Identify and diagnose: Liver disease in horses
The liver is the largest organ in the body and has many important jobs. It constitutes approximately 1 percent of the horse’s body weight and is located in the front of the abdomen between the diaphragm and stomach.
According to Dr. Jared Janke (Sturgis Veterinary Hospital at Sturgis, S.D.) the main functions of the liver include filtration and storage of blood, metabolism of carbohydrates, proteins, fats, hormones and foreign chemicals (including medications); formation of bile (for digestion), vitamin and iron storage, and formation of coagulation factors.
“When discussing abnormalities of the liver, we lump together the liver and the biliary system which includes the microscopic internal ducts that transport bile to the external ducts. Unlike you and I, the horse does not have a gallbladder and is unable to store bile,” says Janke, a large animal internal medicine specialist, board-certified by the American College of Veterinary Internal Medicine.
The small amount of bile needed for equine digestion (feeds containing very little fat) is continually put into the digestive system through bile ducts.
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Plants that cause liver problems – Since the liver is the main filtering system for the body, any toxic substances ingested by the horse, such as toxic plants, may damage the liver. Alsike clover and red clover may also cause problems in horses. “The toxic principle in clover is unknown, but a diet containing 20 percent alsike clover for two weeks will induce signs of liver disease in horses,” Janke says.
“The most common pasture or feed-related toxins include pyrrolizidine alkaloids (PA), such as found in various species of Crotalaria, Senecio, and Amsinckia, and in Heliotropium europaeum, Echium plantagineum and Cynoglossum officinal. Though all of these plants are potentially present in our region, Senecio species (Prairie Ragwort, Lambstongue/Groundsel, and Riddell’s Ragwort) are the most common. There are at least 17 species of Senecio on the Great Plains,” says Janke.
“These plants are generally unpalatable to horses, but palatability increases with weather extremes such as drought or frost. The alkaloid remains toxic despite drying,” he explains. Horses may suffer liver problems if they eat these plants baled in hay.
“Toxicity is cumulative over the horse’s lifetime. Consumption of two to five percent of the horse’s body weight may result in liver disease. The horse can have liver disease/damage without having liver failure. The liver is resilient and can perform adequately until about 80 percent of its function is impaired. Regeneration of damaged liver is possible, as long as fibrosis (scarring) has not developed or worsens—but this is generally what happens with pyrrolizidine alkaloid toxicosis and leads to a poor prognosis,” he says.
Another group of plants that may cause liver damage is Pancium. “Pancium coloratum (Kleingrass) was introduced to Texas in 1952 from South Africa. Pancium virgatum (Switchgrass) and witchgrass are common in our region. Pancium dichlotomilflorum is native to North America and can be found throughout most of the U.S. It grows in moist areas along streams, flood plains, and wet clearings but also grows as a weed in cultivated areas. This plant is a bright green annual that grows six to seven feet tall,” says Janke. With modern agricultural practices and hay movement, almost any plant can be transferred into a new region.
Mycotoxins and aflatoxins – Mycotoxins may be present in moldy feeds, and are not necessarily a pasture problem. “Most mycotoxins occur in stored grains. Most commercially available feeds are screened for these, but not all. Aflatoxins are produced by two strains of the fungus Aspergillus,” says Janke. Aflatoxin B1 is the most toxic, reacting with the body cells and disrupting organ function. “Feed quality and the nutritional plane of the horses play a role in severity of disease,” he says.
Signs of toxicity – “Liver disease can be challenging to diagnose. In most cases clinical signs are vague and non-specific until advanced liver dysfunction occurs. Early signs are often missed. As the disease progresses, the more classic signs appear such as yellowing (jaundice/icterus), photosensitization, abnormal blood clotting (tiny hemorrhages), abdominal distention, limb swelling, mental/behavioral changes, seizures, blindness,” Janke says.
Clinical signs of liver damage from PA develop several weeks to months after the horse consumes sufficient quantity of the toxic plant to cause chronic liver disease. The alkaloid damages the DNA in liver cells, preventing normal regeneration, and leads to fibrosis. “No specific treatment exists, but the horse’s life may be prolonged with good supportive care and appropriate nutrition,” says Janke. “Although there are reports of horses surviving PA intoxication, the prognosis is generally poor.”
