Chapter 2 Diseases of the Digestive System
Food is vital for the life of the animal because it provides the source of energy that drives all the chemical reactions in the body. Consumed food is not in a form readily usable by the body. The digestive system breaks down the consumed food to a point where it can be absorbed and used by the animal.
The organs of digestion can be divided into two main groups: the gastrointestinal tract, a continuous tube beginning at the mouth and ending at the anus, and the accessory structures—the teeth, tongue, salivary glands, liver, pancreas, and gallbladder (Fig. 2-1).
(From Christenson DE: Veterinary medical terminology, Philadelphia, 1997, WB Saunders, by permission.)
A discussion of diseases that affect the gastrointestinal system can best be approached by dividing the system into regions: oral cavity and esophagus, stomach, small bowel, large bowel, liver, pancreas, and rectum and anus.
Diseases of the oral cavity most frequently seen in small animals include gingivitis/periodontal disease, lip-fold dermatitis, trauma, salivary mucocele, and oral neoplasms. Clinical signs for these diseases are similar; the animals are reluctant to eat and have oral pain, halitosis, and excessive salivation.
Periodontal disease results from infectious inflammation of the gingiva, and it affects all the structures involved with tooth attachment (Fig. 2-2, see also Color Plate 1). This condition is a continuum of disease beginning with gingivitis and progressing to periodontitis and tooth loss. Gingivitis, a reversible process that involves inflammation of the margins of the gums is caused by accumulation of tartar on the teeth, which acts as a nidus for bacterial multiplication. Enzymes produced by these bacteria damage the tooth attachments and result in inflammation. Without intervention, gingivitis will progress to periodontitis, an irreversible condition resulting in loss of gingival epithelial root attachment and alveolar bone reabsorption. Periodontal disease is estimated to occur in 60% to 80% of dogs and cats.
Periodontal disease is a collective term for plaque-induced inflammation of the gums. This inflammation is progressive and includes gingivitis, gingival hyperplasia, periodontitis with vertical bone destruction, and periodontitis with horizontal bone destruction. The final outcome of periodontal disease is loss of the tooth.
Gingivitis (inflammation of the gingiva) is the earliest sign of periodontal disease. It results from the buildup of dental plaque (tartar) in the gingival sulcus. Bacteria seldom invade the gingival tissues directly; however, anaerobic bacteria that comprise much of the subgingival plaque secrete enzymes that result in inflammation to the surrounding gum. The inflammatory response of the host animal results in gingival hyperplasia. (Gingival hyperplasia may be breed related or drug induced as well.) As plaque is mineralized, it becomes dental calculus, which protects the bacterial environment.
Gingivitis is limited to the soft tissue of the gingiva, with sulci depths remaining within normal limits for both dogs and cats. As the disease progresses to periodontitis, pathologic periodontal pockets are formed. The coronal portion of the periodontal ligament is destroyed by inflammation and alveolar bone reabsorption begins. If treated early, gingivitis is reversible; however, once periodontal disease progresses, the changes are irreversible. Prevention and treatment of periodontal disease is of utmost importance in the health of companion animals.
Lip-fold dermatitis is commonly seen in breeds with pendulous upper lips and prominent lower lip folds. Breeds such as spaniels, setters, bulldogs, and bassets are most commonly affected. Constant moisture of these folds from saliva results in increased bacterial growth. Together with the collection of food and hair in the area, the saliva causes increased irritation, erythema, and a fetid odor.
Oral trauma is common in small animals. Falls, fights, burns, blunt trauma, penetration of foreign objects, and automobile accidents account for many injuries to the oral cavity of pets. Head injuries from “high-rise syndrome” (e.g., cats falling from windows of buildings) or from other types of accidents often result in fracture of the mandibular symphysis, maxillary dysfunction, and/or separation of the hard palate of cats. The tongue is frequently injured by self-trauma (biting its own tongue), dog fights, penetration of foreign bodies (e.g., splinters, needles, bullets), or strangulation by elastic or stringlike materials. Cats that play with needles and thread may injure the tongue or frenulum or have a linear foreign body lodged somewhere in the oral cavity. Tongue lacerations have occurred as a result of dogs and cats attempting to eat from discarded tin cans to which the lids are still attached.
Electrical and chemical burns are often seen in young, curious animals that cannot resist biting an electric cord or tasting unusual plants or liquids. Electrical burns not only involve the mucosal surface of the oral cavity, but progress deep into the tissue along vessels and wet tissue planes. Contact with caustic chemicals and plants can result in erosion of the oral mucosa, producing pain, inflammation, secondary infection, and necrosis.
