Certain criteria help to differentiate the source of diarrhea; they are listed in Table 35-1. Not every case will have every sign. It is also important to remember that mixed forms can occur. This is especially true if small-bowel diarrhea has been present for some time. The dysfunction/malabsorption in the small intestine leads to secondary changes in the large bowel.

Table 35-1 Small Bowel versus Large Bowel Diarrhea

Diarrhea has many causes. Many times it is due to primary gastrointestinal (GI) tract disease. At times, however, diarrhea can be due to non-GI tract disease such as uremia, liver disease, or hypoadrenocorticism.

Diarrhea can have various causes such as infections, dietary indiscretions, food allergy and non-GI disease (uremia). The mechanisms by which diarrhea develops can be grouped into secretory mechanisms, osmotic mechanisms, permeability changes, and motility disorders.

Secretory diarrhea usually develops with various infectious agents (Escherichia coli, Salmonella spp., etc.) and toxins. This form of diarrhea will not resolve with fasting. The predominant clinical problem with this form of diarrhea is significant fluid and electrolyte loss.

Osmotic diarrhea occurs when increased concentrations of hyperosmolar particles are present in the small and large intestine, overwhelming the ability of the GI tract to resorb water. Common causes include dietary indiscretion and maldigestion. This form of diarrhea is responsive to fasting. The predominant clinical problem is fluid loss.

Changes in permeability can result in more severe clinical signs. The increased permeability leads not only to loss of fluids and electrolytes but can also lead to the loss of proteins, blood, and mucus. Generally, changes in permeability are associated with GI tract inflammation.

Although in theory motility disorders can lead to diarrhea, this has been poorly documented in dogs. Hypomotility disorders, which are more common than hypermotility disorders, can be caused by many diseases or other conditions.

There is no easy answer to this question. A fecal float test is indicated in all cases of diarrhea. This may be the only test needed in uncomplicated cases of acute diarrhea when the dog is not visibly ill. In chronic cases or cases where symptoms are more severe, a complete blood count, serum chemistry profile, urinalysis, and fecal culture are advisable. Imaging studies may be indicated as well. Especially with chronic disease, other diagnostic laboratory studies such as determination of trypsin-like immunoreactivity (TLI), an adrenocorticotropic hormone (ACTH) stimulation test (to rule out hypoadrenocorticism), measurement of serum folate and cobalamin concentrations, and intestinal biopsies may be indicated.

GI tract biopsies are often necessary with chronic disease to establish a more definitive diagnosis. Either full-thickness biopsies or pinch biopsies can be taken. Full-thickness biopsies are usually obtained during exploratory surgery, although they can also be obtained laparoscopically. Pinch biopsies are obtained with endoscopy.

Surgical exploration has certain advantages and disadvantages. Biopsy specimens obtained by this method can be more helpful in diagnosis because they may include all layers of the GI tract. It is also possible to biopsy areas that cannot be reached by endoscopy. Another advantage is that other organs can be examined and biopsied at the same time. The predominant disadvantages of exploratory surgery are that it is more invasive and there is the risk of biopsy site dehiscence. This risk increases considerably when the colon is biopsied or the dog has hypoproteinemia.

Endoscopy often leads to a diagnosis in cases of GI tract disease. The procedure is less invasive than exploratory surgery and there is no risk of wound dehiscence. Endoscopy is preferable for biopsy of the large bowel and in dogs with significant hypoproteinemia. Endoscopy also has the advantage that it allows visualization of the lumen of the GI tract and any abnormal appearing areas. Endoscopy is, however, limited to certain areas of the GI tract (esophagus, stomach, duodenum, proximal jejunum, distal ileum, large intestine) and the biopsy specimens obtained are smaller. Additionally, endoscopy does require the use of special instruments and skill is required to perform a complete endoscopic exam.