If alsike clover is the problem, clinical signs of acute toxicity include lethargy, depression, altered mental status, head-pressing, and behavioral changes. “Signs of chronic disease include ill thrift, lack of appetite, jaundice, and lethargy,” he says.
“Hepatic encephalopathy is a syndrome associated with neurologic dysfunction resulting from any type of severe liver dysfunction. One of the most important roles of the liver is the conversion of ammonia to ammonium. Ammonia is a byproduct of protein digestion. The liver converts the ammonia (which is toxic to the brain) to ammonium to be safely excreted from the body. If liver dysfunction/failure occurs, this process is not efficient and blood ammonia levels rise—ultimately causing toxic injury to portions of the brain.”
“Treatment is focused on supportive care and removal of clover from the pasture and/or hay. The prognosis is fair, depending on the degree of fibrosis,” says Janke. If fibrosis is not excessive the liver may regain function.
Liver disease due to mycotoxins and aflatoxins produces subtle clinical signs that include weight loss, lack of appetite and depression. “These signs may progress to anemia, jaundice and perhaps hemorrhage beneath the skin,” he says.
“No antidote or specific treatment exists for aflatoxicosis beyond prompt removal of the contaminated feed. Optimizing quality of diet (with particular attention to protein, vitamins and trace minerals) will aid in recovery but won’t undo the damage already done. The effects of repeated or long-term exposure may leave permanent organ damage. Treatment will depend on the clinical condition and liver function support,” Janke says.
Diagnosis and treatment – “Liver disease is suspected from results of various blood screening tests, imaging such as ultrasound, and a liver biopsy. In most cases this suspicion is made in conjunction with appropriate clinical signs and evidence of liver injury on lab work, or evidence of liver dysfunction on specific liver function tests. A liver biopsy is almost always required to determine the underlying cause. A biopsy can be obtained in the standing horse with ultrasound guidance. Unfortunately, most toxic liver insults have come and gone by the time of diagnosis. Identifying a specific toxin is almost always impossible,” says Janke.
Liver biopsy is also the best tool for prognosis. “Prognosis is guarded in the short-term and grave in the long-term, especially if the biopsy reveals severe fibrosis. Treatment is largely supportive and includes dietary management, and removal of the offending toxins (if still present). Diets low in protein and high in energy are desired.”
Feeding a horse that has poor appetite can be a challenge. “The major goals are to meet the horse’s energy requirements without exceeding dietary protein needs. This may mean feeding a high-starch diet,” Janke says. Supplementation with water-soluble vitamins (B complex and C) should be considered.
“Forage choices should include grass hay or mature pasture, avoiding lush green grass and alfalfa with high protein content. The ideal diet for the liver patient may not be very palatable; it may be difficult to get the horse to eat enough if he has poor appetite. I prefer a critically ill horse to eat something—even if it’s not the recommended diet. In severe cases, the horse may initially require forced feeding or intravenous nutrition.”
“Numerous homeopathic remedies and nutraceuticals have been used in treating horses with liver disease, but most of these lack clinical evidence of efficacy and many have no safety data,” he says.
Certain medical management strategies may help stabilize the patient and prolong the horse’s life, even though they won’t cure the underlying problem. “If liver failure has not yet occurred, the horse may continue to function with minimal outward signs. Many horses continue a relatively normal athletic life in the early stages, but this picture changes once clinical signs are noticed.”
Preventing liver disease is usually a matter of awareness and avoidance of toxic plants. “Local extension agents can help you identify suspicious pasture plants and eradicate them. Horse owners should also inspect all hay for toxic plants and purchase from reputable suppliers,” says Janke.
“Any confirmed case of pasture or feed-related liver toxicosis should warrant evaluation of pasturemates. Interestingly, multiple cases of pasture-associated liver disease are uncommon even though other horses in the pasture have similar exposure to toxic plants. A band of horses on poor pasture might be supplemented with quality hay, for instance, but some horses at the bottom of the pecking order may not get their share of the hay and may be forced to consume unpalatable and possibly toxic pasture plants.”
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