The effect of damage to the liver by Steve Dean
LIVER disease is often seen as a chronic disease but on occasions it is an acute problem where rapid, severe symptoms develop, usually in the young animal. However, no matter if the symptoms are caused by an acute or chronic effect, in both cases it is only when the liver function falls below 25-30 per cent of its normal capacity that the organ failure becomes apparent.
The cause of the damage may not always be understood for the liver is at risk of many insults that may damage its cells. Of those that cause acute damage the most common are infection and poisoning, although events such as severe heat stress can also result in an acute event.
An acute infection such as viral hepatitis (adenovirus) and leptospirosis (a bacteria) will massively overwhelm the liver cells. The adenovirus infection destroys the cells of the liver through viral reproduction and the leptospiral infection produces a bacterial toxin that is directly damaging for cells. Either infection in the very young animal can so severely damage the liver, the effects follow the infection quite rapidly. Thus, just as the animal appears to be controlling the infection, liver function collapses.
Acute infections are rare today because of vaccination and better husbandry but acute illnesses do still occasionally occur. However, acute liver disease is not limited to infection. On rare occasions medicines, plant toxins, fungicides, herbicides and even a failure of blood oxygen supply can all result in acute liver failure. Some of the better-known examples include some old anaesthetics, blue-green algal toxins in ponds and lakes and heatstroke. All are capable of causing acute liver damage.
A similar pattern may be seen with chronic damage except the damage takes place overtime and is not so easily linked to causal events. The classic medicinal risk that illustrates this is the effect of phenobarbitone used to treat epilepsy. Over a long period of use this drug may cause sufficient damage to liver cells to eventually produce clinical signs of liver failure.
Inherited conditions can be another root cause. For example, a decrease in the normal ability to metabolise dietary constituents is best illustrated by copper toxicosis in the Bedlington Terrier. Much has been done to detect this genetic mutation that results in a reduced ability to handle copper in tissues. The resulting accumulation of copper in the liver gradually reduces function.
Indeed this is a common feature of many poisons. In low doses their effect is insufficient to damage the liver to an extent that causes symptoms, but liver function is nevertheless reduced. Further toxic events continue to damage the liver until the effect becomes noticeable (ie once the damage exceeds 70 per cent of the organís capacity). It is perhaps therefore not surprising for it to be more common to see liver failure in old age.
The liver is often directly subjected to blood circulatory defects. A developing heart failure results in blood pressure changes in the wider circulation. An increase in the pressure in the hepatic vein draining blood from the liver may result in congestion of blood in the organ. Equally problematic is poor cardiac output reducing blood flow to the liver from the hepatic artery. A consequent decrease in oxygen supply to the cells creates damage.
Either problem will result in a reduction in liver function and death of liver cells. A different circulatory issue arises with the congenital liver shunt, directing blood flow from the hepatic portal vein directly into the main vein (caudal vena cava) draining blood back towards the heart.
The portal vein is carrying nutrients from the bowel to the liver for further metabolism and a liver shunt effectively allows these nutrients to bypass the important metabolic functions of the liver. This exposes sensitive tissues to unprocessed nutrients and this has further consequences typically for the function of the brain and kidney.
It is of course quite common to assume an acquired liver malfunction arose from a single insult but in fact for chronic liver damage it is possible that several events contribute to the point where liver function finally fails to keep up with the demands of the body. Furthermore, it may not always be a single causative agent. Multiple factors may add together to cause accumulated liver damage.
A typical scenario could involve a mild infection at a young age leading to reduced liver function. Perhaps an event follows where the dog suffers heat stress by being left in a car and further damage is done. Subsequently a road traffic accident results in surgery where the anaesthetic or other medication places further stress on the liver cells. Finally a heart condition deranges blood circulation sufficiently to increase the damage to the level that triggers the symptoms of liver failure.
Only the infection, causing a mild hepatitis, directly affected the liver. All the other events had an indirect, but nevertheless serious, effect on liver function. Yet the final summation of the damage to the liver finally yields signs of organ failure.
Symptoms are often vague in chronic damage and more immediate and dramatic in acute forms. The classic sign of liver disease is jaundice as the liver fails to effectively clear bile salts and these deposit in tissues turning, for example, the mucous membranes of the eye yellow. However, before this, a dog may vomit on occasions, increasingly drink more water, show a poor appetite or simply demonstrate an unpredictable digestion with occasional diarrhoea.
A failing liver creates many problems for the body and thus many different symptoms. A failure in the ability to store glucose may create energy and stamina issues. The liver is important in maintaining the integrity of the blood and especially clotting factors and so bleeding disorders may arise where liver function fails. The immune system may be less efficient as the liver fails to handle bacteria that might find their way to the bowel.
Physiological deficiencies might occur as either certain elements such as potassium or toxic elements may build up yielding a variety of symptoms as the body struggles to clear toxins from the body. A final twist is that the affected dog may be more likely to suffer the potential side-effects of medicines because the liver is so often the centre of drug metabolism and now the normal dose is too much for the liver to handle. As AVV said before the liver may not be top of the list of important organs but no mammal can live without one.