Liver health is paramount to the overall well-being and performance of horses. The liver is one of the largest and most vital organs in the equine body. [1]

This organ plays a crucial role in numerous physiological processes including metabolism, digestion, detoxification, and immune function. [2]

Although the equine liver possesses a significant ability to regenerate itself, it is still susceptible to a variety of conditions and diseases that can significantly impact a horse’s health and quality of life.

Liver dysfunction in horses can arise from various causes, including exposure to toxins, infectious agents, inflammatory conditions, and other health issues such as Theiler’s disease. [2]

Recognizing signs of liver disease, such as jaundice or behavioral changes, is key to early intervention. Because some forms of liver disease are life-threatening, caretakers must also take steps to prevent infectious and toxic forms of liver damage in their horses.

Keep reading to learn everything you need to know about symptoms of liver disease in equines and steps you can take to protect your horse’s health in the long-term.

Liver Health in Horses

Healthy liver function is essential in horses due to the crucial role this organ plays in maintaining overall well-being.

The liver is a large organ housed in the abdominal cavity adjacent to the stomach. It carries out more than 500 functions in the horse’s body, including processing nutrients, synthesizing important compounds, and regulating hormone levels. [3]

Any compromise in liver health can lead to a multitude of issues, including poor digestion, impaired immune function, and metabolic disorders. [2]

Key Functions of the Liver

The liver performs a wide range of functions that help maintain health and well-being, including: [2][3][4]

  • Metabolism: the liver excretes bile, which helps break down fats, carbohydrates, and proteins in the gastrointestinal tract
  • Filtration: the body’s blood supply is constantly filtered through the liver where toxins are neutralized and broken down
  • Storage: converts vital nutrients into different forms and stores them, including vitamins, minerals, amino acids, glucose, and triglycerides
  • Synthesis: plays a pivotal role in protein synthesis; produces coagulation factors necessary for blood clotting; makes non-essential amino acids
  • Immune support: influences immune responses and contributes to the development of blood cells
  • Blood sugar regulation: serves as a major storage site for glycogen, the stored form of glucose the body uses for energy
  • Homeostasis: adjusts levels of carbohydrates, proteins, and lipids entering the bloodstream from the gastrointestinal tract to ensure consistent nutrient levels in the blood

Liver Anatomy

The liver is the second largest organ in horses. It is positioned on the right side of the abdominal cavity and typically sits about three to four inches above the abdominal floor, near the ventral end (underside) of the seventh or eighth ribs. [7]

The equine liver is underdeveloped at birth, which makes foals more vulnerable to toxins. [5] In adult horses, it accounts for approximately 1.5% of the total body weight at approximately 5 to 6 kg (11 – 13 lb). [6]

The liver receives 70% of its blood supply from the hepatic portal vein, which brings in nutrients from the stomach, intestines, and other organs for processing. [1][6]

The liver is made up of tiny units called hepatic lobules. [1] These lobules are like small compartments made up of liver cells and blood vessels. Blood from different sources mixes in these compartments and provides oxygen and nutrients to the liver cells. [1]

Causes of Liver Health Issues in Horses

A range of health issues can result in liver damage and dysfunction, and these issues can disrupt the critical biological functions that the liver plays.

Toxins

Toxicity or poisoning is one of the most recognizable forms of liver disease in horses. Common poisonous hazards for horses include: [2][8][9][10][11]

  • Plants containing pyrrolizidine alkaloids, such as ragwort
  • Plants with unknown or poorly understood toxicology, such as alsike clover
  • Mycotoxins (e.g. fungal or mold toxins such as aflatoxin, zearalenone, and fumonisin)
  • Drugs (phenylbutazone, flunixin, acetaminophen, salicylates, and antifungals)
  • Other chemicals

Infectious Agents

Some forms of infectious disease are associated with liver damage, including: [2][9][11][12][13]

 

Theiler’s Disease

Also known as serum sickness, Theiler’s disease can lead to a sudden onset of liver failure. It may occur several weeks following exposure to a product containing horse serum such as tetanus antitoxin.

