Digestive System

The digestive tract includes the oral cavity and associated organs (lips, teeth, tongue, and salivary glands), the esophagus, the forestomachs (reticulum, rumen, omasum) of ruminants and the true stomach in all species, the small intestine, the liver, the exocrine pancreas, the large intestine, and the rectum and anus. Gut-associated lymphoid tissue (tonsils, Peyer’s patches, diffuse lymphoid tissue) is distributed along the GI tract. The peritoneum covers the abdominal viscera and is involved in many GI diseases. Fundamental efforts to manage GI disorders should always be directed toward localizing disease to a particular segment and determining a cause. A rational therapeutic plan can then be formulated.

Congenital cysts, sinuses, or fistulae of the branchial arch apparatus or thyroglossal duct have been reported in horses, dogs, cats, and ruminants, yet are very rare. These structures arise from persistent embryologic pharyngeal pouches, arches, or clefts, or the thyroglossal duct. Animals typically present with nonpainful, fluid-filled masses in the cervical region. Clinical signs are typically due to the space-occupying mass and include dyspnea, respiratory stridor, intermittent esophageal obstruction, and coughing. Animals may present later in life; it is not known why a cyst may suddenly enlarge but may be associated with respiratory infection. Diagnostic imaging includes radiography, ultrasonography, video endoscopy, and contrast CT to determine whether there is communication with the pharynx. Branchial cysts (also called lateral cervical cysts) in horses are typically seen on the right side, although a bilateral case has been reported. Surgical excision is curative, although complications include right laryngeal hemiplegia, seroma formation, and pneumonia. Alternatively, some horses have been treated with marsupialization and iodine sclerotherapy with good results. In dogs and cats, few complications have been noted. Embryologic origin of branchial cysts has been mostly of the third pharyngeal pouch, although anatomic location has also suggested origin of the fourth and sixth pouches in some cases.

Odontogenesis, or tooth development, starts in the early embryonic developmental stages and continues for some time after birth. Teeth form in a corono-apical direction and are derived from ectoderm from the first arch, along with ectomesenchyme from neural crest cells. Size, shape, and location are genetically and independently determined for each tooth. By day 23 of gestation, the paired mandibular and maxillary, as well as the medial nasal process, have become distinct. The dental lamina first appears at 25 days of gestation in the dog embryo. By day 30, the laminae of the left and right arches fuse to make a continuous arch. The components of the tooth germ that later develop are the dental papilla, enamel (or dental) organ, and dental follicle. The enamel organ, oral epithelium, and dental lamina originate from the outer embryonic germ layer known as ectoderm. The dental papilla and sac appear in coordination with the enamel; however, they originate from the mesoderm.

Most large animals are herbivores, and efficient dental function is the key to food intake and to the maintenance of normal body condition. The variations in anatomic structure, dental formula, and eruption schedule for deciduous and permanent teeth is fundamental veterinary knowledge and should be reviewed before performing dentistry on any species (All.see table Dental Formulas of Various Animal Species).

Pharyngeal paralysis may be the result of a central or peripheral nervous system disorder or may develop secondary to severe local disease that may cause collapse, obstruction, or malfunction of the pharynx. Of the CNS disorders, rabies (All.see page Rabies) is the most important of the viral causes of encephalomyelitis, although perhaps not the most frequent. CNS intoxication, lead poisoning, cranial trauma, intracranial abscessation, and neoplasia may also result in pharyngeal paralysis in many species.

There are several types of anal sac diseases, including impaction, inflammation, and abscessation. Clinical signs are related to discomfort associated with sitting or defecating. Diagnosis is typically made by physical examination, but may require microscopy, ultrasonography, or biopsy. Treatment may include manual expression, antibiotics or anti-inflammatory medications, or surgical drainage or removal.

Campylobacter spp are spiral, microaerobic, gram-negative bacteria that cause gastroenteritis in people and animals. Several Campylobacter spp are zoonotic. Many domestic animals develop acute gastroenteritis after ingestion of Campylobacter spp, including dogs, cats, calves, sheep, pigs, ferrets, mink, monkeys, and several species of laboratory animals. (All.See also page Bovine Genital Campylobacteriosis, All.see page Zoonotic Diseases, and All.see page Avian Campylobacter Infection.) Infection with C jejuni is one of the most common causes of gastroenteritis in people worldwide and is the most extensively studied Campylobacter species.