Gunshot wounds often result in dental or other oral injuries, as well as shattered bones, teeth, and penetrating wounds of the tongue. Fishhooks of all types attract both dogs and cats. Hooks can become embedded in the lips or the tongue (sometimes both at the same time), resulting in a frantic animal that may require sedation or general anesthesia to properly remove the hook. Round steak bones present a special challenge. These bones typically become lodged behind the canine teeth, over the end of the mandible. As the tissue swells, it becomes painful. General anesthesia is required in most cases. Bones must be cut in sections for removal. Cats also have problems with bones; flat chicken bones can become lodged across the upper dental arcade, requiring sedation for removal.
The salivary mucocele is the most common clinically recognized disease of the salivary glands in dogs, although it may also occur in cats secondary to trauma. A mucocele is an accumulation of excessive amounts of saliva in the subcutaneous tissue and the consequent tissue reaction that occurs. This disease occurs most often in dogs between the ages of 2 and 4 years; German shepherds and miniature poodles are most commonly affected. The initial cause of the accumulation usually is unknown. Owners report a history of a slowly enlarging, fluid-filled, nonpainful swelling on the neck. The animal may have respiratory distress or difficulty swallowing secondary to the partial obstruction of the pharynx. In cats, a ranula (a large fluid-filled swelling under the tongue) may be seen.
Oral neoplasms are relatively common in the dog and the cat, with malignant melanomas and squamous cell carcinomas being the most common. In general, older animals are more commonly affected, and male animals are at an increased risk for malignant melanoma and fibrosarcoma. Dogs with heavily pigmented oral mucosa are also at greater risk for malignant melanoma than dogs having pink oral mucosa.
Benign neoplasms such as papillomas and epulides are seen in the dog (see Color Plate 8). Papillomas, pale-colored cauliflower-like growths, are viral in cause and may be removed surgically or may regress spontaneously. Epulides occur in the gingiva near the incisor teeth. They are generally slow growing but some may be locally invasive and involve bone destruction.
Malignant melanomas are rapidly growing tumors characterized by early bone involvement. They metastasize early to the lungs and regional lymph nodes. These lesions are dome shaped or sessile and may be black, brown, mottled, or unpigmented. Squamous cell carcinomas are ulcerative, erosive neoplasms. They invade the mucosa and often the bone and metastasize to regional lymph nodes.
Esophagitis is an inflammation of the esophageal wall and is most often associated with contact of irritants with the mucosa of the esophagus. Acids, alkalis, drugs, and hot materials can produce lesions of varying severity in the esophagus. The extent of the lesion will depend on several factors: (1) the type of material, (2) the length of contact with the mucosal surface, and (3) the integrity of the esophageal mucosal barrier. Physical trauma by foreign bodies or chemical damage from chronic vomiting may predispose the esophagus to further damage.
The esophagus has a great ability to withstand injury. The mucosa-gel barrier, tight cell junctions, and a bicarbonate-rich layer all serve to protect the mucosal lining from damage. One of the most common causes of esophagitis is GER. In healthy animals, the lower esophageal sphincter prevents reflux of gastric contents back into the esophagus. Gastric acid, pepsin, and trypsin found in gastric fluid will damage the mucosal lining if allowed to remain in contact with it for prolonged periods. Once inflammatory damage to the esophagus has occurred, lower esophageal sphincter function becomes abnormal, perpetuating the problem.
Ingestion of nondigestible foreign objects is more common in the dog than in the cat, with young animals being more likely to be affected. Bones and small play toys commonly lodge at the thoracic inlet, cardiac base, or distal esophagus. The degree of damage to the esophagus depends on the size of the object, the shape, and the time spent in contact with the esophageal mucosal lining.
Prompt removal is important to prevent serious damage. Endoscopy will allow the clinician direct visualization of the object, and most foreign bodies can be removed by endoscopic retrieval. Those that cannot be removed orally can often be pushed into the stomach and removed surgically. Surgical removal of foreign objects directly from the esophagus has a less favorable prognosis because of the poor healing qualities of the esophagus and the potential for stricture formation.
The stomach is located in the left cranial abdomen and stores ingesta, mixing it with gastric juices and then propelling it into the duodenum at a controlled rate. Anatomically, the stomach wall is composed of three layers of tissue: the mucosal (epithelial) lining, the muscular (smooth muscle) layer, and the serosa. The mucosal lining contains glands that secrete mucus, parietal glands that secrete hydrochloric acid, chief cells that secrete pepsinogen, and argentaffin cells that secrete gastrin. Gastric mucosal cells also secrete bicarbonate.
Gastric motility depends on two motor control centers in the stomach plus external control from the autonomic nervous system. Emptying of solids depends on caloric density and pyloric resistance. The presence of fats and proteins will delay gastric emptying.
The normal bacterial flora of the stomach consists of spirilla (Helicobacter sp. and Gastrospirillum hominus). In addition, cats may have a nonpathogenic Chlamydia living in the mucosal lining of their stomachs.
