The liver is the powerhouse of the body. It has a vital role to play in almost every bodily function and is the most important single organ in the body, because every other organ depends on it. The various roles of the liver include metabolic functions such as regulation of carbohydrate, fat and protein building blocks for the body. These are delivered to other organs so they can function, repair and regenerate. The liver also 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 intestines are removed from the blood by the liver before they can affect the rest of the body. The liver also manufactures many essential micro-chemicals, such as clotting factors and vitamins.
The liver has a large reserve capacity. Individual liver cells are arranged in columns around a central draining blood vessel. Each cell can be rapidly regenerated and the whole mass of the liver is replaced within a few weeks. Over 70% of the liver's function must be damaged before liver failure occurs.
Common Causes of Liver Disease
Common causes of liver disease include toxicities (plant/pasture, mycotoxins, cyanobacteria, heavy metals, agrochemicals) and infectious agents (viruses, fascioliasis, macrofungi, Clostridia). The commonest cause of liver toxicity in the UK is pyrrolizidine alkaloid poisoning. In the UK, ragwort is the commonest plant containing these toxins.
Ragwort is a very common weed found on pastures and roadsides. The plant is very bitter and unpalatable to horses. However, if there is no alternative food source and grazing is sparse, they may be tempted to eat it. When ragwort is wilted and dried, as in hay, it is no longer bitter and horses will happily eat it. Ragwort causes irreversible damage to liver cells, including fibrosis (cirrhosis), and can take weeks, to months, to even years to develop. These cause irreversible damage to liver cells. Access to ragwort should be avoided, particularly in its dried palatable state. Checking your hay source is important as it is palatable in this form. Removing ragwort plants from the field by hand and disposing of the pulled-up plants away from livestock is also important. Leaving pulled up ragwort plants to wilt and dry in the field will encourage horses to eat them. Alternative forage should be made available in fields with large quantities of ragwort in, or ideally these fields should not be grazed or all plants should be removed.
Serum Hepatitis (Theiler’s Disease)
Serum hepatitis is an uncommon disease in horses, mostly seen in horses approximately 4-12 weeks after administration of an equine biologic, such as a dose of tetanus anti-toxin. The disease causes acute liver failure, and can be rapidly fatal, with reports of 50-90% mortality for symptomatic horses. It is important to ensure your horses are regularly vaccinated for tetanus to avoid the use of tetanus anti-toxin, in emeregency situations where a wound requires cover.
Clinical Signs of Liver Disease
The clinical signs of liver disease vary greatly, are non-specific and depend on the extent and duration of hepatic disease. Usually, greater than 70% of liver mass must be lost before clinical signs become apparent, regardless of the cause of liver disease. Therefore, despite the duration of hepatic disease, the onset of clinical signs is often abrupt.
More severely affected cases may show signs of Hepatic encephalopathy which include neurological abnormalities such as depression, head-pressing, ataxia (incoordination), blindness, seizures, coma and death.
How do we Investigate Liver Disease
Diagnosis is made by a combination of clinical signs, blood tests, ultrasound examination and liver biopsy. As the effects of liver diseases such as ragwort can develop over months to years, evidence of liver disease is often picked up on routine blood screens at annual health checks. If you think your horse may have been exposed to ragwort, or a horse in the same field is diagnosed with liver disease, it is prudent to have a blood test performed to check levels of liver enzymes.
Samples are sent to an equine pathologist for interpretation. Ragwort poisoning is easily detected under the microscope. Other diseases often show less definitive pathology. Frequently, biopsies provide us with information which allows us to give you a prognosis and may also help the vet chose the appropriate treatments for your horse.
What is the Latest Research in Liver Disease in Horses
Recently, there has been much research looking at new equine viruses that can cause liver disease. Viral causes of hepatitis are common in humans and noninvasive serological tests can be used to diagnose them and prevention with vaccines is possible. We hope further research in this interesting area will enable us to better diagnose liver disease and formulate effective treatments. Ultrasonography of the liver is relatively simple to perform. However, only a small proportion of the horse’s liver can be seen on ultrasound. It is most useful for guiding the vet when performing a liver biopsy. Liver biopsy is a relatively simple procedure performed in the standing sedated horse. It is performed as an outpatient appointment and we ask horses to stand for an hour after the procedure prior to travelling home.