The condition is also associated with viral agents and other unknown causes, but these contributing factors require further study to be fully understood. [2][9][14]

Other Conditions

Liver damage is sometimes secondary to other conditions, or it can be vague and non-specific. Other possible causes associated with liver disease in horses include: [2][9]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants | Mad Barn USA

Symptoms of Liver Dysfunction

Symptoms of liver dysfunction in horses are often nonspecific and vary depending on the severity and duration of the underlying condition. Typically, clinical signs become apparent only when 80% or more of the liver is damaged. [2]

Early signs of liver dysfunction that may be subtle and overlooked include: [2]

More obvious indications of liver disease in horses include:

Jaundice

Jaundice, also known as icterus, is characterized by yellowing of the skin, mucous membranes, and the whites of the eyes. It occurs when there is an excess of bilirubin, a yellow pigment produced during the normal breakdown of red blood cells.

Jaundice can be a symptom of various underlying medical conditions in horses, such as liver disease, hepatitis, or obstruction of the bile ducts. [2]

Behavior Changes

Hepatic Encephalopathy (HE) is a neurological disorder that occurs because of liver dysfunction. HE leads to disruptions in blood chemistry that affect typical brain functioning. [15] While the exact cause of the condition is poorly understood, elevated levels of ammonia found in these cases are believed to damage nervous system cells. [15]

HE is characterized by a range of symptoms such as lethargy, head pressing, circling, lack of coordination, difficulty swallowing, and even seizures or coma. [15] If Hepatic Encephalopathy leads to liver failure, affected horses may display loud breathing sounds and difficulty breathing due to throat structure collapse. [15]

Skin Changes Due to Photosensitization

Photosensitization describes increased skin sensitivity to sunlight due to increased levels of phylloerythrin in the bloodstream, which is normally kept at safe levels by the liver. Photosensitization can lead to itching, skin inflammation, ulceration, and even blindness or death in severe cases. [9][16]

It can result in skin damage upon exposure to ultraviolet light, particularly in unpigmented or light-skinned areas. [16]

Colic

Many horses with liver disease show symptoms of colic, the hallmark of equine abdominal pain.  Abdominal pain from liver disease typically results from inflammation in the abdominal cavity. [1]

Inflammation activates nerve cells to produce pain signals, causing discomfort. [1] In some cases, obstruction of bile outflow can also trigger pain and discomfort, as bile accumulates within the liver and causes compression. [1]

Other Symptoms

Less common signs of liver dysfunction in horses include: [2][17]

  • Fever
  • Difficulty breathing
  • Diarrhea
  • Constipation
  • Abnormal bleeding

Diagnostic Methods

In addition to a physical exam and medical history, a veterinarian has multiple tools to diagnose and guide treatment of liver disease in horses.

Blood Tests

Blood tests are crucial for diagnosing liver diseases in horses. These tests include serum testing for presence of specific liver enzymes, including: [2][18][19][21]

  • Sorbitol Dehydrogenase (SDH)
  • Aspartate Aminotransferase (AST)
  • Lactate Dehydrogenase (LDH)
  • Gamma-Glutamyl Transferase (GGT)
  • Alkaline Phosphatase (ALP)

Serum bile acid concentrations, bilirubin levels, and blood ammonia levels also provide valuable insights into liver function and the presence of liver disease. [18]

Elevated levels of these markers may indicate liver dysfunction, but further evaluation may be needed to determine the specific type of liver disease present. [18]

Liver Ultrasound

Ultrasound of the liver is a non-invasive and safe diagnostic tool used to assess liver health in horses presenting with clinical signs of hepatic dysfunction.

