The digestive system includes all of the organs that are involved in taking in and processing food. It begins with the mouth and includes the esophagus, stomach, liver, pancreas, intestines, rectum, and anus.
People, dogs, and cats have simple stomachs that are good at breaking down meat, fruits, and vegetables. Most animals that eat grass (including cows and sheep) have a more complex system consisting of several stomachs, including a large fermentation vat called a rumen. Grasses (such as hay) in the rumen are digested by billions of bacteria that break down roughage into volatile fatty acids. These fatty acids are absorbed for energy further down in the digestive tract. The equine digestive system combines features of both the simple stomach and the multiple-stomach digestive systems.
The beginning of the digestive tract (called the foregut) includes a simple stomach and the small intestine. The stomach is relatively small considering the size of the horse and can hold only about 2 to 2.5 gallons (8 to 10 liters) of food and water. Food passes quickly through this small stomach, which allows horses to graze continuously throughout the day. The small intestine consists of the duodenum, jejunum, and ileum. The jejunum is approximately 65 feet (19.5 meters) long. Enzymes break down food within the foregut.
The hindgut, which is located after the foregut, is what makes it possible for horses to eat grasses for energy. It includes the cecum, large colon (right ventral colon, left ventral colon, left dorsal colon, and right dorsal colon), and small colon (transverse colon and descending colon). It is in the hindgut that bacterial digestion breaks down fibrous plant material into volatile fatty acids that can be used for energy. The cecum (or caecum) is a large fermentation vat that can hold 7 to 8 gallons (27 to 30 liters) of food and water. The equine digestive tract bends and narrows in multiple locations, which increases the risk of impactions and blockages by dense, fibrous plant material.
The main functions of the digestive system include grasping and chewing food, creating saliva, drinking water, swallowing food and water, digesting food, absorbing nutrients, maintaining the proper balance of fluids and electrolytes (salts) in the body, and eliminating waste products. Dysfunction of the digestive tract can be divided into 4 main categories: digestion, absorption of nutrients, motility (movement through the digestive tract), and elimination of feces.
When treating a digestive system problem, the veterinarian’s goal is to first identify the part of the system where the problem lies and then to determine the specific cause and appropriate treatment.
Signs of digestive system disease can include:
Diarrhea is often a sign of digestive system disorders, but it can have many causes. Large-volume, watery diarrhea usually is associated with hypersecretion, a condition in which excess fluid is secreted into the intestines. This can be caused by bacterial infection.
Diarrhea can also be caused by malabsorption, the failure to properly absorb nutrients. Malabsorption can be due to a defect in the intestinal cells responsible for absorption. This can be caused by viral or bacterial infection or by a defect that limits the ability of the intestines to absorb liquids (such as severe inflammation or cancer within the wall of the intestines). Other causes of malabsorption include inherited conditions, parasite infections, or defects in the pancreatic secretions needed for effective digestion. Dehydration and electrolyte (salt) imbalance, which may lead to shock, are seen when large quantities of fluid are lost (for example, from diarrhea).
Digestive problems can also show up as changes in the number or character of bowel movements. Normal horse feces are apple-sized lumps that are well formed but somewhat moist. Digestive problems can result in feces that are too soft or too hard. Hard, dry feces can predispose horses to colic. The number of bowel movements per day, the color of the feces, and any tell-tale signs of blood should be reported to your veterinarian.
Colic is one of the most common digestive problems in horses. It usually involves intense abdominal pain, which your horse may show by pawing repeatedly at the ground, kicking at the abdomen, laying down and rolling, and looking at the flank. Other signs of colic include intense sweating, a distended or swollen abdomen, curling the upper lip and arching the neck, stretching out as if to urinate, loss of appetite, depression, decreased amount of feces, and straining to defecate. Colic is a serious condition that can have several causes; if you suspect that your horse has colic, prompt veterinary attention is necessary.
A diagnosis can only be made after the horse is thoroughly examined. Your complete, accurate description of your horse’s history (age, signs of illness, current diet, deworming schedule, when teeth were last floated, past problems, travel history, and so on) combined with a veterinarian’s physical examination can often determine the cause of a digestive system problem. When a digestive system disorder is suspected, your veterinarian’s initial examination might include the following: a visual inspection of the mouth for abnormalities or changes in the color and moistness of the mucous membranes; a determination of heart rate; a “hands on” examination of the abdomen and rectum to evaluate the shape, size, and position of the abdominal organs; tapping the abdomen or using a stethoscope to listen for sounds that indicate gas or for other abnormal sounds, such as splashing; and a visual examination of feces.
Depending on what the initial examination reveals, additional tests might include performing bacterial culture, parasite examination, and virus isolation from feces; passing a stomach tube; using a long needle to collect fluid from distended abdominal organs or from the abdominal cavity for analysis; creating x‑ray images, which may include use of special dyes, to detect blockages and other problems; passing an endoscope (a specialized camera to examine the inside of the digestive tract); performing abdominal ultrasonography; obtaining a biopsy (sampling and microscopic analysis) of liver or intestinal tissue; or conducting additional blood tests to detect possible malabsorption or maldigestion.
