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Rhubarb, or Rheum rhabarbarium, is a perennial vegetable plant found in fields and gardens. Grown for its shiny, red stalks, it boasts large heart-shaped leaves that can be up to 2 feet long and 1 ½ feet wide. Leaves have a shiny, smooth surface with five main veins. In late spring, flower clusters in green and white form at the end of the stems.
Although rhubarb is known to many as a wonderful pie additive, it can be lethal if ingested by horses. While the stems are edible, the sour leaves contain potent toxins that can irritate the digestive system, and cause severe kidney damage, resulting in kidney failure and death.
Ingestion of rhubarb leaves causes damage to the digestive and urinary systems, as the electrolyte balance in the body is disrupted and the kidneys fail. Chronic consumption of low doses of the leaves can be difficult to notice, but can often cause swellings in the head. Signs include:
Rhubarb poisoning can be defined as acute or chronic.
Acute Rhubarb Poisoning
This is when large amounts of rhubarb leaves are consumed at once, or in a very short amount of time. The toxins directly affect the digestive tract, the kidneys, and urinary tract, and can lead to kidney failure and death. Symptoms that are common for this type of poisoning are jaundice, increased drinking and urinating, depression, tremors and convulsions, and other signs that the kidneys are shutting down.
Chronic Rhubarb Poisoning
This type of poisoning occurs when rhubarb leaves are ingested over a long period of time, generally in small doses. Decreased levels of calcium caused by the toxins in rhubarb cause overactivity of the parathyroid glands as they try to balance the body’s levels. This causes secondary hyperparathyroidism, seen in symptoms such as lameness, weight loss, and various swellings in the head and face.
The cause of rhubarb poisoning is ingestion of the leaves, which are the only toxic part of the plant. The leaves contain oxalic acid, which combines with calcium in the blood. This produces insoluble calcium oxalate crystals. These crystals are deposited into the kidneys and can form kidney stones, which then cause damage to the kidneys and the urinary tract. The calcium oxalate crystals also bind up the available calcium, which upsets the balance of electrolytes in the body. This lack of available calcium causes the parathyroid gland to overproduce the parathyroid hormone, and the body begins to leach calcium from the other areas of the body, such as bones. Other components in rhubarb that may contribute to toxicity are anthraquinones and other glycosides.
A diagnosis is based on the symptoms and a history of rhubarb consumption, along with the results of testing. Tests can include blood and serum tests, a urinalysis, fecal tests, and microscopic examination of tissues, such as biopsied kidney tissue. Usually, these types of tests can reveal the presence of calcium oxalate crystals, or can determine if calcium levels are unusually low. If you know your horse has ingested rhubarb, bring a sample in to your veterinarian for a both a positive identification, and a gas chromatography test which can show the presence and amounts of oxalates.
If you do not know that your horse has eaten rhubarb, your veterinarian may run more tests to determine the reason for the kidney problems, such as X-rays, ultrasounds, and the collection of more samples to be analyzed.
Treatment is often symptomatic and supportive, and seeks to remove the source of the oxalates while stabilizing your horse. Removal of rhubarb from areas your horse frequents is paramount to prevent further damage.
If a diagnosis is made before the kidneys fail, fluid diuresis is administered for up to 48 hours. Parenteral calcium is given to correct low calcium levels. Continued oral calcium is given to prevent oxalate absorption by binding to them, thereby reducing kidney stones, and may be needed for many months if hyperparathyroidism is present. The diet may be adjusted to ensure the electrolyte levels remain balanced as your horse recovers.
Recovery of your horse from a case of rhubarb poisoning is good if treatment is quick and early in the progression of the condition. In severe cases where the kidneys have been compromised, prognosis can be poor. In cases where secondary hyperparathyroidism is present, it can take many months to reverse the damage. You may need to adjust your horse’s diet and provide supplementation during that time, and your veterinarian may also need to monitor your horse’s progress through testing. Your horse can recover from hyperparathyroidism, but the swellings may not completely resolve.
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