Clinical Reasoning in Veterinary Practice

In cases of diarrhoea, the problem‐based system we discussed in Chapter 2is applied, but in a slightly different order. Identification of the location occurs first , which then assists in defining the system. This is because, almost always, large bowel diarrhoea reflects primary GI disease, whereas small bowel diarrhoea can occur with either primary or secondary GI disease. Thus, defining the location first aids in defining the system.

A thorough history is essential to differentiate small from large bowel disease. It is important to carefully question the owner as to the character of the faeces and to elicit information regarding consistency, colour, frequency and presence of blood or mucus. Related abnormalities should also be assessed, such as whether there has been significant weight loss, loss of appetite or vomiting. The characteristics of small and large bowel diarrhoea are detailed in Table 4.1.

Because large bowel diarrhoea has fewer and more specific characteristics than small bowel diarrhoea, it is often easiest to note if there is any fresh blood, mucus and small amounts of faeces passed frequently. If the diarrhoea has none of these characteristics, then the patient has small bowel diarrhoea. Note also that diarrhoea may have features of both small and large bowel, which indicates either primary small bowel with secondary effects on the lower bowel or diffuse disease involving both the small and large intestine.

Table 4.1 Characteristics of small and large bowel diarrhoea.

Diarrhoea can be due to primary disorders of the small bowel and/or large bowel or due to other systemic, secondary GI disorders such as hepatic disease, exocrine pancreatic insufficiency, pancreatitis, hyperthyroidism or hypoadrenocorticism. As discussed earlier, large bowel diarrhoea is almost always due to primary GI disease, whereas small bowel diarrhoea may occur with either primary or secondary GI disease.

Severe systemic diseases such as sepsis and uraemia can cause large bowel diarrhoea, but this will be a very minor clinical sign in relation to the patient’s other systemic clinical signs. Thus, secondary disorders are not usually likely differentials when considering the work‐up of a patient whose primary problem is large bowel diarrhoea.

Diarrhoea due to primary GI disease is more common than diarrhoea due to secondary GI disease. In animals with secondary GI disease, with the exception of exocrine pancreatic insufficiency and some dogs with hypoadrenocorticism, diarrhoea is not usually the primary presenting complaint.

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.

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