Chronic diarrhea is a common presenting complaint for small animal patients. The causes include infectious etiologies as well as noninfectious conditions involving dietary factors, idiopathic inflammatory enteropathies and neoplasia. Initial diagnostic efforts should differentiate between small intestinal and large intestine disease and rule out extra-intestinal diseases. Physical examination and clinical signs generally can distinguish small bowel from large bowel diarrhea. Baseline data from a complete blood count, serum chemistry panel and urinalysis are used to identify underlying extra-intestinal disease. Primary diseases in other organ systems that can result in diarrhea to be ruled out include liver failure, renal failure, and endocrinopathies such as hyperthyroidism in cats, hypoadrenocorticism and exocrine pancreatic insufficienty. Ruling out thyroid, adrenal or pancreatic disease would require thyroid panel, ACTH stimulation test and serum trypsin-like immunoreactivity (TLI) tests, respectively.
A fecal examination is most valuable for identification of parasitic agents. Microscopic examination of serial fecal samples concentrated by flotation can identify nematode eggs, oocysts of coccidian and cryptosporidia, as well as the trophozoites of Giardia sp. and Tritrichomonas sp. Giardia is more readily detected when zinc sulfate flotation is used. Both Giardia and Cryptosporidia can be detected by more sensitive methods such as ELISA, PCR and immunofluorescent antibody staining.
Fecal culture for bacterial agents is often part of the diagnostic approach to diarrhea. The bacteria most often associated with diarrhea in dogs include Clostridium perfringens, Cl difficile, Campylobacter sp., pathogenic E. coli and Salmonella sp. Isolation of these organisms from clinically normal animals is not uncommon and so, interpretation of laboratory findings is not without controversy. These organisms can be demonstrated by fecal culture as well as by more sensitive techniques. Cl. perfringens enterotoxin (CPE), a specific toxin associated with canine disease, can be demonstrated by ELISA tests and the toxin associated genes can be demonstrated by PCR testing. As with positive culture for Cl. perfringens, CPE can also be demonstrated in dogs without diarrhea. The pathogenicity of E. coli isolates requires molecular testing by PCR. Many questions remain concerning bacteria-associated diarrhea and the interpretation of laboratory demonstration of these organisms and their toxins.
Cytologic examination of stained rectal or colonic mucosal scrapings can be used to determine if leukocytes are present, indicating an inflammatory or infectious etiology, and to identify etiologic agents. This is especially useful for cases of large bowel diarrhea.
Determination of serum folate and cobalamin concentrations can be used to localize intestinal damage to either the proximal or distal small intestine. Low serum folate suggests a proximal lesion and is sometimes associated with dietary sensitivity. Low levels of cobalamin suggest a distal lesion and have been associated with small intestinal bacterial overgrowth (antibiotic responsive diarrhea), as well as exocrine pancreatic insufficiency. Increased folate levels also suggest bacterial overgrowth. Decreased levels of both folate and cobalamin suggest a diffuse lesion. In cats with intestinal disease, determination of serum cobalamin concentrations is particularly important in that response to therapy is optimized only when cobalamin status is corrected. Generally, folate and cobalamin tests are combined with TLI testing as a panel.
Analysis of feces for α1-proteinase inhibitor is rarely used in suspected cases of protein losing enteropathy. This serum protein can leak into the intestinal tract but is resistant to digestion and can be detected in feces. This is a specialized test which has limited availability and is technically difficult to perform.
The final and most invasive diagnostic technique for chronic diarrhea is intestine biopsy, obtained either by endoscopy, laparoscopy or laparotomy. Endoscopic biopsy is the least invasive technique and allows multiple biopsies, but its use limits sampling to the stomach, duodenum distal ileum (dogs only), and colon and only mucosal specimens can be obtained for histologic examination. Full thickness biopsies from any segment of the gastrointestinal tract can be obtained by laparoscopy and laparotomy. Full thickness biopsy specimens allow diagnosis of conditions where characteristic changes are most evident in the deep aspect of the lamina propria, submucosa, or even the tunica muscularis. This often includes gastrointestinal neoplasia. Biopsy may allow visualization of etiologic agents such as fungi and parasites or characteristic histologic changes that are diagnostic, but the technique has limitations. Some gastrointestinal diseases have similar histologic changes and cannot be differentiated on the basis of pathology alone. Final diagnosis may be dependent upon interpretation of a combination of clinical impressions and laboratory results as well as response to therapy.