Disruption of the gastric mucosal barrier (the mucosal lining, mucous coating, and bicarbonate layer) or motility disorders can result in damage to the stomach. The most commonly seen disorders include gastritis, both acute and chronic; ulceration; foreign body obstruction; gastric dilatation/volvulus; hypermotility; and neoplasia.
One common cause of vomiting in dogs and cats is acute gastritis. Causes of acute gastritis include diet (spoiled food, change in diet, food allergy, or food intolerance), infection (bacterial, viral, or parasitic), and toxins (chemicals, plants, drugs, or organ failure). Ingestion of foreign objects may also result in gastritis. Whatever the cause, once the mucosa is damaged, inflammation occurs and clinical symptoms develop.
Immune-mediated inflammatory bowel disease (IBD) is the result of the accumulation of inflammatory cells within the lining of the small intestine, stomach, or the large bowel and is seen most commonly in cats, although it occurs in dogs as well. The range of symptoms reported with this disease is related to the location and type of infiltrate. The cause of the inflammatory infiltrates within the lining of the gastrointestinal tract is unknown but is most likely related to chronic antigenic stimulation of some type. Diagnosis is made from intestinal and gastric biopsies. Treatment begins with daily administration of corticosteroids together with metronidazole. If the patient has a good response, as measured by decreased clinical symptoms, then the amount administered may be slowly decreased over 1 to 2 weeks. More severe cases may require azathioprine or cyclophosphamide. Hypoallergenic diets should be prescribed for these animals.
Gastric ulceration and erosion is commonly the result of drug therapy in the dog and cat. Nonsteroidal antiinflammatory drugs (NSAIDs) are the most commonly implicated drugs, producing ulceration in humans, as well as in dogs. These drugs, which include aspirin, ibuprofen, flunixin meglumine, and phenylbutazone, disrupt the normal gastric mucosal barrier, resulting in ulceration. Stress, as seen in severely traumatized animals or animals in strenuous training, can also result in gastric erosion.
Gastric dilation/volvulus (GDV) is primarily a disease of 2- to 10-year-old large and giant breed, deep-chested dogs, but it can occasionally occur in small breeds. The exact mechanism for the disease remains unclear, but diet and exercise have been implicated in its development. Delayed gastric emptying, pyloric obstruction, aerophagia, and engorgement may predispose dogs to dilation and volvulus. Recent studies indicate that affected dogs may have gastric dysrhythmias that predispose them to GDV.
The stomach is similar to a bag with openings at each end. As the stomach fills with air, food, and/or fluid, the outflow tracts can become occluded. Further distension results in simple dilation (an air-filled stomach), or the air-filled stomach may twist along its longitudinal axis (volvulus). The pylorus usually passes under the stomach and comes to rest above the cardia on the left side of the abdomen. The enlarged tympanic stomach pushes against the diaphragm, making breathing difficult and blocking venous return of blood through the hepatic portal vein and the posterior vena cava. The increased luminal pressure within the gastric wall results in ischemia and subsequent necrosis of the wall.
The spleen may also be involved and can become congested. Endotoxins that accumulate in the gastrointestinal tract activate the inflammatory mediators. The end result is the development of hypovolemic, endotoxic shock in patients with GDV.
Figure 2-3 Lateral radiograph of a dog with gastric dilation/volvulus. Stomach is dilated (large arrows) and there is a “shelf” of tissue (small arrows), demonstrating that the stomach is malpositioned. Radiographs obtained from the right lateral position appear superior to those of other views in demonstrating this shelf. If the stomach were similarly distended but not malpositioned, the diagnosis would be gastric dilation.
(From Nelson RW, Couto CG: Small animal internal medicine, ed 3, St Louis, 2003, Mosby, by permission.)
The most common malignant canine gastric neoplasm is the adenocarcinoma. This type of tumor is most commonly found in older animals, and because the clinical signs are relatively nonspecific (vomiting and weight loss), the tumor may be well advanced before it is diagnosed.
Disease of the small intestine involves impairment of the absorptive villous surface of the small intestine, which causes diarrhea, malabsorption, and weight loss. Types of intestinal damage may include villous atrophy, disruption of the microvilli, and disruption or defects in villous proteins. Diarrhea, defined as an increase in frequency, fluidity, and volume of defecation, may result any time the flux of fluid or nutrients across the absorptive membrane is altered. It may be classified in several ways: acute or chronic; osmotic (resulting from decreased digestion or absorption that increases the osmotic solute load in the bowel), secretory (caused by hypersecretion of ions), exudative (resulting from an increased permeability with loss of plasma proteins), or dysmotility (resulting from abnormal motility).
Acute diarrhea is one of the most commonly seen types of diarrhea in the small-animal clinic. Frequently, the cause is a change of diet, drug therapy, or any stressful situation that may result in disruption of the normal bacterial flora within the bowel. Acute diarrhea is easily managed with supportive and symptomatic treatment.