Liver Disease in the Horse

The Liver

The liver is the largest organ in the horse’s body, weighing approximately 5- 7kg for the average 500kg horse!

The liver is vital for life and health. Here are just a few of its important functions:

• Digestion and metabolism of nutrients e.g. carbohydrates, fats and proteins.
• Production of bile to help digestion of fats.
• Detoxification and elimination of waste products and harmful toxic substances.
• Synthesis of Vitamins (e.g. Vitamin C), blood proteins and clotting factors.
• Storage of vitamins and minerals, including Iron, Copper and Vitamin A, D, E, K and B12.

Symptoms of Liver Disease

The liver is incredible and can continue to function normally until over 70% of it is damaged. If 70% or more of the liver is damaged the horse will start to show signs of disease due to liver failure. Despite being relatively common, liver disease in the horse can be difficult to spot as often there are no external signs. Symptoms are usually vague, such as being off colour, just a bit quiet or the horse is under performing. Symptoms may not show until the disease is very advanced, and sadly it may be too late for successful treatment. The good news is that horses with mild liver disease can make a full recovery in time with the right treatment, providing the initial cause of the problem is removed.

The most common presenting complaint is weight loss, but there is a whole range of symptoms of varying severity that can be seen. Other symptoms include: reduced appetite, jaundice (yellowing of mucous membranes), photosensitivity (reaction of skin to sunlight), fever, depression, laryngeal paralysis (very noisy or difficulty breathing), swelling of the limbs and underneath the belly, diarrhoea and bizarre neurological symptoms such as head pressing, aimless walking or circling.

How can liver disease affect the skin?

Liver disease can present with photosensitisation on the skin. This looks like sunburn, and usually affects white or pink skinned areas such as the muzzle or white socks. Photosensitisation due to liver disease occurs because specific toxins increase in the body as the liver struggles to remove them. These then circulate to the skin where they react with sunlight inside the skin cells which cause cell damage, resulting in inflammation and ulceration of the skin, which may also be very itchy.

Common Causes of Liver Disease

There are a wide range of causes of liver disease in the horse, with some more common than others, but frustratingly it can be difficult to be certain of the initial cause in many cases.

The pasture, forage, feed and water source should be carefully examined (and tested) for evidence of possible causes of liver disease. This is particularly important if more than one horse is affected as it may indicate the toxin has been eaten or is accessible in the shared environment.

1. Toxic Causes of Liver Disease:

• Mycotoxins

Mycotoxins are toxins produced by certain moulds (fungi) that can contaminate feed and forage. Laboratories have seen an increase in mycotoxin contamination over the past few years, due to the weather, improved testing and detection in feed, and more sustainable and ecologically improved farming practices for example lower fungicide usage. Mycotoxins are known to cause liver disease in the horse.

-Iron

Iron is an important mineral, and its role in red blood cells is well known but too much iron can be harmful. In horses, iron deficiency is rare, as equine diets usually contain enough iron and so we should be careful not to over supplement it. Iron overload can be toxic to the liver where it is stored. There have also been examples of iron overload from water contaminated with iron.

- Poisonous plants

Ragwort (Senecio jacobaea) is the most well-known cause of liver disease in the horse in the UK. Most horse owners are very aware of the harmful effects of Ragwort. Ragwort contains pyrrolizidine alkaloids which are toxic to the liver and often the effects of poisoning are not seen until months after eating the plant. Ragwort is bitter but horses will eat it if the grazing is bare, and ragwort thrives on overgrazed pastures. Ragwort also becomes more palatable when dried, so it is important to ensure grass being baled for hay or haylage does not contain ragwort.

2. Infectious Causes of Liver Disease:

• Viruses

Viruses are thought to be a significant cause of liver disease, but they are still relatively poorly understood.  It is an exciting area of research, including recently finding new viruses that are linked to liver disease, but not all have been seen in the UK population of horses.  

