The liver is one of the largest organs in the body, second only to the skin. It equates to approximately 1.5% of the body weight of an adult horse.  It performs many essential functions and it has been described as the body's 'powerhouse'.

Among other things, it plays a major role in the regulation of nutrients (by balancing carbohydrates, proteins and fats) and in detoxifying drugs and poisons absorbed from the intestines.  As a result, liver failure is a serious and life-threatening disorder.  

Fortunately, the liver has a large reserve capacity; this means that over 70% of the liver's function must be damaged / lost before liver failure occurs.  In addition, the liver is very effective at regenerating and repairing itself when it is damaged.

Common causes of liver disease include toxicities (poisonings) and infectious diseases.  The commonest cause of liver toxicity in the UK is pyrrolizidine alkaloid poisoning.  Pyrrolizidine alkaloids are found in various plants (there are believed to be over 3000 different plant species in the world that contain these poisons), the most important in the UK being ragwort.

Ragwort is a very common weed found in pastures, waste ground and verges.  Fortunately the plant is bitter and most horses will not eat it when it is growing in the pasture.  However, if grass is scarce, horses will sometimes be forced to eat the plant, which can then result in liver disease.  More commonly however, horses will eat this plant when it is wilted and dead (for example in hay). In this state the plant is no longer bitter and horses will willingly eat it.  

The poison contained in ragwort (pyrrolizidine alkaloids) accumulates in the liver and causes severe damage that can result in irreversible scarring (cirrhosis), which eventually leads to liver failure.  Once liver failure has occurred within this disease, most affected horses will die within a period of weeks to months.

One of the major problems in diagnosing liver disease, especially in ragwort poisoning which is one of the more common causes of liver disease in the UK, relates to the fact that the poison accumulates over a period of time - weeks, months or even years.  A slow deterioration occurs that may only cause mild symptoms such as weight loss, until liver failure; once this happens, sudden onset of severe and usually fatal liver disease usually follows.

Unless blood samples are taken to check for the presence of liver damage, this disease may go unnoticed until it is too late to treat.  So if you think that it is possible that your horse(s) may have been exposed to ragwort in hay or in their pasture, it would be worth considering having a blood test to see if the liver has been damaged.

Other diagnostic techniques that can be helpful in diagnosing the cause of liver disease include ultrasound examination and liver biopsy.

Liver biopsy is a relatively simple procedure in most cases that can be undertaken at the hospital on an out-patient basis with the horse standing (and sedated if necessary).  It involves passing a thin needle through the skin and into the liver (usually on the right hand side of the chest).  In this way, a tiny sample of liver can be obtained which is sent off to a specialist pathologist who will examine the structure of the liver under a microscope. If early liver damage due to ragwort poisoning is detected, then specific treatments can be started which will hopefully prevent progression of the disease.

Theiler's Disease (aka serum hepatitis):-

Another important, but thankfully uncommon, liver disease is Theiler's disease (also known as serum hepatitis).  This disease was first recognised and described by Sir Arnold Theiler in South Africa in 1914, when a large group of horses became ill with acute liver disease which resulted in the death of 22 of the group.

Today the disease has been reported most commonly in horses that have recently (usually within 6 to 8 weeks) received tetanus anti-toxin.  The disease causes acute and severe liver damage and failure, that can progress to death within a matter of a few days, or even a few hours.  The clinical disease is usually characterised by severe neurological symptoms (such as severe depression, walking in circles, wobbliness, blindness and pressing the head against stable walls).  These neurological symptoms are caused by the liver failure.

Recently, three different viruses have been identified in horses suffering from this disease; although it is not yet certain whether one or more of these viruses is the cause of Theiler's disease, it seems likely that the disease is similar to viral hepatitis in humans.  If further research proves that one of these viruses causes the disease, then hopefully we will be able to find effective ways of preventing it.