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The liver is able to tolerate injury and insult

better than most organs, and repair itself

remarkably well[ following damage


From Horse and Rider - UK - January 2016

The liver supports almost every organ in the body and is vital for survival. It is the largest internal organ and a complex chemical factory that that works 24 hours a day, with numerous functions. It has been estimated that the liver is responsible for more than 1,500 essential processes on a biological level. Its primary function is detoxification, protein synthesis and the production of biochemicals needed for digestion, and it plays an important role in the horse's coagulation (clotting), endocrine (hormone) and immune systems. Because of its strategic location and multidimensional functions, the liver is prone to decease

The equine liver

The horse's liver is contained entirely in the rib cage, and is found behind the diaphragm and in front of the stomach and intestines. The bulk of the liver is on the right of the horse's midline. Like other vertebrates, the horse's liver is divided into lobes, but horses do not have gall bladders, despite still producing bile acids.

The liver is made of very soft, pinkish-brown tissue that is encapsulated in a connective tissue capsule. This capsule is covered and reinforced by the peritoneum, which holds the liver in place in the abdomen. There are two main cell types in the liver - the hepatocytes (liver cells) and the Kupffer cells (cells involved in immune response).

The causes of disease

Liver disease is relatively common in horses. However, liver failure is much less common because the liver has a large reserve capacity and good capacity to regenerate. It is able to tolerate injury and insult without failing better than most organs, and repairs itself remarkably well following damage. Liver failure will not occur until 75% or more of the organ has been damaged.

There are a number of potential causes of liver disease in the horse, including...

* Infectious agents 

Viral and bacterial diseases can be responsible for liver disease and even liver failure. A common reason for a bacterial caused liver failure is when overwhelming amounts of bacteria from the digestive tract enter the liver. This bacterial infection may arise from a sudden injury to the intestines or when there is bacterial overgrowth of particularly aggressive types of bacteria.

* The consumption or administration of toxic
drugs or poisons 

Toxic substances and plants can damage the liver. These may include pyrrolizidine-alkaloid containing plants (such as ragwort), mouldy alfalfa, mycotoxins (fungal/mould
toxins that contaminate bedding or feed), blue-green algae (which may contaminate drinking water) and heavy metals (such as iron and copper - inappropriate supplementation, such as feeding a supplement containing copper when the horse is eating copper-rich grass. Always ask your vet for advice before feeding supplements).

The liver is often the first organ to handle drugs given orally, so an overdose of some drugs - including antibiotics (such as trimethoprim-sulfa antibiotics and tetracycline), some anticonvulsants and inhaled anaesthetic gases - can lead to liver disease or failure.

* Hepatitis 

This condition can arise in adult horses who have been administered tetanus antitoxin - a drug that provides quick, short-term protection against tetanus in horses who have not been vaccinated and are at risk of contracting the disease. It maybe seen in mares who have received tetanus antitoxin 1-3 months after foaling.

Taking a closer look

Because of the vague and subtle clinical signs of early onset liver disease, it can be difficult to recognize. It is not until the disease progresses that it's possible to recognize a significant clinical picture that may be suggestive of liver disease and prompt further diagnostic investigation.

It is important that your veterinary surgeon gathers a thorough and detailed history of your horse's symptoms, as well as his management. This will be followed by an extensive clinical examination. Your vet will often take a blood sample to confirm the liver disease, however it does not reliably indicate the severity and possible causes of the disease.

Laboratory diagnosis of liver disease can be challenging due to the fact that a large amount of damage to the liver must be present and the progress of the disease can alter different liver functions. With a blood sample, certain intracellular enzymes are examined, such as alkaline phosphatase (AP), gamma glutamyl transferase (gGT), aspartate aminotransferase (AST) and glutamate dehydrogenase (GLDH). In addition, the blood sample will also evaluate hepatic function by evaluating the blood levels of serum globulins, serum bile acids, serum albumin and bilirubin. However, for complete diagnosis and to determine the type of liver disease, the severity of the disease, the prognosis and to select an appropriate treatment, ultrasound scan of the liver and ultrasound-guided liver biopsy should be performed.

In recovery

The prognosis for recovery depends on multiple factors, such as the severity of the liver damage, the vet's ability to identify and remove the contributing cause of the disease, and the general health of the horse.

An older horse with a poor body condition may struggle to recover, whereas a younger horse who is in good health may be able to recover fully, despite experiencing the same amount of liver damage. ■