many situations when gut flora is disturbed by a variety of GI disorders.
Toxins	ToxinsLeadOrganophosphatesPlants *Ingestion of many toxins can cause acute diarrhoea sometimes in combination with other systemic signs	Plants that may cause diarrhoea if ingested (as well as other clinical signs) include:Lily of the valleyDaffodil bulbsAloe veraAsparagus fernChrysanthemumsCyclamens.
Unknown	Acute haemorrhagic diarrhoea syndrome (AHDS)	Previously called haemorrhagic gastroenteritis (HGE). This is a syndrome causing acute onset of vomiting and bloody diarrhoea.Characteristically the patient will have significant haemoconcentration (increased packed cell volume [PCV]) with a normal or low plasma protein due to protein loss into the bowel. It is typically described in small breed dogs but any size/breed can be affected.It is now believed that the cause may be novel necrotising toxin produced by Type A Clostridia perfringens.However, antimicrobial therapy does not improve outcome unless the patient is septic.
Secondary GI disease	Acute pancreatitisSevere systemic disease	See previous comments regarding presence of other clinical signs.

Table 4.3 Causes of chronic small bowel diarrhoea in dogs and cats.

Cause	Examples	Comments
Diet related	Diet‐responsive disease or food‐responsive enteropathy (FRE)	Food intolerance is a non‐immunological reaction to a component of food, for example, gluten, preservative, bony material and other irritants.Food allergy/hypersensitivity is an immunological reaction to a component of food, for example, beef, chicken and dairy.Diagnosis of dietary allergy or intolerance is usually a process of trial and error by using elimination diets, as there is no sensitive or specific diagnostic test.Current evidence would suggest that blood tests for anti‐food antibodies are not specific and not clinically useful for dogs and cats with FRE.
Parasites/protozoa	Intestinal parasites (as previously mentioned)Protozoa (as previously mentioned)	See comment in Table 4.2.
‘Infection’/bacterial/viral	Campylobacter/SalmonellaFeline infectious peritonitis	Note comments in Table 4.2 in relation to bacterial causes of diarrhoea.
Antibiotic responsive	Antibiotic‐responsive diarrhoea (ARD)Secondary small intestinal bacterial overgrowth (SIBO)	ARD: small intestinal diarrhoea that is responsive to antibacterials but no underlying cause can be identified.SIBO: secondary to an underlying problem such as exocrine pancreatic insufficiency, inflammatory bowel disease, partial obstruction or motility disorders.
Infiltrative	Immunosuppressive‐responsive or non‐responsive chronic enteropathy (IRE and NRE) – also termed inflammatory bowel disease (IBD), for example,Lymphocytic‐plasmacytic enteritisEosinophilic enteritisDiffuse or focal lymphomaAdenocarcinoma/adenomaMast cell tumour (feline)Smooth muscle/stromal cell tumours (canine)	Diarrhoea is the most common clinical signs of IBD in dogs.Vomiting is the more common clinical sign in cats.Intestinal biopsy is primarily required to characterise infiltrative inflammatory gut disease (IBD vs. neoplasia) and/or protein‐losing enteropathy.
Miscellaneous	LymphangiectasiaInherited selective cobalamin deficiencyChronic intussusceptionChronic partial obstruction	Lymphangiectasia is usually secondary to IBD but primary forms exist.
Secondary GI (not an exhaustive list)	Hypoadrenocorticism –dogsHyperthyroidism – catsExocrine pancreatic insufficiencyChronic pancreatitisLiver disease (hepatocellular failure, cholestasis, portal hypertension)Severe systemic disease

Table 4.4 Causes of acute and chronic large bowel diarrhoea in dogs and cats.

Parasites/protozoa	Giardia spp. (more commonly small bowel but can also cause large bowel diarrhoeaTritrichomonas foetus (cats)Entamoeba spp.Trichuris vulpis (whipworm)Ancylostoma caninum (hookworm)
Infection (bacterial/viral)	Campylobacter spp. (interpret positive results with caution)Clostridium perfringens; Clostridium difficile*Salmonella spp.*Yersinia enterocolitica*Feline infectious peritonitis (FIP)Granulomatous colitis (boxers, French bulldogs) – caused by invasive and adherent E. coli *These bacteria are rarely identified as causal agents in canine chronic diarrhoea; they may be more commonly identified in faecal samples from raw‐fed pets, but raw food is not currently proven to cause diarrhoea more often than commercial pet food.
Diet related	Food‐responsive enteropathy (FRE)Passing foreign materialFibre‐deficient diet
Inflammatory	Immunosuppressive‐responsive or non‐responsive enteropathy = inflammatory bowel disease (IBD), for example,Lymphocytic‐plasmacytic enteritis (colitis)Eosinophilic enteritis (colitis)
Neoplasia	Diffuse or focal lymphomaAdenocarcinoma/adenomaMast cell tumour (feline)Smooth muscle/stromal cell tumours (canine)
Miscellaneous	Stress‐induced colitis can occur relatively commonly in hospitalised or newly kenneled dogs, which may be due to an overgrowth of Clostridium perfringensStructural disease such as caecocolic intussusception (rare)