Hepatitis in horses refers to inflammation of the liver, which can result from various causes, including infections, toxins, or autoimmune diseases.
The liver is a vital organ responsible for numerous functions, including detoxification, metabolism, and nutrient storage, so liver disease can have serious consequences for a horse’s health.
Symptoms of hepatitis in horses include jaundice, lethargy, weight loss, behavioral changes, exercise intolerance and photosensitivity (a skin reaction when exposed to sunlight).
The prognosis for horses with hepatitis depends on the underlying cause, the severity of liver damage, and how early treatment begins. If you suspect your horse may have hepatitis, it’s essential to seek veterinary care promptly to diagnose and treat the condition effectively.
Hepatitis in Horses
Hepatitis refers to inflammation of the liver, and is a non-specific term used to describe liver disease. [2] The liver is the largest organ in the horse’s body, making up one percent of its total body weight. [1]
This vital organ is responsible for many critical functions including metabolism of toxins, medications, and fats. It also aids in the production of red blood cells (hematopoiesis) and stores glucose and other sugars in the form of glycogen. [2][3][4]
Some forms of liver disease are progressive and lead to liver failure. [4] The liver has the ability to regenerate; if cell loss is slow, liver failure does not necessarily occur. [1]
If liver cell loss occurs gradually, liver failure may be avoided. The regenerative capacity of liver cells means that certain liver diseases can be reversible, depending on the location, duration, pattern, and extent of the damage. [1]
By understanding the different underlying causes of hepatitis, horse owners and caretakers can take proactive steps to prevent liver failure, keeping their herd healthy and happy for years to come.
Liver Anatomy
The liver is located between the gastrointestinal tract and the heart of the horse, contained within the rib cage. The front portion of the liver is located in the front third of the intercostal spaces (i.e. space between the ribs) and reaches towards the back of the right kidney. [1]
The liver is made up of two sides: visceral and diaphragmatic. It is divided into four lobes by fissures. Within the visceral surface, the hilum (also referred to as “the door”) allows nerves, blood vessels and lymphatics to enter, while the hepatic duct exits the liver. [1]
The hepatic portal vein, bile duct and hepatic artery make up the hepatic triad. The hepatic portal vein brings deoxygenated blood into the liver for processing while the hepatic artery brings oxygenated blood to the liver to support its functions. [5]
Liver Disease
Liver cells (hepatocytes) have the ability to self-multiply (proliferate); however, when the liver experiences chronic injury this process can be compromised. Instead, biliary and progenitor cells begin to proliferate and form ductules around the portal veins. These cells may differentiate into hepatocytes as they develop. This process is the liver’s way of trying to regenerate and repair itself. [5]
If the liver cannot repair itself, different forms of tissue damage can arise, including: [2]
- Fibrosis: refers to the scarring of the liver. Fibrosis occurs when there is an increase in fibroblasts (cells that produce connective tissues and collagen) and collagen in the liver tissue
- Cirrhosis: refers to extensive fibrous deposits in the liver, typically during end-stage liver disease
- Necrosis: refers to cell death; this occurs when liver failure has progressed to the point where the cells are no longer able to regenerate and repair the hepatic tissue
Hepatic Encephalopathy
Hepatic Encephalopathy (HE) is a neurological disorder in horses that can be caused by a number of liver diseases. When the liver is diseased and unable to detoxify the blood, toxins such as ammonia build up in the bloodstream resulting in neurological damage. [2]
Symptoms of HE include: [2]
- Ataxia (incoordination)
- Aggressive or vicious behavior
- Difficulty swallowing
- Sleepiness
- Circling
- Seizures
- Coma
- Pica (eating non-food items)
Causes
The literal definition of hepatitis is “liver inflammation.” In veterinary medicine, the term is often used as a general term for any form of liver disease, since inflammation is the primary effect of most hepatic disease processes and leads to further damage if left untreated.
Horses are at risk of a number of conditions associated with hepatitis, including chronic and acute causes.
Chronic Conditions
Chronic hepatitis refers to cases where the inflammation is present consistently for an extended period of time. There are a number of specific chronic conditions associated with equine hepatitis including:
- Chronic active hepatitis
- Pyrrolizidine alkaloid toxicity (Ragwort poisoning)
Chronic Active Hepatitis
Chronic active hepatitis (CAH) is a progressive, chronic liver disease in horses. CAH is idiopathic, meaning the underlying cause is unknown or cannot be determined.