Salmonellosis is infection with Salmonella spp bacteria. It affects most animal species as well as humans and is a major public health concern. The clinical presentation can range from a healthy chronic carrier state to patients with acute or chronic enteritis to septicemia. Diagnosis is made through isolation of the pathogen, which can be challenging because bacterial shedding in chronic carriers occurs intermittently and because a one-time isolation of the pathogen from a healthy individual does not prove infection. Vigorous antibiotic and supportive treatment is required in patients with signs of systemic disease. The use of antimicrobials in healthy carriers but also in patients with localized enteritis is controversial because this approach is thought to increase the risk for the patient of becoming a chronic shedder.

Tyzzer disease is caused by Clostridium piliforme. The disease affects a variety of animals, including mammals and birds. It is characterized by a triad of lesions that include colitis, hepatitis, and myocarditis, although the three are usually not present together in a single animal. Treatment is supportive and nonspecific, with a low success rate.

Amebiasis is an acute or chronic colitis, characterized by persistent diarrhea or dysentery, that is prevalent in tropical and subtropical areas worldwide. Its prevalence has declined in the USA over the past several decades, but the disease is still important in many tropical areas, particularly in times of disasters. It is common in people and nonhuman primates, sometimes seen in dogs and cats, and rare in other mammals. Several species of amebae are found in mammals, but the only known pathogen is Entamoeba histolytica. People are the natural host for this species and the usual source of infection for domestic animals. Mammals become infected by ingesting food or water contaminated with feces containing infective cysts. E dispar is a noninvasive, nonpathogenic ameba that is molecularly distinct but morphologically indistinguishable from the pathogenic species E histolytica. E invadens of reptiles is also morphologically identical to E histolytica, but it is not transmissible to mammals.

Coccidiosis is usually an acute invasion and destruction of intestinal mucosa by protozoa of the genera Eimeria or Isospora. Clinical signs include diarrhea, fever, inappetence, weight loss, emaciation, and in extreme cases, death. However, many infections are subclinical. Coccidiosis is an economically important disease of cattle, sheep, goats, pigs, poultry (All.see page Coccidiosis), and also rabbits, in which the liver as well as the intestine can be affected (All.see page Coccidiosis). In dogs, cats, and horses, coccidiosis is less often diagnosed but can result in clinical illness. Other genera, of both hosts and protozoa, can be involved (All.see page Cryptosporidiosis, All.page numonly Sarcocystosis, and All.page numonly Toxoplasmosis).

Cryptosporidiosis is a highly prevalent gastrointestinal parasitic disease caused by protozoan species of the genus Cryptosporidium that infect a wide range of animals, including people, throughout the world. Cryptosporidiosis is of considerable importance in neonatal ruminants, in which it is characterized by mild to severe diarrhea, lethargy, and poor growth rates. In infected individuals, Cryptosporidium oocysts can be detected in Ziehl-Neelsen–stained fecal smears. Treatment is supportive, with antidiarrheal remedies and replacement of fluids and electrolytes. Strict hygiene is cardinal to preventing infections.

Giardiasis is a chronic, intestinal protozoal infection seen worldwide in most domestic and wild mammals, many birds, and people. Infection is common in dogs, cats, ruminants, and pigs. Giardia spp have been reported in 0.44%–39% of fecal samples from pet and shelter dogs and cats, 1%–53% in small ruminants, 9%–73% in cattle, 1%–38% in pigs, and 0.5%–20% in horses, with higher rates of infection in younger animals. Farm prevalences in production animals vary between 0% and 100%, with the highest prevalence in younger animals. The cumulative incidence on a farm where Giardia has been diagnosed is 100% in cattle and goats and nearly 100% in sheep.

Wounds of the lips and cheeks occur frequently in horses. The most common cause is external trauma or secondary to the use of inappropriate bits or restraint devices. Lip lacerations may be accompanied by mandibular or incisive bone fractures with or without dental fractures and tooth avulsions. These occur when a horse grasps objects with its mouth and then pulls back when startled. Lip lacerations without bone or teeth involvement can be sutured, usually with a good result. Healing is rapid because of the good blood supply to the head. Lacerations left to heal by second intention can result in orocutaneous fistula, which may require resection and primary wound closure. Rarely, skin grafts or mucosal flaps are required to manage orocutaneous fistula.

Esophageal obstruction (choke) occurs when the esophagus is obstructed by food or foreign objects. It is the most common esophageal disease in large animals. Horses most commonly obstruct on grain, beet pulp, or hay. Esophageal obstruction can also occur after recovery from standing chemical restraint or general anesthesia. Cattle tend to obstruct on a single solid object, eg, apples, beets, potatoes, turnips, corn stalks, or ears of corn.

Gastric ulcers are important in adult horses, foals, and pigs. Abomasal ulcers (All.see page Abomasal Ulcers in Cattle) in mature cattle and calves appear to be increasing in importance.

