The equine liver looks similar to that in most other species. This smooth, dark red organ is situated near the front of the belly, between the lungs and the intestines. Most of the liver is found on the right-hand side of the horse’s body, to allow room for the stomach on the left (see diagram, above).
The liver is quite small in the horse, however, compared with the rest of his body. In an average adult horse it weighs about 5kg — roughly four times that of the average human liver. Unlike the liver of many other mammals, the equine version does not have a gall bladder to store bile.
The hard-working liver processes nutrients that the horse absorbs from food — making proteins, for example, from the plant proteins the horse has eaten. It can store some useful nutrients, such as vitamins. It cleanses the blood to remove toxins the horse may have eaten, as well as dealing with some normal waste products made by the horse’s body. In humans, the liver also detoxifies alcohol after we have had a few drinks — not normally a job for the equine liver, unless the horse has gorged on fermented apples.
This versatile organ helps to control the levels of fats, sugars and proteins in the blood so that they are ready when other parts of the horse’s body need them — for doing exercise, for example, or building muscle. The liver produces many of the proteins that are needed for blood-clotting and to maintain the consistency of blood. It also produces bile, which is used in the intestines to help digest food.
All in all, the liver is a busy organ. Luckily, it has a lot of spare capacity, so it can often perform all of these jobs even when parts of it are damaged or diseased. In fact, the liver only really struggles to fulfil its many roles when at least three-quarters of it is damaged.
Poisons and parasites
The plant we commonly associate with liver damage is ragwort. Horses do not usually eat ragwort when it is growing because of its bitter taste, but will consume the plant when it is dried in hay. Ragwort is a dangerous toxin that can continue to damage the liver for months or even years after it was originally eaten.
There is more to liver disease than ragwort poisoning, however. The liver can be affected by many other toxins; moulds in hay are often suspected and even soils with very high levels of iron can cause damage. The liver can also be infected by bacteria, viruses and parasites.
Detecting viruses can be particularly difficult; there are probably viruses that affect the horse’s liver that have not yet been discovered. The liver can also become inflamed, for other reasons that are still poorly understood, or filled with fat — usually in the case of ponies that have other severe illnesses. Liver disease can cause varied symptoms.
If damage is mild, the liver can often continue to function and the horse may show no signs of disease at all. In more serious cases the horse may lose weight, become quieter, go off his food or show signs of colic.
An affected horse may develop diarrhoea or his skin may become sensitive to sunlight or itchy. Severe cases of liver disease can affect the function of important nerves and the brain, even causing death.
What’s the prognosis?
Liver disease may be diagnosed when a horse starts to show obvious signs, but can also be indentified before these become apparent. Blood tests are a very useful diagnostic tool, highlighting damage to liver cells and indicating how well the liver is able to perform its usual functions.
The liver has a great capacity to repair itself; mild liver damage often resolves and blood tests can return to normal within a few weeks. In more serious cases or those with long-term problems, further investigations are often needed.
Ultrasound scans allow us to examine certain parts of the liver, but many areas cannot be seen because of their position hidden behind pieces of intestine or lung. An ultrasound scan will help to identify whether the liver is a normal size and shape, however, allowing the vet to diagnose some types of disease. In some older horses, the liver shrinks and becomes even harder to see. Often an ultrasound scan will not provide the whole story and so a liver biopsy may be needed.
A biopsy (pictured, below) involves taking a small piece (or pieces) of liver tissue to examine under the microscope. This enables pathologists to identify important changes to the liver cells, such as scarring, evidence of ragwort poisoning or other signs of inflammation.
Biopsies are usually performed using local anaesthetic and sedation. An ultrasound scan is often used to pinpoint the best place to perform the biopsy, before a needle measuring around 15cm long and 2mm wide is inserted through the skin to take tiny samples. This quick and simple procedure can provide helpful information.
Treatment depends on the type of disease detected. Often, the horse is better at “treating” his own liver than we are — the liver can usually recover from mild damage very well, assuming the cause of the damage goes away. In other cases, the horse may benefit from changes to his diet and the possible addition of a vitamin supplement. Many liver supplements are available for horses but very few have been properly tested.
Treatment with anti-inflammatories or antibiotics may also be required. Humans with serious liver disease are sometimes given a liver transplant, but this is not something we can do with horses.
Ref Horse & Hound; 23 February 2017