The signs of CAH are subtle and consistent with progressive liver failure. Further study is required to determine the underlying disease process of CAH. One theory is equine CAH may be linked to an autoimmune disease, similar to CAH in humans. [1]
Symptoms of CAH in horses include: [1]
- Lethargy
- Weight loss
- Loss of appetite
- Colic
- Jaundice
- Fever
- Exercise intolerance
- Moist, exfoliative coronary dermatitis (uncommon)
Diagnosis of the CAH typically involves a combination of bloodwork, liver biopsy, and immunohistologic staining to accurately identify and assess the extent of disease. Treatment primarily consists of supportive care and administration of corticosteroids aimed at managing symptoms and preventing further deterioration.
The prognosis for affected horses can vary significantly depending on the severity of the condition. If cirrhosis is present, the prognosis is more guarded, as it indicates advanced liver damage that is often irreversible.
Ragwort Poisoning
There are a number of plants that can be toxic to horses and cause liver damage. Plants containing pyrrolizidine alkaloids typically lead to liver damage. However, a horse must consume a large quantity or have ongoing, chronic intake for toxicity to occur. [6][7]
Pyrrolizidine alkaloid-containing plants include: [7]
- Ragwort
- Groundsel
- Rattleweed
- Yellow Tarweed
- Ryegrass
Clinical symptoms include: [7]
- Loss of appetite
- Weight loss
- Photosensitivity (skin reaction to sunlight)
- Jaundice
- Exercise intolerance
- Oral ulcers
- Bad breath
- Diarrhea
The diagnosis of the Ragwort poisoning in horses is challenging because symptoms usually do not begin until months after ingestion has occurred. Further, the severity of poisoning often necessitates medical intervention before a full diagnosis is possible, since clinical signs do not arise until liver damage has already set in.
If diagnosed, ragwort poisoning is confirmed through bloodwork and liver biopsy. Treatment focuses on supportive care, with some cases benefiting from a low-protein diet during recovery. The prognosis depends on the extent of liver damage, but in many cases, prognosis is grave and humane euthanasia is the only option. [7]
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Acute Conditions
Acute hepatitis refers to cases where the inflammation has a sudden and severe onset. There are a number of specific acute conditions associated with acute equine hepatitis including:
- Theiler’s disease
- Cholangiohepatitis
- Infectious Necrotic Hepatitis
- Tyzzer’s Disease
- Viral Hepatitis
- Parasitic Hepatitis
Theiler’s Disease
Theiler’s Disease (TD) is the most common cause of acute hepatitis in adult horses. TD is also known as: [1]
- Serum sickness
- Acute hepatic necrosis
- Serum-associated hepatitis
The exact cause of Theiler’s disease is not well understood. It was previously believed to be a complication of biologic therapies, such as tetanus antitoxin (TAT) or other serum or plasma injections that contain a commercial equine plasma. [1][6]
More recently, researchers have discovered Theiler’s is likely caused by equine parvovirus-hepatitis (EqPV-H), but more study is required to determine how it is transmitted between horses. [8][9][10]
Symptoms of Theiler’s Disease in horses include: [1]
- Jaundice
- Loss of appetite
- Fever
- Photodermatitis
- Colic
- Sudden death
Diagnosis is suggested if the affected horse recently received a biologic. Diagnosis is confirmed using bloodwork, urinalysis, and liver biopsy. Treatment focuses on supportive care, including IV fluids and antibiotics. [1]
The prognosis varies depending on severity. In cases where affected horses have developed secondary hepatic encephalopathy, the prognosis is poor. [1]
Cholangiohepatitis
Cholangiohepatitis refers to inflammation of the bile ducts in the liver, which can lead to liver failure. Cholangiohepatitis is associated with parasitic migration, bacterial infection and small intestinal ulcers. [2]
Common causes of this condition in horses include: [1][2]
- Intestinal obstruction
- Intestinal inflammation (enteritis or colitis)
- Toxins
- Crystals (stones) in the bile duct
