The horse's liver
The liver is the powerhouse of the body. It has a vital role to play in almost every bodily function. The various roles of the liver include some metabolic functions. The liver produces energy and protein building blocks for the body. These are delivered to other organs so that they can function, repair and regenerate cells and organs that die off.
The liver plays an important role in digestion. Bile and salts secreted into the bile, help change the acidity of the food as it enters the gut from the strongly acidic stomach.
A lack of bile production alters digestion, particularly the way in which fats are broken down. The horse has no gall bladder and bile is discharged in response to eating, so if a horse is starved for 12-24hrs the bile pigments can accumulate in the blood, giving a false impression of jaundice.
Failure to produce and secrete bile salts into the bile duct is a common indication of liver function – if the functional reserve of the liver is exhausted, bile salts will accumulate in the bloodstream, an invaluable test for the overall function of the liver.
The liver acts as a filtration system to protect other organs from the effects of toxin build-up. It is the body's own detox system. Poisons absorbed from the gut are removed from the blood by liver before they can affect the rest of the body. Also, naturally produced poisons such as ammonia are converted into safe chemicals that can be excreted. Ammonia is converted into urea for excretion by the kidney.
Liver infections range from transient to serious or even life threatening. The initial signs are often mild, but fortunately liver damage is usually detectable through blood analysis.
By far the most common cause of serious liver damage in horses in the UK is ragwort poisoning. The toxin hits the cells nearest the gut and these are the focus of the early damage. This is an insidious killer; each tiny amount of the poison absorbed from the gut causes a corresponding additional amount of damage. There is no such thing as a safe amount of ragwort. This damage is usually irreversible because the poison destroys the liver tissue so the liver cannot regenerate or repair itself in the normal way.
Usually the liver is able to maintain most of its functions, even when large proportions of its tissues are damaged. Only when more than 75% of the liver is damaged will any outward evidence appear, at which point the prognosis is poor.
If the liver is damaged, it may lead to a hypersensitivity to sunlight, resulting in a special type of sunburn or photosensitivity. Typically, light-coloured areas, such as on the muzzle and the white lower limbs, are affected.
The liver also manufactures many essential micro-chemicals, such as clotting factors and vitamins. When the functional reserve of the liver is exhausted these are deficient and the horse may bleed from the nose or into the gut or urine.
Iron and other essential vitamins and minerals are stored in the liver until they are required; vitamin and mineral deficiency diseases are a common reflection of liver failure.
Damage to the liver, arising from any cause, has the potential to be life threatening. The most significant harm is caused by disease that either prevents the liver from healing or damages too much of the liver, so death intervenes before repair can take place.
Often a horse may be suffering from more than one condition. One condition may be manageable, but a general illness plus liver disease can push the horse into liver failure.
This is why, in some cases of ragwort poisoning, the horse may seem to recover reasonably well, usually the transient problem resolves and restores the function of the liver to over the minimum 25% required for it to work.
Diagnosis – and the future
The diagnosis of liver disease is a major challenge. One way to make a categorical diagnosis is by taking a small liver sample (biopsy) and examining it under a microscope. However, this is only effective for diseases that affect the whole organ. Where there is a focal problem, there may be no indication of the disease unless the biopsy is taken from the affected area. While biopsy provides very useful information, it is not necessarily an easy or safe procedure.
Signs of liver disease
· Diarrhoea or constipation
· Weight loss
· Fluid accumulation under belly and in hind limbs
· Tendency to bleed without clotting
· Repeated yawning
· Poor or reduced performance
· Reduced appetite
· Poor coat quality
· Severe debility
· Head pressing
· Swelling of the head and all legs, as well as fluid accumulation in the belly
· Central nervous signs including depression, mania, seizures, bizarre behaviour, blindness, coma and death