Small intestinal disease can be acute or chronic. Acute small intestinal disease is most commonly dietary due to ingestion of food that leads to adverse reactions, infectious due to Parvovirus enteritis or other enteric pathogens, or mechanical due to foreign bodies, intussusceptions, or torsions. Chronic small intestinal disease can mainly be infectious, inflammatory, mechanical, or neoplastic.

Dogs with acute small intestinal disease usually present with diarrhea and/or vomiting, and depending on the severity of the condition a variety of systemic clinical signs. The most important aspect of the successful management of acute small intestinal disease is supportive care as most acute small intestinal disorders are self-limiting. However, it is important to quickly rule-out serious small intestinal disorders that require swift definitive therapy.

Dogs with chronic small intestinal disorders usually present for chronic diarrhea and may or may not display other clinical signs such as vomiting, weight loss, flatulence, or borborygmus. The most common cause for chronic small intestinal disease in dogs and cats is idiopathic inflammatory bowel disease (IBD). Dogs also commonly have intestinal dysbiosis (also known as small intestinal bacterial overgrowth (SIBO), antibiotic-responsive diarrhea (ARD), or tylosin-responsive diarrhea (TRD)). Other common small intestinal disorders are intestinal neoplasia, infectious causes such as chronic infections with a pathogenic organism, fungal infections, enteric protozoal or helminthic parasites, or partial obstructions.

IBD in human patients defines a syndrome of chronic diarrhea due to ileitis or ulcerative colitis. Both ileitis and ulcerative colitis are idiopathic disorders but are defined by specific criteria. In dogs there is no universally accepted definition for IBD. The author defines IBD as any inflammatory condition of the intestine no matter what the predominant cell type or the underlying etiology. In some cases of IBD the cause may be obvious. For example, intestinal parasites may cause eosinophilic inflammation and some bacterial infections may cause neutrophilic inflammation. However, the majority of cases of inflammatory bowel disease remain idiopathic. Some investigators include only cases of idiopathic inflammation of the intestine to define the term IBD, while others distinguish between idiopathic IBD and IBD due to a specific cause.

Clinical presentation and diagnosis

Chronic diarrhea is the most common clinical sign observed in dogs with IBD. Weight loss is also commonly observed. Vomiting is observed when the stomach is involved but can also occur in cases that are limited to the intestinal tract.

A diagnosis of IBD is usually made based on histopathologic evaluation of intestinal biopsy specimens most commonly collected during gastroduodenoscopy. Histopathology is accepted as a gold standard for diagnosing IBD. However, histopathology is not without limitations. There is a large degree of variability of the number of inflammatory cells seen in biopsy specimens of normal dogs and objective criteria for a diagnosis of IBD have not been agreed upon. Still, histopathology can be useful in the clinical evaluation of dogs and cats with suspected IBD. However, it is of utmost importance to critically assess the quality of the biopsy specimens. Also, the histopathologic diagnosis needs to be carefully evaluated in light of all the other patient information available. If the histopathologic diagnosis of the biopsy does not fit the patient a second opinion should be sought.

There are an increasing number of non-invasive tools to assess function and pathology of the gastrointestinal tract. Serum folate can be decreased in proximal and diffuse small intestinal disease. Serum cobalamin can be decreased in distal or diffuse small intestinal disease, exocrine pancreatic insufficiency, or small intestinal bacterial overgrowth. Fecal α₁-proteinase inhibitor concentration can be used to assess gastrointestinal protein loss. Serum C-reactive protein can be used as a marker for intestinal inflammation. Also, evaluation of serum and tissue levels of cytokines may be useful in the definition of specific subsets of IBD in dogs and cats.