A metabolic master

The liver is tough and versatile, but signs of disease in horses are often subtle until it is advanced, and so it can be devastating. Angela Jones MRCVS explains

LIVER disease is relatively common within the UK equine population. However, its signs are typically subtle, which results in diagnostic challenges.

When trying to understand liver disease in horses, it helps first to understand both the anatomy of the liver and its many functions.

ANATOMY OF THE EQUINE LIVER

A HUB of metabolic activity, the liver sits in the cranial (front) abdomen, tucked behind the diaphragm. It is entirely enclosed in the ribcage.

Accounting for almost 2% of a horse's body weight, it is made un of many lobes and millions of hexagonal, microscopic lobules.

As grazing animals, horses don't have a gallbladder - as they eat constantly rather than intermittently they have no need for an organ that stores bile. Instead they continually secrete bile into the duodenum (the first section of the small intestine).

The liver has two blood supplies. It receives oxygenrich blood from the hepatic artery and oxygen-poor, nutrient-rich blood from the gastrointestinal tract via the hepatic portal vein.

THE FUNCTION OF THE LIVER

THE liver wears many hats, performing a myriad of vital functions in the horse, including:

• metabolism of fats, carbohydrates and proteins received from the gastrointestinal tract via the hepatic portal vein

• storage of glycogen, triglycerides and minerals

• protein synthesis (albumin, globulins, clotting factors, fibrinogen and enzymes)

• production of bile, which aids fat digestion and absorption of vitamins

• biotransformation (detoxification) of medications, ammonia, hormones and other potentially harmful substances, allowing safe excretion in the urine

• red blood cell development (extramedullary haematopoiesis)

• hormone production and regulation