Digestive System - Merck Veterinary Manual

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Digestive System Introduction

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 and Inherited Anomalies Involving the Digestive System

Congenital cysts, sinuses, or fistulas of the branchial arch apparatus, or thyroglossal duct, have been reported in horses, dogs, cats, and ruminants, yet they are very rare. These structures arise from persistent embryologic pharyngeal pouches, arches, or clefts, or the thyroglossal duct. Patients 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. Patients may present later in life; it is not known why a cyst may suddenly enlarge, but such enlargement could be associated with respiratory infection. Diagnostic imaging includes radiography, ultrasonography, video endoscopy, and contrast CT to determine whether there is communication with the pharynx.
Dental Development and Anatomy

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.
Dentistry

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

Pharyngeal paralysis is generally a disease of the central or peripheral nervous system. Symptoms of pharyngeal paralysis include dysphagia, nasal discharge of food, ptyalism, coughing, and upper respiratory tract noise. Diagnosis is based on clinical signs, endoscopic examination (rhinolaryngoscopy), and diagnostic imaging (eg, radiography and CT). Treatment is mainly symptomatic; however, treatment options can include medical and surgical management. Prognosis for affected animals is fair to guarded.
- Pharyngeal Paralysis in Animals
Diseases of the Rectum and Anus

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.
Enteric Campylobacteriosis

Campylobacter spp cause GI disease in humans and several animal species. Although Campylobacter jejuni and Campylobacter coli are the best-characterized species within the genus, other fastidious Campylobacter spp have been discovered that have a pathogenic role in humans and animals. Several Campylobacter infections are zoonotic, and infections in humans are frequently acquired through ingestion of contaminated dairy products, meat, or other animal products. Clinical signs include vomiting and bloody, mucoid diarrhea; treatment is supportive and, in some cases, requires antimicrobial treatment. Diagnosis of campylobacteriosis is via culture, which can be challenging; or by PCR assay in conjunction with clinical signs.
- Enteric Campylobacteriosis in Animals
Salmonellosis

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.
- Salmonellosis in Animals
Tyzzer Disease

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.
- Tyzzer Disease in Animals
Amebiasis

Amebiasis due to Entamoeba histolytica can result in profuse watery or bloody diarrhea. This protozoal parasite infects human and nonhuman primates and occasionally dogs and cats. Amebiasis can be diagnosed by microscopic examination of wet mounts of fresh feces to detect either mature trophozoites or cysts. Infections can be treated with several drugs, including metronidazole and paromomycin.
- Entamoeba histolytica Amebiasis
Coccidiosis

Coccidia are single-celled obligate intracellular protozoan parasites in the class Conoidasida within the phylum Apicomplexa. The main clinical sign of coccidiosis is diarrhea. Oocysts can be identified in feces by use of salt or sugar flotation methods, direct intestinal smears, or a McMaster counting chamber. Antiprotozoal treatment can shorten the length of illness, decrease discharge of oocysts, alleviate clinical signs, and reduce likelihood of secondary infections and death.
Cryptosporidiosis

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.
- Cryptosporidiosis in Animals
Giardiasis (Giardia)

Giardiasis is an intestinal infection with the protozoan flagellate parasite Giardia spp. Most infections of veterinary health importance are by G duodenalis, a species complex of different genotypes that tend to show host specificity; production animals, companion animals, and wildlife can all be infected. In general, the clinical signs of giardiasis are abdominal discomfort and diarrhea, often with steatorrhea. Diagnosis is usually by demonstration of cysts or antigens in fecal samples. Treatment regimens are variable and depend on specific indications. Hygiene measures are an essential component of control.
- Giardiasis in Animals
Diseases of the Mouth in Large Animals

Wounds of the lips and cheeks occur frequently in horses. The most common cause is external trauma, which can be 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 outcome. Healing is rapid because of the good blood supply to the head. Lacerations left to heal by second intention can result in an orocutaneous fistula, which may require wound revision with resection and primary wound closure. Rarely, skin grafts or mucosal flaps are required to manage orocutaneous fistulae.
Disorders of the Esophagus in Large Animals

Esophageal obstruction, commonly known as choke, occurs secondary to obstruction of the esophagus with food or foreign objects. Symptoms include nasal discharge of feed, coughing, bloat, and dysphagia. The diagnosis is confirmed with passage of an oro- or nasogastric tube or with endoscopy of the esophagus. Most cases can be resolved by means of sedated lavage of the esophagus, with water delivered by oro- or nasogastric tube to remove the impacted feed material. Rare cases require general anesthesia or surgery to resolve the obstruction. The prognosis for uncomplicated cases is good; however, complications such as esophageal stricture or aspiration pneumonia can worsen the prognosis.
Gastrointestinal Ulcers in Large Animals

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.
Diseases of the Ruminant Forestomach

Simple indigestion occurs most commonly in hand-fed cattle and is the result of cattle being fed an abnormal diet. Diagnosis is based on multiple animals having decreased appetite and forestomach motility, abnormal rumen pH, and an abnormal diet. Treatment focuses on feeding a typical ruminant diet.
Diseases of the Abomasum

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.
Acute Intestinal Obstructions in Large Animals

