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Veterinarian Technician June 2009 (Vol 30, No 6)

Nutrition Know-How — Dealing with Diarrhea Woes

by Ann Wortinger, BIS, LVT, VTS (ECC, SAIM, Nutrition)

    The causes of diarrhea range from self-limiting to life-threatening, making diagnosis and treatment largely a process of elimination.

    Diarrhea is any change in the frequency, consistency, or volume of bowel movements or stools.1 It is one of the most common clinical signs of gastrointestinal (GI) disease but also can reflect disorders outside the digestive system that affect the small or large bowel and lead to diarrhea. There are four major types of diarrhea with different mechanisms of action.

    Large volumes of water are normally secreted into the small intestinal lumen, but a majority of this water is efficiently absorbed before reaching the large intestine. Diarrhea occurs when secretion of water into the intestinal lumen exceeds absorption.

    Diarrhea can present as either a small bowel disorder involving the duodenum, jejunum, and ileum or a large bowel disorder involving the colon. Presenting signs are the easiest way to distinguish the two causes. Small bowel disorders typically present with weight loss, poor body condition, vomiting, borborygmus, and flatulence, whereas large bowel disorders are generally characterized by mucus, blood, tenesmus, severe cramping, discomfort, and straining to defecate.1

    Diarrhea can be caused by anything that disrupts the normal function of the GI tract, usually affecting one of the four mechanisms of action. These can be divided into several categories, depending on whether the diarrhea is acute or chronic (see Causes of Canine and Feline Diarrhea).1

    Osmotic Diarrhea

    Osmotic diarrhea is associated with the retention of solutes within the intestines because of failure associated with absorption or digestion.1,2 This type of diarrhea can occur with pancreatic exocrine insufficiency (PEI) and diseases of the small intestines, such as small intestinal bacterial overgrowth (SIBO). Diseases that cause severe inflammation can alter the permeability of the GI tract and can raise the hydrostatic pressure, not only increasing fluid loss but also diverting proteins from the bloodstream into the intestines (protein-losing enteropathy [PLE]).2

    Osmotic diarrhea typically results from one of two situations:

    • Ingestion of a poorly absorbed substrate — The offending molecule is usually a carbohydrate or divalent ion. Common examples include mannitol or sorbitol.
    • Malabsorption — Inability to absorb certain carbohydrates is the most common deficit in this category of diarrhea, but it can result in virtually any type of mal­ab­sorp­tion. A common example of malabsorption afflicting many adult humans and pets is lactose intolerance.

    Inflammatory/Infectious/Infiltrative Diarrhea

    The GI tract is composed of epithelial cells, lymphatic vessels, blood vessels, and the local immune system. When any of these fail to perform properly, diarrhea can occur. Failure can be the result of diseases that cause erosion, ulceration, inflammation, or infiltration of the intestinal mucosa.

    The epithelium of the digestive tube is protected from insult by a number of mechanisms constituting the GI barrier, but like many barriers, it can be breached. Disruption of the epithelium of the intestine from microbial or viral pathogens, inflammatory bowel disease (IBD), and diffuse neoplasia is a common cause of diarrhea in all species.1 Destruction of the epithelium results not only in exudation of serum and blood into the lumen but often is associated with widespread destruction of absorptive epithelium. In such cases, absorption of water occurs inefficiently and diarrhea results.

    Examples of pathogens and conditions frequently associated with infectious diarrhea include:

    • Bacteria — Salmonella spp, Escherichia coli, Campylobacter spp
    • Viruses — Rotavirus, coronavirus, parvovirus (canine and feline), norovirus
    • Protozoa — Coccidia spp, Cryptosporidium spp, Giardia spp
    • IBD — causes unknown
    • Diffuse neoplasia — lymphoma and other diffuse neoplasms

    The immune response to inflammatory conditions in the bowel contributes substantially to the development of diarrhea. Activation of white blood cells leads to the secretion of inflammatory mediators and cytokines that can further stimulate secretion, in effect imposing a secretory component on top of inflammatory diarrhea. Diseases that cause severe inflammation can alter the permeability of the GI tract and raise the hydrostatic pressure, not only increasing fluid loss but also diverting proteins from the bloodstream into the intestines, thereby leading to PLE.2

    Abnormal GI Motility

    Abnormal GI motility can occur in the form of ileus, causing a "pipe effect," with little resistance to ingesta passing through the GI tract, or increased peristaltic contractions pushing the ingesta through the GI tract before complete absorption of water.1 The pipe effect may be secondary to infiltrative disease, severe abdominal pain, parvoviral enteritis, or surgery. Increased peristaltic contractions generally accompany IBD. This change in GI motility also can predispose the patient to SIBO.1

    For nutrients and water to be efficiently absorbed, the intestinal contents must be adequately exposed to the mucosal epithelium and retained long enough to allow absorption. Disorders in motility that accelerate transit time could decrease absorption, resulting in diarrhea, even if the absorptive process was proceeding properly.

