Liver damage in horses
Organ acts as a filter to remove poisons. The liver may be the most important single organ involved in maintaining the body. It is the storehouse for many things, including glycogen (a form of sugar), certain enzymes and proteins, copper, iron, and vitamin A.
The liver also helps regulate blood volume. Some of its other functions are destruction of old red blood cells, manufacture of certain blood-clotting factors and blood proteins, and metabolism of proteins, carbohydrates, and fats. The liver is also the primary filtering system for the bodyworking as a specialized detoxification unit to remove the poisonous effects of foreign substances in the bloodbacterial toxins, poisons that enter the bloodstream from the digestive tract, and others.
When blood reaches the liver from the small intestine, its carbohydrate content is adjusted as needed, either by removal of excess sugars (converting them to glycogen to be stored) or by the addition of sugar through breakdown of previously stored glycogen.
Any toxic substances carried by the blood are detoxified by special cells that ingest and destroy them, or are removed from the blood and stored. Partially broken down red blood cells arrive from the spleen at the rate of about 3-million per second and are picked up by special cells that break them down further. The breakdown products are either recycled or excreted through bile. The liver also synthesizes ureaa waste product that is subsequently excreted by the kidneys as one of the components of urine.
The liver is a very large organ, and the body can continue to function even with some of it damaged or destroyedas long as there is still enough healthy tissue to filter out wastes and poisons. About three-quarters of it must be rendered inactive before clinical signs of liver impairment are obvious.
The livers great reserve capacity and its ability to regenerate itself usually prevent serious trouble. Some types of damage or disease, however, can overwhelm the liver and cause death.
Horses with chronic liver failure seldom live more than six months regardless of treatment. Horses with acute liver damage have only a slightly better chance of recovery if prompt treatment and supportive therapy can be givento get the animal through the danger period of acute liver insufficiencyin hopes that the liver will be able to regenerate itself enough to restore its function. Death may still occur due to very low blood sugar; the blood glucose level must be maintained by oral or intravenous injections of glucose.
Primary disease of the liver is rare in horses except as a result of poisonings, such as eating toxic plants or ingesting toxic chemicals. Secondary disease of the liver is more common and usually is due to a generalized disease process in the body or the spread of infection from another organ.
The liver is one of the bodys main defenses; one of its major functions is detoxification of harmful substances in the blood. The liver can modify almost any chemical, breaking down toxic moleculesmaking them harmless. But, sometimes in the process of protecting the body by filtering out poisons, the liver itself becomes damaged or infected.
Signs of liver damage
The first sign of acute toxic hepatitis (inflammation of the liver) is dullnesslethargy and lack of appetite. Pulse, respiration, and temperature are usually normal, but the horse may have abdominal pain due to rapid swelling of the liver, and the problem may be mistaken for colic. Sometimes there are blood pigments in urine.
Jaundice (yellow color of the skin and mucous membranes such as gums and whites of the eyes) is a common sign of liver disease and results from an accumulation of bile pigment, especially from the breakdown products of old red blood cells. These products are not cleared from the body when the liver is not functioning properly or its bile ducts are obstructed, and the pigments tend to accumulate in the skin and other tissues. Jaundice can also be caused by other problems in which there is no liver involvement, so jaundice alone is not a sure sign of liver problems.
Toxic substances normally processed by the liver build up in the bloodstream and affect the nervous system. The horse may be dull (standing with head drooping and feet wide apart) or overly excited; he also may exhibit weakness or muscle tremors, staggering or dragging his feet. His mental condition rapidly deteriorates and he may behave strangely, such as not responding to signals, pressing his head against a wall or fence, or walking into obstacles. Convulsions followed by a coma may occur.
One of the most common effects of severe and acute liver damage is a sharp drop in blood sugar, accompanied by nervous signs and hyperexcitability. For instance, the horse may become violent or unmanageable, running wildly and crashing into fences. If damage is occurring more slowly, the low blood sugar is less dramatic; the horse shows milder symptoms such as drowsiness, inability to work, yawning, and lethargy.
When the liver loses its detoxifying ability, ammonia and other toxic byproducts of protein breakdown are not filtered out of the blood and have an adverse affect upon the brain.
Muscle wasting can be another sign of liver damage. Failure of the liver to process amino acids and protein results in tissue wasting and a drop in plasma protein. This is sometimes severe enough to cause edema (fluid seeping out of the blood vessels and into the tissues to create enlargement and swellings). Diarrhea or constipation may also occur with certain types of liver disease.
Photosensitization in horses (in which unpigmented areas of skin are affected by sunlight, with skin cells dying and patches of skin sloughing off due to photodynamic agents brought to the skin from the digestive tract via the bloodstream) is often a sign of liver damage.
