The role of the liver
The liver has a wide variety of functions. One of these functions is clearing the blood from toxins. When the amount of toxin exceeds the capacity of the liver to detoxify this compound, the liver function may be impaired or damaged and a liver disease may occur. Horse owners may be familiar with Tansy ragwort or “stinking willie” (Senecio jacobaea) poisoning. These may not directly be associated with liver disease. In this article we use Tansy ragwort poisoning as an example to illustrate the role of the liver in detoxifying the blood and how a liver disease may develop.
The role of the liver in the degradation of feed compounds
The aim of feeding is to provide the horse with all substances needed for the survival and health of all tissues in the horse body. During the process of metabolism rest products may be formed that would be toxic if the horse liver function would be impaired. Ammonia for example is a toxic compound that is formed when amino acids are degraded. In the liver, toxic ammonia in the blood is transformed to urea and subsequently excreted by the kidneys in the urine. This is a normal process in the horse body and does not cause problems when the liver is healthy.
When the Horse ingests feed, the horse may also ingest substances that may harm the health of the horse. For example, the horse may ingest grass or silage contaminated with moulds that produce toxins (mycotoxins) or may be provided hay containing poisonous plants, e.g. Tansy ragwort that can be harmful to the health of the horse.
The liver plays an important role in detoxifying or the removal of (toxic) compounds from the blood. The liver can remove hormones, drugs and other biologically active substances from the blood. The liver has several ways to detoxify the blood or inactivate toxic compounds. The most important tools are excretion of compounds in the bile; inactivate harmful substances by special cells or by alteration of the chemical structure that results in less harmful compounds. The extent to which the liver can detoxify or inactivate toxic compounds is limited and some toxins may affect liver function directly. If the amount of toxin exceeds the capacity of the liver to deactivate these toxins the liver (function) will be damaged. Below we use Tansy ragwort poisoning as an example to illustrate how hepatic failure may develop.
Hepatic failure due to Tansy ragwort (Senecio Jacoba)
Tansy ragwort is a common plant (especially on sandy soils) that contain the poisonous pyrrolizidine alkaloïds"(PA). The uptake of PA's is one of the most reported causes of chronic liver impairment in horses. Other plants may also contain significant amounts of PA's, e.g. Hound's tongue. The PA's even remain when Tansy ragwort is dried. When Tansy ragwort is present in hay it may therefore be more harmful compared to the presence of Tansy ragwort growing at the pasture. Normally horses will not eat from fresh plants when there is sufficient grass available at the pasture. Horses are unable to recognize the plant in the dried form. The risk for Tansy ragwort contamination in roughage seems higher when hay is derived from pastures with a low fertilization status (nature conservation areas). This seems at least valid for Western-European countries.
A liver impairment may be induced due to an acute or a chronic intake of a poisonous substance. In most cases toxity occurs due to a "long" term intake of a low dose. The liver is not able to transform the PA's in non-toxic compounds. The compounds that are formed when PA's are ingested, damage the liver cells in such a way that these cells cannot divide and die. The liver is not able to form new liver cells at affected sites and liver fibrosis may occur. The liver looses not only detoxifying capacity but also a loss in capacity of other liver functions. PA's are excreted in milk and urine and are able to cross the placenta. The fetus and foal may therefore be at risk for poisoning when the mare ingests PA's.
What (clinical) signs does my horse show if it has a liver problem?
The clinical signs of liver problems are often non-specific (e.g. weight loss, depression, abnormal behaviour, jaundice, anaemia and photosensitivity). It's therefore difficult for horse owners as well as vets to diagnose a liver related problem in an early stage. Pre-mentioned signs may also occur when PA toxity has occurred. Horses may show clinical signs of PA poisoning after 2 weeks- 5 months when 1-2% of Tansy ragwort on body weight basis has been ingested. Photo sensibility of the unpigmented (white) skin and skin problems (poor coat condition, alopecia, itching, damaged coronet) may also occur in case of PA poisoning. Photo sensibility occurs because the liver is unable to sufficiently remove and excrete phylloerythrin, a brake down product of chlorophyll (a plant substance) from the body. Phylloerythrin in the blood reacts with ultraviolet rays at places where the skin is not pigmented, as a result tissue damage may occur. Thus, at a certain point of liver failure the liver is unable to detoxify photosensitive substances. It is noted that there are also other plants that may cause photo sensitivity after ingestion.
When a relative large part of capacity of the liver is impaired, hepatic failure may occur. At this stage of the disease process more specific clinical signs may appear. These signs may appear sudden. If the liver is damaged considerably, changes in behaviour or neurologic (ataxia) problems may occur. The official name for this complex clinical syndrome is "Hepatic Encephalopathy (HE)", and it may occur irrespective of the cause of the liver disease. The disease is characterized by various mental stages. These mental stages comply clinical signs varying from mild depression, decreased attention or irritable (stage 1) to lethargic, drowsiness (stage 2) to somnolence, aggressive and uncontrolled behaviour to coma (stage 4). Owners of horses with a suspected liver problem or have a horse that has a liver problem should contact the vet immediately when changes in behaviour of the horse are observed.
The liver has a certain regenerative capacity. It is therefore important to exclude or diagnose a liver problem in an early stage. The vet can diagnose a liver problem with several diagnostic tools. If the liver has been severely damaged the chances for recovery are small. The survival rates of horses with PA poisoning vary.
The incidence of Tansy ragwort poisoning seems to increase (Western-Europe). Especially hay and silage derived from unfertilized pastures (nature conservation areas) might contain Tansy ragwort. Horse owners may reduce the risk for liver problems by controlling the pasture (and surroundings till about 50 m) for poisonous plants and an appropriate supply/quality of grass. When there are a few Tansy ragwort plants present, these may be eliminated from the pasture by hand. If there is a more persistent growth of the plant, it may be advised to adjust the mowing management. Mowing before flowering (Western European Countries: Mid July) and about 2 months later to prevent regrowth maybe a workable approach to reduce the amount of plants considerably. The feeding of mouldy roughage or concentrates to horses should also be avoided.