    Alterations in intestinal motility can be observed in many types of diarrhea. What is not usually clear, and, therefore, difficult to demonstrate, is whether primary alterations in motility are actually the cause of diarrhea or simply an effect.1

    Secretory Diarrhea

    Secretory diarrhea is relatively uncommon in dogs and cats compared with humans and food animals — millions of people and animals have died from secretory diarrhea associated with E. coli, Salmonella spp, and other bacteria.

    Epithelial crypt cells lining the GI tract produce intestinal fluid, whereas enterocytes lining the villous tips are responsible for absorbing nutrients and water. This results from cAMP enzyme in the epithelial cells, a derangement that causes uncontrolled water secretion.

    The causes of secretory diarrhea include factors that affect intestinal fluid production, such as endogenous hormones, bacterial enterotoxins, certain medications, unconjugated bile acids, and hydroxyl fatty acids. In humans, this type of diarrhea is typically seen with bacterial and viral infections (e.g., cholera, dysentery, salmonellosis).1,3

    Gathering Information for a Diagnosis

    There is no breed, age, or gender predisposition associated with diarrhea.2 The most important step in the diagnosis of both acute and chronic diarrhea is a complete physical examination and history, paying close attention to the number and frequency of defecations. Body condition score and weight also should be evaluated. Large, watery stools are more typical of small bowel diarrhea, whereas bloody or tarry stools can indicate a potentially life-threatening condition, such as GI bleeding or ulcer.1

    Because many cases of diarrhea in small animals are diet-induced, obtaining a dietary history is paramount. There may be recent dietary changes, such as to a moist, high-fat or meat-based diet, as well as the feeding of table scraps or spicy foods and access to garbage or dead animals. Cats that hunt can be exposed to Salmonella when they consume birds.

    Common physical findings include weight loss, dehydration, and the presence or absence of nonspecific signs, including fever, lethargy, inappetence, and change in behavior.1

    The choice of laboratory tests depends on the presenting signs. It is important to determine whether the condition is self-limiting, as with dietary aberrations, or life-threatening, as with parvoviral infection. Initial diagnostic testing may include assessment of dehydration by evaluating packed-cell volume and total solids (PCV/TS) and fecal examination for parasites and bacteria. Advanced testing may involve additional blood tests for PLE, endoscopy, or surgery to obtain intestinal biopsy samples. Other considerations include foreign body obstruction, infection, and dietary imbalances.

    Nursing Plan Considerations

    If there is a history of dietary indiscretion and the patient is still eating and not clinically dehydrated, the diarrhea usually resolves with time. Switching to a highly digestible, low-residue food for 5 to 7 days may help the inflamed GI tract absorb nutrients while healing occurs. The animal can resume its regular diet after the diarrhea has resolved.

    In many cases, however, conservative management is necessary, with restriction of food intake for 8 to 24 hours, followed by introduction of a bland, highly digestible diet fed frequently in small amounts. Failure to implement a conservative strategy is a common reason for poor response and relapse.1

    Sometimes a secondary bacterial or fungal infection can develop, which may require medical therapy in addition to dietary manipulation. Food poisoning secondary to bacterial infection can be life-threatening3 and requires treatment of the bacterial infection and potential endotoxins, as well as a bland, highly digestible diet.

    If a foreign body is involved, the introduction of a highly digestible diet is essential for tissue healing and for restoring a positive nitrogen balance after its removal.2

    In some cases, dietary manipulation may be both diagnostic and therapeutic. For example, if the animal has food sensitivity or a true food allergy, switching to a number of different diets may be necessary. Dermatologic signs may be present, but GI signs can appear alone or with skin changes.2 Intestinal changes are reversible, and normal GI function can return in approximately 6 to 8 weeks. Dietary changes may involve switching to a novel protein or modified protein diet, such as hydrolyzed protein.2

    Some cats are carbohydrate intolerant and develop secondary diarrhea from high-carbohydrate diets. Switching them to a low-carbohydrate, high-protein diet often resolves the diarrhea.1

    Intestinal cancers are generally nonresponsive to dietary manipulation, but a high-quality diet can aid in the ability of the patient to heal, respond to chemotherapy, and maintain adequate nutrition.1 High levels of fish oils may help in the treatment of some forms of lymphoma.


    Dietary fiber predominately affects the large bowel but also can influence gastric, small bowel, and pancreatic structure and function. Fiber modifies gastric-emptying time to ensure normal GI motility and intestinal transport time, buffers gastric acidity and toxins, binds and holds excess water within the GI tract, supports the growth of normal intestinal bacterial flora, alters the viscosity of intestinal contents, and adds nondigestible bulk to the diet.1,4

    Carbohydrates and fibers can be divided into three classes, depending on their digestibility in dogs and cats: highly fermentable fibers, moderately fermentable fibers, and nonfermentable or slowly fermentable fibers.

    Highly fermentable fibers, such as guar gum and pectin, can aggravate existing diarrhea by increasing the osmolality in the GI tract, thereby drawing additional fluid into the intestines. Highly fermentable fibers can be found in some canned and saucy foods.1,4

    Moderately fermentable fibers, although not readily digested, can be broken down by intestinal bacterial flora into short-chain fatty acids (SCFAs), which are a significant energy source for the enterocytes lining the GI tract. One class of moderately fermentable fibers is the fructooligosaccharides. Research has shown that these fibers provide energy for the enterocytes and exert a prebiotic effect on bacterial flora that selects for the presence of certain bacteria over others to maintain an optimal bacterial population.4 Examples include pea fiber, beet pulp, and oat fiber.