The technique involves imaging both sides of the abdomen to capture the entire liver. It provides valuable information on the liver’s size, shape, location and many abnormalities. [2][19]

Ultrasound can also help to identify any changes in liver function and biliary structure (bile ducts and associated structures that are involved in the production and transportation of bile). [2][19]

Liver biopsy

A biopsy is a procedure where a sample of tissue is removed and examined under a microscope. Liver biopsy is often necessary for a conclusive diagnosis in horses showing signs of liver disease.

Since this procedure requires extracting a sample of tissue with a needle, veterinarians often use ultrasound while performing a liver biopsy to minimize the risk of obtaining tissue from unintended organs like the colon or lung. [2][19]

Preventive Measures

There are many feeding and management steps that horse owners can take to reduce the risk of liver issues in their equines. Always work with a veterinarian if your horse is showing signs of illness before making changes to their routine, feed, or medications.

Milk Thistle

5 stars
95%
4 stars
3%
3 stars
2%
2 stars
0%
1 star
0%

Learn More

  • Supports liver function
  • Promotes hoof health
  • Antioxidant support
  • Used in metabolic horses

Nutrition

The liver plays a crucial role in processing ammonia from protein breakdown, but excess protein can strain the liver. If your horse has liver issues, ensuring their dietary protein intake meets their requirement without significantly exceeding it can prevent overtaxing the liver. [20]

Consult with a qualified equine nutritionist to evaluate protein concentrations in your horse’s diet. If necessary, consider replacing high-protein forages like alfalfa and fresh spring grasses with grass hay and fall pasture grazing to reduce the protein load on the liver. [20]

Increasing the frequency of meals throughout the day can also help distribute feed intake and prevent large spikes in nutrients reaching the liver after each meal. This can reduce the strain on liver function following a meal. [20]

Avoid supplements containing added iron, as iron toxicosis can lead to liver damage over time. [20]

Avoid Toxins

Plants known to adversely affect the equine liver include:

  • Alsike clover
  • Pancium grass
  • Plants containing pyrrolizidine alkaloids like ragwort, fiddleneck, and rattlebox

Ingesting significant quantities of these plants can lead to acute liver failure. [8] Prompt veterinary intervention is crucial to assist affected horses in overcoming the toxic exposure. However, the prognosis remains guarded for many cases of liver damage due to toxicity.

Effective pasture management plays a pivotal role in preventing horses from encountering harmful plants. Your local department of agriculture can provide resources on poisonous plants along with visual aids to aid in identification within pasture areas.

Regular Deworming

Following a strategic deworming program can help to prevent parasitic infections that can lead to liver damage in horses.

For example, large strongyles and roundworm larvae migration occur in the liver. [24] This causes significant damage to the organ through inflammation and scarring.

Supplements

Milk Thistle, also commonly known as St. Mary’s Thistle, consists of dried, powdered seeds from the Silybum marianum plant. This Mediterranean herb is rich in silymarin, a group of active compounds known for their beneficial effects on liver health according to research in humans. [21][22][23]

Silymarin acts as a potent antioxidant. It is particularly beneficial during times of liver injury caused by toxins, fat buildup, or excessive iron intake. [21][22][23]

Milk Thistle supplementation also supports normal lipid metabolism to maintain healthy levels of fat accumulation in the liver, which could benefit horses at risk of hyperlipemia. This condition is characterized by excess fat accumulation in the liver. Milk thistle has shown promise in supporting normal liver function in humans facing similar challenges. [23]

Treatment Options

Treatment goals for horses with liver disease or failure include addressing the underlying disease and providing supportive care for liver regeneration. [21]

While initial treatment may begin before the full extent of liver damage is known, specific therapies depend on factors such as the cause, presence of liver failure, and degree of liver fibrosis (scarring) or bile duct obstruction.