Poor blood-clotting ability can be a result of severe liver disease due to deficiency in prothrombin formation.
Diseases of the liver
The term hepatitis is usually used in referring to liver inflammation, injury, or degeneration. Toxic (noninfectious) hepatitis may be due to toxins being carried to the liver, such as through an infection from somewhere else in the body. Toxic hepatitis can also be caused by inorganic poisons such as copper, mercury, arsenic, or excessive amounts of phosphorus from polluted water containing industrial wastes, phosphorus fertilizers, or rat poison.
Dioxin, an ingredient in waste oil that was sometimes used in the 1970s to keep dust under control in riding arenas, was also suspected of causing acute hepatitis. Organic poisons such as carbon tetrachloride and coal tar pitch also can damage the liver. Iron supplements contained in probiotics marketed as a digestive innoculant for newborn foals were a cause of severe liver damage.
Poisonous plants that can cause toxic hepatitis are weeds (including various groundsels, Crotalaria, Heliotropium, fiddleneck, tarweed, fire weed, yellow burr weed, buckthorn, tansy ragwort, rattle weed, wild pea, or puncturevine); pasture plants such as alsike clover and water-damaged alfalfa hay; a few trees and shrubs; and several types of fungi and algae.
Aflatoxin, a fungal toxin that can contaminate corn, peanut meal, and other feeds, can cause acute or chronic hepatitis. Castor beans, often grown as an ornamental plant, are poisonous if eaten by horses. Several legumes can be toxic under certain conditions, including lupines, vetches, and velvet beans.
Most of these are not very palatable, and horses only eat them if feed is in short supply or when seeds get mixed in with grain or hay.
The horse eating these feeds may not get enough of the poison plants to cause acute hepatitis, but if he eats contaminated feed over a long period of time he may develop chronic liver damage. In certain geographic regions, toxic hepatitis from eating poisonous plants and contaminated feed is the most common cause of liver failure.
Moderate degrees of hepatitis can occur with many types of bacterial infections within the body. Liver injury can also be caused by extensive tissue damage elsewhere in the body, as in the case of severe burns, wounds, or tissue death due to obstruction of circulation. The body has to get rid of the toxic products of tissue degeneration, and the liver can be adversely affected.
Infectious hepatitis
Infectious hepatitis is rare in horses, but some types of infection can create liver lesions. One of them is Tyzzers disease in foals, which occurs in foals 1-to-5 weeks old and is characterized by high fever, shock, terminal coma, and death within a few hours. Others are the virus that causes rhinopneumonitis, and severe cases of equine viral arteritis (EVA).
Some diseases do not produce signs of liver damage during the life of the horse, but liver lesions are evident at necropsy. These diseases include salmonellosis, septicemic listeriosis, leptospirosis, and EIA (equine infectious anemia).
Chronic hepatitis can be mistaken for moldy corn poisoning, infectious anemia, or equine encephalomyelitis, since some of the signs are similarunless jaundice or photosensitization is present to give a clue as to liver involvement. Toxic hepatitis can be acute or chronic. Sometimes horses with an acute case will partially recover and develop a chronic liver disorder.
Acute hepatitis appears suddenly and chances of survival are poor. Between 50%-to-90% of horses with acute hepatitis die within 12-to-48 hours. Of those that survive, recovery begins in 4-to-5 days and the damaged liver regains most of its function.
Chronic hepatitis appears more slowly and may not be noticed until symptoms worsen. A chronic liver condition may sometimes be caused by certain toxins or poisons in small doses. If a horse is continually subjected to a poison, the liver may eventually become fibrous (cirrhosis) and lose its ability to function.
Local infections in the liver itself generally do not cause signs of liver dysfunction unless the lesions are quite large or spreading. Occasionally a liver abscess may occur, as in cases of strangles that form internal abscesses. Inflammation of bile ducts can also occur, causing fever, pain over the liver, jaundice, and photosensitization. The liver also may be damaged by migrating parasites that obstruct the bile ducts, causing intermittent colic or continuous pain, and sometimes jaundice.
In rare instances, a foal may be born with a congenital liver abnormality such as absence of bile-duct components or a duct closure. The foal seems normal for the first 2-to-3 weeks but then becomes listless and stops nursing. Feces become gray and pasty, the foal becomes jaundiced, and death occurs about a week later.
Hepatitis is a life-threatening disease with very little effective treatment available. Preventive measures probably are the best treatmentsuch as being careful with chemicals, using feed supplements and medications wisely, and feeding horses well so they will not be tempted by toxic plants.
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