    Nonfermentable or slowly fermentable fibers, which are typically added to foods to dilute calories and provide bulk, include cellulose, peanut hulls, and soybean hulls. Although these fibers are not particularly water soluble, they do have significant water-holding capacity and can be useful in treating some cases of fiber-responsive diarrhea.1,4

    Many diets contain a combination of fiber sources to provide the benefits of each without the problems associated with the use of a single fiber source. Other diets may contain minimal fiber sources to increase digestibility and decrease stool volume.


    Probiotics are a class of supplements that can be used in animals with diarrhea. Probiotics, which are typically live bacteria or yeast, can be beneficial to the animal, but a primary problem is keeping the probiotic organisms alive when they pass through the gastric acids. Most, but definitely not all, bacteria are killed by the high acidity found in the stomach.5

    Probiotics may be recommended after a course of antibiotics to allow the gut microflora to reestablish healthy numbers of beneficial bacteria rather than allowing the "bad" bacteria to flourish. Problems that stress the immune system also can affect bacterial numbers in the intestines and may benefit from a course of probiotics.5

    Certain probiotics have been evaluated for use in animals and have been documented to survive the stomach's high acidity and will grow in the host animal.5 A number of veterinary products meet these criteria.


    Protein quality determines the bioavailability of various amino acids. A poor-quality protein has fewer available amino acids. Egg, which is the highest-quality protein, has a biologic value of 100; all other proteins are compared with this value (Table 1).

    Protein quality is determined by the amino acid that is found in the lowest quantity, that is, the limiting amino acid. Every species has a different essential amino acid profile. As obligate carnivores, cats have a larger amino acid profile and a higher protein requirement than dogs have.1,6 The cat's amino acid and fatty acid requirements mandate a meat-based source of protein in its diet; a vegetarian diet cannot meet a cat's nutritional needs.1,6

    Protein-energy malnutrition also can occur when the diet is inappropriate for the animal's age or condition, as with cats that eat dog food and pregnant or nursing animals that eat low-protein diets.1

    Fatty Acids

    Fats are classified by their state at room temperature and the location of the first set of double bonds within the molecule. Lipids are solid at room temperature, and fats are liquid at room temperature.7

    The main fatty acid families are n-3, n-6, and n-9, in which the double bonds are located at the "n" designation within the molecule. The n-3 and n-6 fatty acids are essential fatty acids (EFAs) because they cannot be synthesized from other fats. Fatty acids cannot be converted from one family to another.1,7

    Increasing the dietary amount of n-3 fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), has been found to help decrease the inflammatory response. Because the n-6 EFAs are proinflammatory, increasing the amount of n-3 EFAs gives the n-6 EFAs less space to exert their effects.1,7


    The term "highly digestible" does not have a regulatory definition but is generally used to describe a diet that has a protein digestibility of 87% or higher and a fat and carbohydrate digestibility of at least 90%.1,4 Digestibility of diets for patients with GI disease should be higher because normal digestion and absorption of nutrients are often compromised. In addition, because GI disease may cause tissue damage and immune dysfunction, the use of high-quality nutrients is critical.1,4 Veterinary therapeutic diets formulated for patients with GI disease usually contain meat and carbohydrate sources that have been highly refined to increase digestibility.1,4 Digestibility can be determined by consulting the product reference guide or contacting the manufacturer.

    1. Davenport DJ, Remillard RL, Simpson KW, Pidgeon GL. Gastrointestinal and exocrine pancreatic disease. In: Hand MS, Thatcher CD, Remillard RL, Roudebush P (eds). Small Animal Clinical Nutrition, ed 4. Marceline, MO: Walsworth; 2000:750-782.

    2. Simpson JW, Anderson RS, Markwell PJ. Dietary management of clinical diseases. In: Clinical Nutrition of the Dog and Cat. Cambridge, Mass.: Blackwell; 1993:59-62.

    3. LeJeune JT, Hancock DD. Public health concerns associated with feeding raw meat diets to dogs. JAVMA 2001:219(9):1222-1225.

    4. Case LP, Carey DP, Hirakawa DA, Daristotle L. Carbohydrates in Canine and Feline Nutrition, ed 2. St. Louis: Mosby; 2000:16-18.

    5. Walker WA. Mechanisms of action of probiotics. Proceedings of the 2008 ACVIM Meeting; San Antonio, Texas.

    6. Case LP, Carey DP, Hirakawa DA, Daristotle L. Protein and Amino Acids in Canine and Feline Nutrition, ed 2. St. Louis: Mosby; 2000:23-28.

    7. Case LP, Carey DP, Hirakawa DA, Daristotle L. Fats in Canine and Feline Nutrition, ed 2. St. Louis: Mosby; 2000:19-22.

    References »

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