Simple indigestion is a minor disturbance in ruminant GI function that occurs most commonly in cattle and rarely in sheep and goats. Simple indigestion is a diagnosis of exclusion and is typically related to an abrupt change in the quality or quantity of the diet.

Abomasal disorders include left displaced abomasum, right displaced abomasum, abomasal volvulus, abomasal ulceration, and impaction. Displacement or volvulus occurs most commonly in dairy cows but can also occur in dairy bulls and calves. Except for abomasal volvulus, abomasal displacement is rare in beef cattle and essentially undiagnosed in small ruminants. Less-common disorders include abomasitis caused by ingested sand or gravel (abomasitis geosedimentosa), clostridial abomasitis, incarceration of the abomasum in umbilical hernias, and abomasal neoplasia. Abomasal ulcers are seen in dairy and beef cattle, in calves, and in South America camelids; they are rarely diagnosed in small ruminants. Impactions can be primary, which is most frequent in beef cattle, or secondary, which develop most often in dairy cows as a form of vagal indigestion. Impactions may have a hereditary basis in Suffolk sheep.

Intestinal obstructions are functional or mechanical in nature and result in a decrease or absence of feces. Diagnosis is based on abnormal findings during examination via palpation per rectum, transabdominal ultrasonography, clinicopathologic testing including peritoneal fluid analysis, and exploratory laparotomy. Treatment depends on the cause of the obstruction.

In its strictest definition, the term “colic” means abdominal pain. Throughout the years, it has become a broad term for a variety of conditions that cause a horse to exhibit clinical signs of abdominal pain. Consequently, it is used to refer to conditions of widely varying etiologies and severity. To understand these etiologies, make a diagnosis, and initiate appropriate treatments, veterinarians must first appreciate the clinically relevant aspects of equine GI anatomy, the physiologic processes involved in movement of ingesta and fluid along the GI tract, and the extreme sensitivity of the horse to the deleterious effects of the structural components of the bacteria that reside within the lumen of the intestine.

Determination of the cause of intestinal disease in cattle is based on clinical, epidemiologic, and laboratory findings. Nonspecific therapy includes oral and parenteral fluid therapy to restore the fluid, electrolyte, and acid-base homeostasis. Specific therapy and prevention are detailed under the individual disease headings. Intestinal diseases of neonates are discussed separately, although some of the causes also affect older animals.

Intestinal disease in horses and foals is suggested by diarrhea, weight loss, hypoproteinemia, and abdominal pain. (Also All.see page Overview of Colic in Horses.)

Pigs of all ages are susceptible to intestinal diseases, and diarrhea is the clinical sign common to nearly all such disorders. Infectious agents that cause enteropathies are typically transmitted via the fecal-oral route. More than 20 etiologic agents, including bacteria, viruses, and parasites, can cause primary intestinal disease in pigs. Some other viruses, including sapovirus, adenocirus, and enterovirus, also have been isolated from the intestines of pigs but are not associated with economically consequential disease.

Many species of nematodes and cestodes cause parasitic gastritis and enteritis in sheep and goats. The most important of these are Haemonchus contortus, Teladorsagia (Ostertagia) circumcincta, Trichostrongylus axei, intestinal species of Trichostrongylus, Nematodirus spp, Bunostomum trigonocephalum, and Oesophagostomum columbianum. Cooperia curticei, Strongyloides papillosus, Trichuris ovis, and Chabertia ovina also may be pathogenic in sheep; these and related species are discussed under GI parasites of cattle (All.see page Gastrointestinal Parasites of Cattle).

Horse bots are the parasitic larvae of the botflies, Gasterophilus spp. Adult females deposit their eggs onto hair shafts of horses. Bot larvae are eventually ingested through grooming and can cause inflammatory reactions during migration within the oral cavity and by attachment to the stomach wall. In general, bot larvae are considered benign, even though some pathology is observed. Bot larvae can be identified during oral inspection or gastroscopy. Treatment of choice is ivermectin (0.2 mg/kg), which has documented efficacy against both oral and gastric stages.

Also All.see page Coccidiosis of Pigs.

Fasciola hepatica, the most important trematode of domestic ruminants, is the most common cause of liver fluke disease in temperate areas of the world. In the USA, it is endemic along the Gulf Coast, the West Coast, the Rocky Mountain region, and other areas. It is present in eastern Canada, British Columbia, and South America and is of particular economic importance in the British Isles, western and eastern Europe, Australia, and New Zealand. Fasciola gigantica is economically important in Africa and Asia and is also found in Hawaii. Fascioloides magna has been reported in at least 21 states (USA) and in Europe. In North America, Dicrocoelium dendriticum is confined mainly to New York, New Jersey, Massachusetts, and the Atlantic provinces of Canada. It is also widespread in some areas in Europe and Asia. Eurytrema spp, the pancreatic flukes, parasitize sheep, pigs, and cattle in Brazil and parts of Asia. Several species of paramphistomes or rumen flukes are found throughout much of the world.

