Introduction and classification
Diarrhoea is a common clinical sign in animals presented to veterinarians in small animal practice. Similar to vomiting, the clinical consequences can range from insignificant to life threatening, although the latter is less common than the former. Many acute cases require little diagnostic intervention and resolve with or without symptomatic treatment. Chronic diarrhoea, however, can be a diagnostic challenge and the source of much frustration for the client and veterinarian. It is defined as diarrhoea that lasts for more than three weeks or intermittent diarrhoea over a period of one month or more. Animals can have chronic diarrhoea for months to years. Often, the animal may not be particularly unwell, and the diarrhoea may be chronic but intermittent and may respond partially but not entirely to different therapeutic interventions.
Diagnostic investigation of chronic diarrhoea can involve various procedures that range from the inexpensive to the expensive and the non‐invasive to the invasive. Unlike many clinical problems, therapeutic trials often play an important role in helping the clinician reach a probable diagnosis. However, trials need to be conducted logically and the outcomes reviewed critically.
The temptation to give multiple treatments aimed at different aetiologies in the hope that something will work is understandable. However, even if there is a positive response to multi‐modal therapy, if the diarrhoea recurs once treatment stops (as it often does), the clinician is no wiser about the underlying cause and how to manage the patient long term. Patience is needed by all parties, and excellent communication between the veterinarian and the client is imperative. The clinician needs to have a rational diagnostic and therapeutic approach to chronic diarrhoea in the dog and cat, and this will be dependent on the classification of the type of diarrhoea that is present and its potential causes.
Pathophysiology
Diarrhoea can be due to osmotic or secretory mechanisms, increased gut permeability or altered gut motility. Although many types of diarrhoea are due to more than one mechanism, an understanding of the mechanisms of diarrhoea facilitates a rational approach to its diagnosis and treatment.
In general, osmotic and secretory mechanisms occur with small bowel disorders, whereas increased permeability and altered motility can relate to both small and large bowel disorders. Altered motility occurs in most diarrhoeal diseases. However, primary motility disorders are very uncommon in dogs and cats.
Classification of diarrhoea
Although symptomatic therapy (or no therapy!) is appropriate for the majority of animals with acute diarrhoea, chronic diarrhoea does not usually respond to non‐specific symptomatic treatment and will often present the veterinarian with a diagnostic challenge where the more routine laboratory aids are not useful.
The diagnostic work‐up, differential diagnoses and therapy for small and large bowel diarrhoea may differ, although there are some causes common to both. Therefore, it is of utmost importance that before embarking on invasive diagnostic procedures or extensive therapy, an assessment is made as to whether the diarrhoea is:
Acute or chronic
Relatively mild or more severe, with the presence of secondary systemic effects
Small bowel or large bowel origin, or mixed
Due to primary or secondary gastrointestinal (GI) disease.
Failure to elicit sufficient information from the client about the characteristics of the diarrhoea, so as to allow appropriate classification as small bowel, large bowel or mixed, may result in inappropriate diagnostic procedures or therapeutic trials with increased expense to the client and frustration of the veterinarian and client.
Diarrhoea is defined as an alteration in the normal pattern of defaecation, resulting in the passage of soft, unformed stools with increased faecal water content and/or increased frequency of defecation. It is important to consider the animal’s previous pattern of defecation, as the frequency of defaecation and the nature of faeces vary between individuals.
There are a few uncommon situations where it may not be obvious to the owner that his/her animal has diarrhoea. Occasionally, the owner may mistake anal or vaginal discharges for diarrhoea, or see vomitus on the floor and think it is diarrhoea. The patient with constipation may pass small amounts of liquid faeces, which the owner thinks is diarrhoea. Conversely, the patient who is straining to defaecate and attempting to defaecate frequently because of large bowel disease may be interpreted by the owner as being constipated. Therefore, it is important that the clinician is cognisant of these issues and aims to define the problem as a first priority in the consultation.
In cases of diarrhoea, the problem‐based system we discussed in Chapter 2 is 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.
| Small bowel diarrhoea | Large bowel diarrhoea | |
|---|---|---|
| Consistency, volume and pattern | Faecal volume and/or water content is increased. Diarrhoea may be projectile and does not usually involve significant tenesmus or urgency. | Small amounts of faecal material are passed frequently. Tenesmus and urgency are often present, particularly if the lower colon or the rectum is involved. |
| Blood | If blood is present, it is usually digested (melaena) or, in acute diarrhoea, reddish‐brown. | If blood is present, it will be undigested (haematochezia). |
| Appearance |
Colour may be
|