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.
- Acute Intestinal Obstructions in Large Animals
Colic in Horses

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 gastrointestinal 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 intestinal lumen.
- Overview of Colic in Horses
- Diseases Associated with Colic in Horses by Anatomic Location
Intestinal Diseases in Ruminants

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.
Infectious Diarrheal Diseases in Horses

Diarrhea in adult horses can be acute or chronic. Infectious agents that have been cited as potential causes of acute diarrhea in adult horses include numerous Salmonella serovars, Neorickettsia risticii, Clostridium difficile, C perfringens, Aeromonas spp, coronavirus, and cyathostomiasis.
Diarrheal Diseases in Foals

Diarrhea in foals can result from overfeeding (eg, when a foal is reunited with the mare after a period of separation) and improper nutrition (eg, orphan foals being fed calf milk replacer or sucrose). Lactose intolerance in foals is rare and can be determined by lactose tolerance challenge tests or clinical response to supplemental lactase. Diarrhea can also develop when foals consume indigestible substances such as roughage, sand, dirt, and rocks. Diarrhea in foals has been reported to be associated with infection by Strongyloides westeri, Parascaris equorum, and Cryptosporidium spp. (Also All.see page Gastrointestinal Parasites of Horses.)
Miscellaneous Intestinal Diseases in Horses

Consumption of large amounts of sand can lead to accumulation, abrasion and impaction in the large intestine; diarrhea, weight loss, or colic may result. Sand is ingested when horses or foals are kept or fed in a sandy area (paddock, stall, or pasture). Drought conditions can lead to multiple animals being affected.
Intestinal Diseases in Pigs

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.
Gastrointestinal Parasites of Ruminants
Gastrointestinal Parasites of Horses

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.
Gastrointestinal Parasites of Pigs

Also All.see page Coccidiosis of Pigs.
Fluke Infections in Ruminants

Fluke infections cause economic losses in production systems throughout the world. 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 US, it is endemic along the Gulf Coast, the West Coast, the northern Rocky Mountain region, and other areas of high rainfall with poorly drained or heavily irrigated pastures. 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 US states and in Europe. In North America, Dicrocoelium dendriticum is confined mainly to the northeastern US and the Atlantic provinces of Canada. It was recently introduced into Alberta, Canada. It is widespread in some areas in Europe and Asia. Eurytrema spp, the pancreatic flukes, parasitize sheep, pigs, buffalo, and cattle in Brazil and parts of Asia. Several species of paramphistomes, or rumen flukes, are found throughout much of the world.
Hepatic Disease in Large Animals

Diseases of the liver produce clinical signs of depression, anorexia, icterus, and sometimes photosensitization. Chronic liver disease is often accompanied by weight loss. Abdominal ultrasonography enables documentation of liver enlargement (more common in acute disease) or atrophy (more common in chronic disease), and it provides guidance for liver biopsy as a diagnostic and prognostic test. Depending on the specific cause, treatment often includes the administration of antimicrobials (if an infectious process is suspected), anti-inflammatories, and intravenous fluids with glucose, as well as sedation if neurologic signs are present from hepatic encephalopathy.
Malassimilation Syndromes in Large Animals

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.
- Malassimilation Syndromes in Large Animals
Abdominal Fat Necrosis

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.
- Abdominal Fat Necrosis in Animals
Diseases of the Mouth in Small Animals

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

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.
Diseases of the Stomach and Intestines in Small Animals

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 Exocrine Pancreas

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.
Gastrointestinal Parasites of Small Animals

Spirocerca lupi is a parasite of dogs, typically acquired by ingestion of dung beetles and associated with development of esophageal nodules that may become neoplastic. Vomiting, coughing, and regurgitation are most commonly observed. Diagnosis is based on imaging and fecal analysis. Macrocyclic lactones (preferably doramectin) should be administered for treatment, combined with imaging to evaluate efficacy.
Hepatic Disease
Liver Structure and Function

The liver is central to health, providing numerous synthetic, metabolic, bioactivating, conjugating, and elimination functions () that are essential to normal homeostasis. It orchestrates synthesis and metabolism of lipids, carbohydrates, and proteins; storage, metabolism, and activation of certain vitamins; and storage of minerals, glycogen, and triglycerides. It provides coagulant, anticoagulant, and essential acute phase proteins and can function as an on-demand site of extramedullary hematopoiesis.
Pathophysiology of Hepatic Disease in Small Animals

Because of the liver's large functional reserve and ability to regenerate, hepatic injury must be considerable or chronic and recurrent to cause overt hepatic dysfunction or failure.
Hepatic Diseases of Small Animals

Nutritional support has a pivotal influence in cats with hepatic lipidosis (HL) and is an important husbandry consideration for home management of animals with progressive hepatobiliary disease. Proper nutritional support demonstrably improves quality of life for animals with hepatic insufficiency prone to hepatic encephalopathy (HE).
Laboratory Analyses and Imaging in Hepatic Disease in Small Animals

Hematologic changes in patients with liver disease are generally nonspecific and overlap with many other systemic disorders. The exception is that icteric plasma may be a hallmark of liver disease in the absence of severe anemia, which would implicate a hemolytic process.

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