Hepatic disease is common in large animals. Increases in serum hepatic enzymes and total bile acid concentration may indicate hepatic dysfunction, insult, disease, or failure. Although liver disease is especially common in horses and foals, progression to liver failure is not.

Malassimilation is the decreased ability of the GI tract to incorporate nutrients into the body, either due to maldigestion or malabsorption. If animals lose weight despite normal appetite and feed intake, malassimilation can be suspected. Malassimilation is usually confirmed by absorption tests. Additional tests such as intestinal biopsies might be used to determine etiology.

Hard masses of necrotic fat are occasionally identified in the peritoneal cavity of mature cattle, especially the Channel Island breeds, Japanese Black cattle, and beef cattle grazing fescue for long periods. Abdominal fat necrosis has also been seen in goats and some species of deer maintained on pastures consisting primarily of tall fescue. These masses are commonly mistaken for a developing fetus on palpation per rectum because they feel like “floating corks,” similar to cotyledons. Clinical signs are uncommon; however, in cases in which many masses are present an extraluminal intestinal obstruction can result in episodes of moderate abdominal pain, distention of the proximal intestine, and decreased fecal production.

For a discussion of developmental diseases of the mouth, All.see page Congenital and Inherited Anomalies of the Mouth. For eosinophilic granuloma complex, All.see page Eosinophilic Inflammatory Skin Diseases.

Cricopharyngeal achalasia is characterized by inadequate relaxation of the cricopharyngeal muscle, which leads to a relative inability to swallow food or liquids. It is seen primarily as a congenital defect but is occasionally seen in adult dogs. The cause is generally unknown, but in adult animals it may be associated with acquired neuromuscular disorders. Repeated attempts to swallow are followed by gagging and regurgitation. Aspiration pneumonia is a common complication. An accurate diagnosis requires fluoroscopic evaluation of swallowing after oral administration of contrast material alone and mixed with food. Abnormal function (lack of relaxation) of the cricopharyngeal muscle results in retention of barium in the posterior pharynx.

Canine parvovirus is a highly contagious virus that commonly causes GI disease in young, unvaccinated dogs. Presenting signs include anorexia, lethargy, vomiting, and diarrhea, which is often hemorrhagic. Diagnosis is typically based on history, physical examination findings, and fecal antigen testing. Treatment is largely supportive on an inpatient or outpatient basis because specific therapies are not available.

The pancreas has both endocrine and exocrine functions. The exocrine pancreas is made up of pancreatic acinar cells and a duct system that opens into the proximal duodenum. Pancreatic acinar cells synthesize and secrete digestive enzymes (eg, amylase, lipase, and others) and inactive proenzymes of digestive enzymes (ie, zymogens; eg, trypsinogen, chymotrypsinogen, proelastase, prophospholipase, and others), which are essential for the digestion of dietary components such as proteins, triglycerides, and complex carbohydrates. The exocrine pancreas also secretes other essential substances, such as large amounts of bicarbonate, which buffers gastric acid, intrinsic factor, which is needed for cobalamin absorption, and colipase, which is an essential cofactor for pancreatic lipase.

Adult Spirocerca lupi are bright red worms, 40 mm (male) to 70 mm (female) long, generally located within nodules in the esophageal, gastric, or aortic walls. Infections are seen in southern areas of the USA as well as in many tropical and subtropical regions worldwide (eg, Greece, India, Israel, Japan, South Africa). Dogs are infected by eating an intermediate host (usually dung beetle) or a transport host (eg, chickens, reptiles, or rodents). The larvae migrate via the wall of the celiac artery to the thoracic aorta, where they usually remain for ~3 mo. Eggs are passed in feces ~5–6 mo after infection.

The liver performs numerous functions, including but not limited to lipid, carbohydrate, and protein metabolism; storage, metabolism, and activation of vitamins; storage of minerals, glycogen, and triglycerides; extramedullary hematopoiesis; and synthesis of coagulant, anticoagulant, and several acute phase proteins. It also influences immunologic responses and contributes to digestion through synthesis and enterohepatic circulation of bile acids and detoxification of many endogenous and exogenous compounds, toxins, and xenobiotics. Because the liver has a large functional reserve and the ability to regenerate, hepatic injury must be considerable or chronic and recurrent to cause overt hepatic dysfunction or failure.