Because it is easy for foreign organisms and other “invaders” to enter the digestive tract through the mouth, this body system is prone to infection by bacteria, viruses, parasites, and other organisms (see Table: Infections of the Digestive System in Horses, below). These infections spread in various ways, but the most common are by direct contact or by contamination of food or water by feces.
Infections of the Digestive System in Horses
People and animals all have small numbers of certain intestinal microorganisms found within the digestive tract—most commonly in the intestines—that become established within a few hours after birth. These so‑called intestinal flora are actually beneficial, in some cases aiding in digestion and in others helping to prevent infection. However, sometimes infections occur when these organisms, normally found in small numbers, suddenly multiply. This can occur after a period of stress, under unhygienic conditions, or in an animal whose immune system is weakened. For example, salmonellosis in horses can develop after transportation, extended anesthesia, or surgery.
Diagnosis of a specific infectious disease depends on finding and identifying the organism suspected to cause the disease. This may require one or more fecal samples, which will be submitted to a diagnostic laboratory.
Parasites are a frequent cause of digestive tract disorders in horses. Many species of parasites can infect the digestive tract and cause disease. The life cycles of some parasites are direct, which means that there is only one host. Eggs and larvae are passed in the feces of an infected horse, develop into an infective stage, and are eaten by another horse. No other animals or organisms are needed to complete the life cycle. Other parasites have complex life cycles that involve an intermediate host, such as an insect. In this situation, infection is acquired when the intermediate host—or parasite shed by that host—is consumed by the horse. The extent to which a parasite affects an animal depends on the parasite itself, the number of parasites involved, as well as the animal’s own resistance, age, nutrition, and overall health.
Parasites can cause severe disease or simply decrease your horse’s overall fitness. Some of these parasites also infect humans. Because parasite infection is easily confused with other illnesses, diagnosis depends on the veterinarian’s knowledge of seasonal cycles of parasite infection, as well as examination of feces for evidence of parasite eggs or larvae. In some cases, blood tests can also be used to detect the presence of parasites.
Many digestive system diseases are not caused by infective organisms. Their causes include overeating, eating poor-quality food, chemicals, obstruction caused by swallowing foreign objects or displacement of the digestive tract, or injury to the digestive system. Digestive system disease can also be caused by digestive enzyme deficiencies, abnormalities of the lining of the digestive tract (such as from stomach ulcers, inflammation, or cancer), or birth defects. Digestive system signs such as diarrhea may also occur because of kidney, liver, or adrenal gland disease. The causes are uncertain in several diseases, including stomach ulcers in foals. Some conditions, such as colic, may have both infectious (such as damage from internal parasites) and noninfectious (such as excessive gas or simple obstruction) causes.
In noninfectious diseases of the digestive tract, usually only a single animal is affected at one time; exceptions are diseases associated with excessive food intake or poisons, in which multiple animals living together can be affected.
Specific disorders and their treatments are described later in this chapter; however, some general principles are listed in this section. Eliminating the cause of the disease is the primary objective of veterinary treatment; however, a major part of treatment is often directed at the signs of the disease and is intended to relieve pain, correct abnormalities, and allow healing to occur.
Elimination of the cause of the disease may involve drugs that kill bacteria, fungi, or parasites; antidotes for poisons; or surgery to correct defects or displacements.
Use of drugs to correct diarrhea or constipation is done depending on the specific case. Although such drugs might seem to be a logical choice, they are not beneficial in every situation. For example, diarrhea can actually be a defense mechanism for the animal, helping it to eliminate harmful organisms and their toxins. In addition, the available drugs may not always give consistent results. For some disorders, veterinarians may prescribe medications to improve intestinal motility.
Replacement of fluids and electrolytes (salts) is necessary in cases where the animal is at risk of dehydration, such as from excessive diarrhea or intestinal obstruction.
Relief of distension (bloating) by stomach tube or surgery may be required if the digestive tract has become distended with gas, fluid, or food.
Pain relief is provided when abdominal pain affects other body systems or when the horse may injure itself due to rolling or kicking. However, a horse being given pain medicine must be watched carefully to ensure that the pain relief is not masking a condition that is becoming worse.
Bacterial and parasitic diseases of the digestive system are often treated with medications designed to kill the infectious organisms. There are currently no specific medications for treatment of viral diseases. Antibiotics (drugs effective against bacteria) are commonly given for several days until recovery is apparent, although their effectiveness in treating digestive system disease is still uncertain, and overuse of antibiotics can be harmful. Antibiotics may be given by injection when septicemia (infection of the blood) is apparent or likely to occur. Your veterinarian will make the decision whether to prescribe antibacterial medication based on the suspected disease, likelihood of benefits, previous results, and cost of treatment.
Advances in understanding the life cycles of parasites, coupled with the discovery of effective antiparasitic drugs, have made successful treatment and control of digestive system parasites possible. Response to treatment is usually rapid, and a single treatment is often all that is needed unless reinfection occurs or the damage caused by the parasites is particularly severe.
Control of digestive diseases and parasites depends on practicing good sanitation and hygiene. This is achieved primarily by providing adequate space for your horse and by regular cleaning of its living areas and removal of manure. In addition, adequate nutrition and preventive medical care will minimize the stress on your horse and help it to stay healthy.
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