• Parasitic Diseases

Liver fluke is a very common parasite in sheep and cattle, but it is rare to see clinical disease in horses, despite the same parasite Fasciola Hepatica being able to infect horses as well as ruminants. Horses appear to be relatively resistant to infection, but donkeys appear to be more susceptible to liver fluke.

The life cycle of the liver fluke is complex and it needs the right climatic and environmental conditions such as wet pastures with the mud snail (Galba (Lymnaea) truncatula) to complete the life cycle. The benefit of co-grazing with ruminants such as sheep will outweigh the risk of liver fluke for most yards, but the risk can vary in different areas and environments.  

3. Other causes of liver disease:

Liver tumours are very rare, they may be primary or secondary (spread from another area) and are more likely to be seen in elderly horses.

Fatty Liver disease can be seen in small ponies and donkeys when they stop eating causing hyperlipaemia. The body mobilises fat into the bloodstream which then accumulates in the liver.

How will my Vet confirm liver disease?

After taking a careful history and examining your horse your Vet will perform a blood test, which will show liver damage and how well the liver is functioning. If there are signs of liver disease, the Vet may recommend further testing including ultrasound scan and liver biopsy. These tests will provide further information about the cause and type of liver disease and help formulate a treatment plan and prognosis. Unfortunately, and rather frustratingly, the cause is not always found.

A liver biopsy is performed under ultrasound guidance using a special long biopsy needle, through the skin. The procedure is performed under sedation, with local anaesthetic and pain relief to ensure the horse is comfortable. The liver samples are then sent to the laboratory to be analysed under the microscope.

It is common for multiple horses in the herd to be affected so your Vet may recommend testing to ensure no others are affected.

Feeding the Horse with Liver Disease

Traditionally diet changes in horses with liver disease focussed on providing low protein and high carbohydrate diets, often with added B Vitamins. We now understand dietary modification may not be required in all cases as the typical modern equine diet is not high in protein. It is a balancing act to ensure enough protein to meet the horse’s requirements and prevent weight loss, whilst ensuring the liver does not have to work too hard to metabolise the protein. High protein diets may only need to be avoided in severe liver disease with signs of hepatic encephalopathy, as the liver is not functioning properly to clear toxic waste products. Ammonia is a by-product of protein metabolism, in healthy horses’ ammonia is processed by the liver, but when the liver is not functioning ammonia and other toxins build up to cause neurological symptoms.

Horses with liver disease may be off their food, so the diet should be palatable and small frequent meals given. Higher carbohydrate diets will aid weight gain if required but may not be suitable for all horses. Grazing can be an excellent way to encourage a horse to eat. B Vitamin supplements can be useful to promote appetite.

Many liver support supplements are commercially available, but unfortunately horse specific evidence either to support or contradict their use is still lacking. There are few reported side effects when supplements are used at the recommended serving rate, however it is always advised to discuss with your Vet or nutritionist before feeding a new supplement to ensure they are appropriate for your horse. A word of warning, some equine supplements contain extra Iron and these should be avoided as they may worsen the liver disease.

Liver disease may cause reduced absorption and storage of vitamins therefore you should ensure that the diet is balanced and meets but doesn’t exceed requirements. B Vitamins and Vitamin E are found in fresh green forage, so continued grazing is important to provide this, providing the pasture has been checked for poisonous plants. Photosensitive animals may need to change their routine to overnight turnout to avoid bright sunlight. Vitamin C is normally synthesised by the liver, but liver damage may reduce the horse’s ability to synthesise it resulting in a requirement for it to be supplemented.

Milk Thistle is the most commonly used plant in supplements to support the liver. Research has shown milk thistle has antifibrotic and antioxidative properties in humans. Antioxidants are natural compounds that can be made in the body or found in the diet. They mop up harmful free radicals reducing oxidative stress and damage to cells. Free radicals are unstable molecules that are naturally produced by the body during normal exercise, but intense exercise, illness and stress can produce excessive free radicals. Vitamin E, Vitamin C and rosemary also have antioxidant properties.

Specific advice for your horse with liver disease should be sought from your Vet and nutritionist.