The liver can be affected by various causes. Liver diseases in adult horses are most commonly brought about by ingestion of mouldy hay and straw as well as poisonous plants. Newly-born foals can suffer from, for instance, EHV (equine herpes virus); from the age of one or two weeks there is Tyzzer’s disease (damage to liver and appendix by the Bacillus pilliformis originating from the mare or the environment) and umbilical infections spreading upwards to the liver.
The liver is the largest equine organ and depending on the size of the individual horse weighs between five and eight kilos. The liver is found just behind the sternum. The membrane enveloping the liver, also called the ‘liver capsule’, has a high intensity of nerves and is therefore very sensitive.
The liver produces vital digestive fluids that help to break down the fat from the nutrients. These enzymes are a component of the gall fluid which the liver secretes from the small intestine via a duct between the liver and the small intestine. Horses don’t have a gallbladder. When the duct becomes obstructed by a gallstone it won’t take long before the effects are felt.
The liver has two vital functions: energy storage and the distribution of energy across the body as well as detoxification of the body. The food substances that are absorbed through the wall of the intestine during digestion of meals are transported directly from the intestine to the liver by a portal-vein system.
Source of blood sugar
The liver is an important source of blood sugar. When the liver is seriously ill this will jeopardise the energy supply of the body. When a horse has used up all his blood sugars, liver and muscle glycogen for energy the body will tap into the fat reserves.
Adrenalin and for one, the thyroid hormone, together dissolve the fat reserves. These dissolved fats are transported to the liver and transformed to optimal energy. Proteins and amino acids are also transported to the liver where they are turned into energy. When proteins are broken down for energy purposes they produce waste products. These too are moulded by the liver so that they can be expelled via the urine. The final product is called ‘urea’. In horses suffering from liver failure the liver production line delivers an unfinished product: ammonia.
Horses on a ‘high’
Horses that are producing an excessive amount of ammonia will become ‘high’. They will display uncharacteristic behaviour such as intense drowsiness, blindness, pushing their head against the wall or they may be extremely nervous. Horses with persistent liver problems must be put on a special feeding regime.
The liver also processes the waste products of the colouring agent of the red blood cells: haemoglobin. Old and exhausted red blood cells are broken down by the body. The haemoglobin is broken down to bilirubin, which is an important factor that causes the green-yellow colouring of the gall fluids. When the liver is malfunctioning this bilirubin is insufficiently removed.
When the removal of gall from the liver becomes problematic this will result in bile duct obstruction (blockage of gall). The bile is not passed smoothly to the small intestine. Affected horses will show jaundice of the mucous membranes and the eyes. Jaundiced mucous membranes may also be caused by feeding high-carotene nutrients (such as carrots) or disorders that trigger an increased breakdown of red blood cells.
Health and functioning of the liver can be examined by means of a blood test, ultrasound scan, biopsy of the liver and testing of the liver function.
Photosensitisation
Finally there is the notion of ‘photosensitisation’, which can be seen in the white pigment-free parts of the equine skin and is therefore very uncommon in Friesians. The white skinny parts will become red, flaky, itchy and painful. This phenomenon can be triggered by the ingestion of specific plants (photosensitising plants), absorbing specific material (primary photosensitisation) or it can be a symptom of plain liver failure (secondary photosensitisation).
In the case of a skinny horse that has gone off its food liver failure might be the root of the problem (secondary photosensitisation). The liver then does not succeed in processing the chlorophyll (green colouring agent of grass) into the right waste product for excretion and an unfinished waste product flows down the production line, particularly phyloerythrine. This substance settles under the skin and will be activated by the UV light from the sun causing as it were, sun burning of the white parts of the skin.
Primary photosensitisation
‘Primary photosensitisation’ is unconnected with liver failure. The horse has ingested specific plants such as buttercups or buckwheat. Substances from these plants settle under the skin and trigger the same effect. Another cause of ‘primary photosensitisation’ could be the licking of railway sleepers or posts used for fencing that have been treated with creosote-related material.
As posted on the KFPS newsletter June 9, 2014
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