Hepatic Encephalopathy in Horses: Neurologic Signs From Liver Failure

Hepatic encephalopathy is a brain and nervous system problem that happens when the liver can no longer process toxins normally. In horses, this most often develops with severe liver disease, liver failure, or abnormal blood flow that bypasses the liver. Ammonia is one of the best-known toxins involved, but it is not the only one.

When these substances build up in the bloodstream, they can affect the brain and cause sudden or progressive neurologic signs. A horse may seem dull, disconnected, unsafe to handle, or dramatically different from normal. Some horses become quiet and depressed. Others become restless, blind, or disoriented.

This is not a condition to monitor at home for long. A horse with suspected hepatic encephalopathy needs urgent veterinary attention because the underlying liver problem may be severe, and the neurologic signs can put both the horse and handlers at risk.

In foals, a congenital portosystemic shunt can cause similar signs because blood bypasses the liver from birth. In adult horses, hepatic encephalopathy is more often linked to acute hepatitis, toxic liver injury, chronic liver scarring, or other causes of major liver dysfunction.

Hepatic encephalopathy is not a disease by itself. It is a consequence of serious liver dysfunction or abnormal circulation around the liver. In adult horses, causes can include acute hepatitis, chronic liver disease with loss of functional liver tissue, severe toxic injury, and advanced fibrosis or cirrhosis-like change. In foals, a congenital portosystemic shunt is an important cause because blood bypasses the liver instead of being filtered normally.

Toxic and inflammatory liver injury are common concerns in horses. Pyrrolizidine alkaloid exposure from plants such as ragwort, groundsel, or fiddleneck can cause chronic liver damage. Contaminated hay or pasture, drought-related plant exposure, and some mycotoxins can also contribute. Viral hepatitis is another important category, including equine parvovirus-hepatitis and equine hepacivirus, although not every infected horse develops clinical liver disease.

Some horses develop high blood ammonia with neurologic signs from unusual metabolic or gastrointestinal conditions, and those cases can look similar. That is one reason your vet may recommend a broad diagnostic workup instead of assuming every neurologic horse has primary brain disease.

Triggers can also make signs more obvious in a horse that already has liver dysfunction. High-protein meals, dehydration, constipation or slowed gut movement, gastrointestinal bleeding, and some medications may worsen toxin buildup and make neurologic signs more noticeable.

Diagnosis starts with an urgent physical and neurologic exam, plus a careful history. Your vet will want to know when the signs started, whether they seem worse after eating, what feeds and supplements your horse receives, whether there has been access to weeds or moldy hay, and whether any biologic products or plasma were given in recent months.

Bloodwork is a key first step. A chemistry panel may show changes in liver-associated enzymes such as SDH, GLDH, AST, and GGT, along with bilirubin abnormalities. Bile acids help assess liver function, and ammonia testing can support the diagnosis when available and handled correctly. Because liver disease can affect clotting and blood sugar, your vet may also check coagulation values, glucose, CBC results, and other supportive tests.

Imaging often follows. Ultrasound can help assess liver size, texture, and whether a biopsy is feasible. In selected horses, liver biopsy gives the most useful information about the type and severity of disease. If viral hepatitis is suspected, your vet may recommend PCR testing on blood and sometimes liver tissue. These tests help separate liver-related neurologic disease from other emergencies such as EPM, rabies, toxicities, trauma, or severe metabolic disease.

In short, hepatic encephalopathy is usually diagnosed by putting several pieces together: neurologic signs, evidence of liver dysfunction, and testing aimed at finding the underlying cause. That cause matters because prognosis can vary widely between reversible inflammation, toxic injury, congenital shunting, and end-stage liver failure.

Prevention focuses on preventing liver disease in the first place. Walk pastures regularly and remove known toxic plants, especially pyrrolizidine alkaloid-containing weeds such as ragwort, groundsel, and fiddleneck. Check hay quality closely, store feed to reduce mold growth, and avoid feeding anything that smells musty, looks contaminated, or has an unknown weed content.

Use medications, dewormers, supplements, and herbal products thoughtfully and only with veterinary guidance. If your horse has known liver disease, follow your vet's feeding plan carefully and do not make sudden diet changes. Some horses need controlled protein intake and closer monitoring after any change in forage, concentrate, or supplement routine.

Routine wellness care also matters. Horses with unexplained weight loss, jaundice, photosensitization, poor appetite, or behavior change should have bloodwork sooner rather than later. Catching liver disease before neurologic signs develop gives your vet more room to act.

There is not one vaccine or one supplement that prevents hepatic encephalopathy itself. The best prevention is early recognition of liver problems, reducing toxin exposure, and building a practical monitoring plan with your vet if your horse already has liver disease or a history of hepatitis.