- Lack of appetite
- Fever
- Jaundice
- Weight loss
- Photosensitivity
- Colic
Diagnosis may include ultrasound, biopsy, and bloodwork. Treatment is primarily supportive care; antibiotics may be prescribed in cases where infection is present. If liver fibrosis is not severe, then the prognosis is favorable. [1]
Infectious Conditions
Infectious causes of acute equine hepatitis may be:
- Bacterial
- Viral
- Parasitic
Infectious Necrotic Hepatitis
Infectious necrotic hepatitis (or black disease) is a fatal infection caused by the bacterium Clostridium novyi type B. Black disease is not commonly seen in horses but occurs frequently in cattle and sheep. Horses that have developed black disease typically live near a large population of sheep. [1]
In cattle and sheep, C. novyi multiply in the liver when there is a parasitic infestation of Fasciola hepatica. Since F. hepatica does not tend to infect horses, it is believed that any parasitic infestation of the liver could make the horse susceptible to black disease. [1]
Symptoms of black disease in horses include: [1]
- Sudden death
- Lethargy
- Fever
- Jaundice
- Colic
- Ataxia (incoordination)
- Petechiae (red spots on gums)
- Rapid breathing
- Reluctance to move
- Tachycardia (rapid heart rate)
Definitive diagnosis before death is not possible, but diagnosis can be confirmed post-mortem on necropsy. There is no treatment for equine black disease and prognosis is grave. [1]
Tyzzer’s Disease
Tyzzer’s Disease is a rare, yet fatal bacterial infection caused by Clostridium piliforme. Nursing foals between 1 – 6 weeks old are at highest risk and usually acquire the infection after eating their dam’s feces. Older foals and adult horses are less likely to develop Tyzzer’s Disease, even if exposed to the bacterium. [2]
Tyzzer’s Disease causes inflammation of the large intestine and heart and damages the liver tissue. [2]
Symptoms of Tyzzer’s Disease in horses include: [1][2]
- Diarrhea
- Coma
- Jaundice
- Lethargy
- Lack of appetite
- Seizures
- Loss of suckle reflex
- Rapid breathing
- Tachycardia (rapid heart rate)
- Fever
Tyzzer’s is difficult to diagnose before death. The only available treatment protocols are supportive care and antibiotics, but the majority of cases are fatal. [2]
Viral Hepatitis
There are four types of viral hepatitis in horses. They include: [1][10][11][12][13]
- Equine herpesvirus-1 (EHV-1): an acute respiratory infection, in pregnant mares EHV-1 can cause late-term abortion or stillbirth. Spreads through direct and indirect contact with placental fluids, placenta, nasal discharge or aborted fetuses.
- Equine infectious anemia (EIA): is a blood-borne virus. Horses that have been infected remain carriers. Infected pregnant mares can pass the infection to foals in utero.
- Equine viral arteritis (EVA): is a togavirus causing abortion, vasculitis and respiratory infection in horses. EVA is spread through the respiratory system as well as venereal routes.
- Giant cell hepatopathy: an idiopathic disease that is suspected to be a viral infection. It is similar to human neonatal hepatitis.
Parasitic Hepatitis
Most cases of parasitic equine hepatitis occur when a horse ingests the larvae of a parasite. The larvae then hatch and mature inside the horse and migrate through the liver. [1]
Species of parasites associated with equine hepatitis include: [1][14]
- Parascaris equorum
- S. vulgaris
- Echinococus granulosa
- S. neurona
- Fasciola hepatica
- Strongylus edentatus
Symptoms
The main symptoms of hepatitis in horses include: [1][2][6]
- Weight loss
- Decreased appetite
- Colic
- Jaundice
- Lethargy
- Head pressing
- Constipation
- Diarrhea
- Photosensitivity
- Abnormal bleeding/clotting
Other symptoms may be present depending on the underlying cause and associated complications.
Diagnosis & Treatment
Diagnosis of hepatitis in horses typically includes: [1]
- Physical exam and medical history
- Bloodwork
- Biopsy
- Diagnostic imaging
Other diagnostics may be recommended based on initial findings and concurrent symptoms. If you suspect your horse has hepatitis, consult with your veterinarian to obtain an accurate diagnosis.