Early intervention yields the most success, particularly when liver fibrosis is minimal, and signs of liver cell regeneration are evident. Horses with severe fibrosis typically respond poorly to treatment due to diminished potential for healthy liver cell regeneration. [21]

Treatment strategies may involve: [21]

  • Removing exposure to liver toxins
  • Administering intravenous fluids
  • Implementing a low-protein diet
  • Minimizing fat in the diet
  • Feeding frequent small meals containing easily digestible carbohydrates predominantly from fiber
  • Providing adequate but not excessive calories
  • Prescribing medications to reduce ammonia production in the gastrointestinal tract
  • Administering broad-spectrum antibiotics if a liver infection is suspected
  • Managing pain with nonsteroidal anti-inflammatory drugs (NSAIDs)
  • Feeding additional antioxidants or vitamin supplements
  • Providing protection from sunlight if photosensitization has occurred

Summary

Protecting and supporting liver health is vital for your horse because the liver is involved in many different biological functions that impact their overall well-being.

  • Key functions of the liver in horses include metabolic processing, detoxification, bile production, and nutrient storage
  • Liver dysfunction in horses can result from exposure to toxins, infectious agents, inflammatory conditions, and other health issues
  • Symptoms of liver dysfunction in horses include jaundice, weight loss, behavioral changes, and skin changes; recognizing these signs is essential for early intervention
  • Diagnostic methods including blood tests, liver ultrasound, and biopsies aid in assessing liver health
  • Preventive measures such as dietary adjustments, avoiding toxins, and regular deworming can help maintain liver health in horses
  • Treatment options for liver conditions in horses may involve supportive care, medication, dietary modifications, and protection from sunlight

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. The Equine Liver in Health and Disease. IVIS. 2020.
  2. Disorders of the Liver in Horses. Merck. 2019.
  3. Kellon E.M. et al., The Horse Liver. Forageplus Talk. 2022.
  4. Table: Functions of the Liver in Horses. MSD Veterinary Manual. 2024.
  5. Wong D. et al., Embryology and Anatomy of the Digestive Tract. In: Equine Neonatal Medicine. John Wiley & Sons, Ltd. 2024.
  6. Mair T.S., Liver Function. In: The Equine Acute Abdomen. John Wiley & Sons, Ltd. 2017.
  7. Poddar S., Horse Liver Anatomy.AnatomyLearner. 2024.
  8. Common Liver Toxins that Affect Horses. | EquiMed. 2021.
  9. Surgeons R.V., Liver Disease. 2024.
  10. Liver Disease in Horses. Horse Sport 2018.
  11. Plant Poisoning and the Liver: Hepatotoxicty. Horseadvice. 2023.
  12. Infectious Hepatitis and Hepatic Abscesses in Large Animals – Digestive System. Merck. 2023.
  13. Prevalent Parasites: Common Types of Equine Internal Parasites. PennState Extension. 2024.
  14. Tomlinson J.E. et al., What Do We Know About Hepatitis Viruses in Horses?. Vet Clin North Am Equine Pract. 2019. View Summary
  15. Divers T., Equine Hepatic Encephalopathy. In: Equine Neurology. John Wiley & Sons, Ltd. 2015.
  16. Photosensitization in Horses. Merck. 2019.
  17. Anemia in Horses – Horse Owners. Merck. 2019.
  18. Satué K. et al., Biochemical and Hematological Indexes of Liver Dysfunction in Horses. Journal of Equine Veterinary Science. 2023. View Summary
  19. Themes U.F.O., Hepatic Diseases in the Horse. Veterian Key. 2016.
  20. Overview of Hepatic Disease in Large Animals – Digestive System. Merck. 2023.
  21. Mulrow C. et al., Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects: Summary. In: AHRQ Evidence Report Summaries. Agency for Healthcare Research and Quality (US). 2000.
  22. Hutchinson C. et al., The Iron-Chelating Potential of Silybin in Patients with Hereditary Haemochromatosis. Eur J Clin Nutr. 2010.
  23. Cacciapuoti F. et al., Silymarin in Non Alcoholic Fatty Liver Disease. World J Hepatol. 2013.
  24. Common Internal Parasites of the Horse . Oklahoma State University Extension. 2017.