Chapter 128 Gastroenteritis
INTRODUCTION
Gastroenteritis is a broad term used to indicate inflammation of both the stomach and the intestinal tract. It is a common cause for acute-onset vomiting, anorexia, and diarrhea in both dogs and cats, but should be differentiated from other problems that may cause similar clinical signs such as pancreatitis, hepatitis, and intestinal obstruction (see additional chapters under the textbook section Intraabdominal Disorders).1 Inflammation in the alimentary tract can be due to a wide variety of underlying causes, including dietary indiscretion, infectious organisms, toxins, immune dysregulation, and metabolic disorders, and may occur in both dogs and cats. A thorough history and physical examination may aid in uncovering an underlying cause, but often a specific cause is not identified. In most cases, supportive therapy, including appropriate fluid support, dietary modification, antiemetics, and gastric protectant agents, are sufficient for resolution of clinical signs. However, in severe cases acute decompensation can occur. This is usually secondary to volume depletion, fluid losses, and acid-base disturbances that occur because the intestinal tract cannot perform its normal hemostatic functions.
ANATOMY AND PHYSIOLOGY
The stomach is the compartment between the esophagus and small intestine that functions as both a storage reservoir for food and a vessel for mixing and grinding food into smaller components that then enter the small intestine.2 The stomach, is made up of muscular layers, glandular portions, and a mucosal barrier. The muscular layers serve to grind food into smaller particles and move it forward into the small intestine through the pyloric sphincter. Of equal importance are the glandular portions of the stomach, which include parietal cells (for secretion of hydrochloric acid), chief cells (for secretion of pepsinogen), and mucous-producing cells (which also secrete bicarbonate). The gastric mucosal barrier is able to keep hydrochloric acid and digestive enzymes within the lumen, and it prevents loss of plasma constituents into the stomach.2 Once the food particles are ground into small enough components, they pass through the pyloric sphincter into the beginning of the small intestine, known as the duodenum.
The small intestine of cats and dogs functions in both digestion and absorption of food and its nutrients, and is divided arbitrarily into the duodenum, jejunum, and ileum.3 The mucosa of the small intestine is involved in both secretory and absorptive functions and contains a single layer of epithelial cells called enterocytes. The mucosa along the length of the small intestine is formed into villi, which are fingerlike projections into the intestinal lumen that enlarge the surface of the small intestine. Microvilli then form the “brush border” to increase the surface area even more for digestion and absorption of nutrients. Enzymes found in this brush border aid in digestion of larger food molecules into smaller, more readily absorbable particles. Absorption may occur via specific transport mechanisms or by pinocytosis. The epithelial cells are also involved with absorption and secretion of electrolytes and water.3 Enterocytes are connected to each other by tight junctions, limiting absorption between cells, as well as preventing backflow of nutrients from the interstitium into the intestine. The life span of these enterocytes is likely somewhere between 2 and 5 days, and they start at the crypt (base of the villus) and migrate toward the intestinal lumen where they are shed. A healthy, intact mucosal lining is important for the integrity of the intestine. Any type of inflammation that disrupts this layer can lead to significant intestinal disease.4 It is also important to remember that the gastrointestinal (GI) tract absorbs about 99% of the fluid presented to it; therefore any damage to it can cause significant alterations in acid-base and fluid balances.5
HISTORY AND CLINICAL SIGNS
A thorough history is extremely important in identifying an underlying cause for gastroenteritis. Questions may be related to the patient’s current diet, recent change in diet, and exposure to unusual food, foreign materials, garbage, or toxins. It is also important to find out about the patient’s environment, including exposure to other animals, and if other exposed animals have similar signs or a history of similar signs. Vaccination status, deworming history, and medication use are also important.
Clinical signs of gastroenteritis are often similar regardless of the underlying cause. Vomiting, diarrhea, and anorexia are most common, and certain combinations of these signs may make one cause more or less likely than another. Severe inflammation or ulceration, depending on the cause, can lead to hematemesis or melena.
Physical examination is often unrewarding in terms of helping to find an underlying cause. Patients may have varying degrees of dehydration, as well as abdominal pain. In severe cases, such as those animals with hemorrhagic gastroenteritis (HGE) or parvoviral enteritis, patients may have signs of hypovolemia and shock due to the severe fluid losses and acid-base disturbances.
ETIOLOGIES
Infectious Gastroenteritis
A variety of infectious agents can affect the GI tract. Viruses, bacteria, parasites, and fungi have all been shown to cause gastroenteritis of varying severity. The descriptions in the text are limited to the most common. Please see Box 128-1 for a more complete list of potential infectious causes of gastroenteritis.
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