Treatment for hepatitis varies depending on the underlying cause, but may include: [1]
- Antibiotics
- Fluid therapy
- Pain management
- Dietary modifications
- Anti-inflammatories
- Liver support medications
Some forms of hepatitis have no available treatment. In cases with extensive cirrhosis, palliative care or euthanasia may be the only options, even if the underlying cause generally has viable treatment protocols.
Prognosis
The prognosis for hepatitis in horses is highly variable and based on several factors, including the duration, location and the extent of damage to the liver.
Key considerations include:
- Type of hepatitis: acute hepatitis has a more guarded prognosis due to rapid progression and the possibility of sudden death. Chronic hepatitis has a variable prognosis, which depends on the degree of damage and treatment response.
- Complications: complications such as hepatic encephalopathy and secondary infections considerably worsen the prognosis.
- Response to treatment: there is a better chance of recovery if the horse responds well to supportive care, proper nutrition and management of symptoms. If the liver cannot repair itself, the prognosis is grave.
- Extent of liver damage: if there is minimal liver damage and the hepatitis was detected early, the liver has a better chance of repair, which can allow for a better prognosis. If there is extensive damage such as severe fibrosis or cirrhosis, then the prognosis worsens accordingly.
Prevention
Preventing hepatitis in horses involves strategies to reduce the risk of liver diseases and damage, which can be accomplished with robust management practices.
Management strategies to promote overall health and wellness include:
- Avoiding toxin exposure
- Routine blood work, including liver function testing
- Parasite control
- Vaccination
- Proper nutrition
- Pasture management
- Regular veterinary examination
- Robust biosecurity and quarantine protocols, especially during travel and when introducing new horses to the facility
Summary
The liver is a vital organ in horses, primary functions include detoxification, metabolizing medications and nutrients, excretion of bile, aiding in red blood cell production and vitamin storage.
- Hepatitis is a general term for liver inflammation and can lead to liver failure.
- Conditions and diseases of the liver can be chronic, acute, toxic, or infectious.
- Symptoms include weight loss, jaundice, and behavioral/neurological and skin changes.
- Diagnosis of hepatitis include bloodwork, biopsy, and diagnostic imaging.
- Treatment of hepatitis include supportive care and antibiotics.
References
- Reed, S. M., et al. Equine Internal Medicine. 3rd ed. Saunders Elsevier, St. Louis, Mo. 2010.
- Bergero, D., & Nery, J. Hepatic Diseases in Horses. Journal of Animal Physiology and Animal Nutrition. 2008.
- Foreman, J.H. Disorders of the Liver in Horses. Merck Veterinary Manual. 2019.
- Tallon, R., & McGovern, K. Equine Liver Disease in the Field. Part 1: Approach. UK-Vet Equine. 2020.
- Divers, T. The Equine Liver in Health and Disease. Proceedings of the AAEP Annual Convention. 2015.
- Foreman, J. Acute Hepatitis in Large Animal. Merck Veterinary Manual. 2023.
- Bildfell, R. Pyrrolizidine Alkaloidosis in Animals. Merck Veterinary Manual. 2022.
- Lascola, K. Equine Herpesvirus Infection. Merck Veterinary Manual. 2023.
- Timoney, P. Equine Infectious Anemia – Generalized Conditions. Merck Veterinary Manual. 2020.
- Tomlinson, J. E., et al. Viral Testing of 18 Consecutive Cases of Equine Serum Hepatitis: A Prospective Study (2014‐2018). Journal of Veterinary Internal Medicine. 2019. View Summary
- Tomlinson, J. E., et al. Viral Testing of 10 Cases of Theiler’s Disease and 37 In‐contact Horses in the Absence of Equine Biologic Product Administration: A Prospective Study (2014‐2018). Journal of Veterinary Internal Medicine. 2019. View Summary
- Divers, T. J., et al. The History of Theiler’s Disease and the Search for Its Aetiology. The Veterinary Journal. 2022. View Summary
- Lascola, K. Equine Viral Arteritis – Respiratory System. Merck Veterinary Manual. 2023.
- Tallon, R., & McGovern, K. Equine Liver Disease in the Field. Part 2: Causes and Management. UK-Vet Equine. 2020.
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