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Many pitted fruits and grasses contain high levels of cyanide, and can be attractive forage for horses who are hungry, or lack adequate healthy plants to graze. All parts of cyanide containing plants are toxic, however damaged or wilted leaves, pits, young growth, and areas of regrowth contain higher amounts. Plants also become more poisonous during times of environmental stress, such as weather or insect damage, if plant parts are cut, frozen, crushed, trampled, or chewed, and if they are grown in high nitrogen and low phosphorus soils.
Cyanide, also called prussic acid or hydrocyanic acid, can be lethal to horses if ingested in toxic amounts. This often occurs when a horse eats a plant high in cyanide. This fast acting poison prevents vital oxygen from being transported to all the cells in the body. Symptoms occur quickly after consumption of the toxic substance, and progress rapidly from excitement and fast breathing to seizures and death by respiratory failure. Cyanide poisoning can be life threatening, and medical care should be sought immediately.
Signs that your horse is being poisoned by cyanide usually occur within 15 minutes to 2 hours after ingestion in acute cases. Symptoms are seen in rapid succession until breathing stops, causing death of the horse, usually within 2 hours. Cases of chronic poisoning can exhibit different symptoms. Signs of an acute cyanide poisoning include:
Signs of a chronic case of cyanide poisoning include:
Acute Cyanide Poisoning
Symptoms are seen within 15 minutes to a few hours after toxic levels of cyanide have been consumed. It begins with excitement and a fast breathing and heart rate, with a bitter almond smell to the horse’s breath. Drooling and voiding of urine and feces follow. The animal begins to lose control of its body, and will stagger and collapse. Breathing paralysis is often the cause of death.
Chronic Cyanide Poisoning
Symptoms are accumulative and include conditions of hypothyroidism and the presence of a goiter. Incoordination and limb paralysis is followed by a kidney infection that can cause death of the horse. Fetuses can also be affected with congenital malformations.
The cause of cyanide poisoning is the ingestion of lethal doses of cyanide. Once ingested, cyanide can quickly be absorbed into the bloodstream, where it travels all over the body. Cyanide then stops the blood cells from delivering oxygen, causing the blood to appear bright red and the oxygen deficient cells to suffocate to death.
Poisoning most often occurs from eating plants laden with cyanogenic glycosides that reside in the outer tissues of the plant. When the plant is damaged, whether from chewing, cutting, wilting, trampling, or freezing, those cyanogenic glycosides mix with enzymes residing in the inner tissues to produce toxic cyanide. A number of plants can contain toxic levels of cyanide, and should be avoided. They include:
Other sources of cyanide can include:
Diagnosis will depend on a history of cyanide exposure, symptoms, a physical examination, and the results of forage testing. Blood work and results of a urinalysis may also lead to a diagnosis. Be sure to bring any suspect plants with you to be identified and analyzed.
The stomach contents of deceased horses, as well as blood samples and liver or muscle tissue, can also be analyzed for a post mortem diagnosis.
Treatment of cyanide poisoning begins with the identification and removal of the source of the cyanide from pastures or other areas your horse frequents. Medical treatment aims to re-establish the oxygen carrying capability of the blood cells. This is done most frequently with the intravenous administration of sodium nitrate or sodium thiosulfate, either separately or simultaneously. Inhaled amyl nitrate could also be used, followed by an injection of the sodium thiosulfate. Other treatments include sodium salts, the oral administration of molasses, glucose or glyceraldehyde, and hydroxocobalamin, which can bind to the cyanide, allowing it to be flushed out of the body through the urine. Supportive treatments are given as needed, such as oxygen therapy.
Recovery is poor, as many animals succumb to death within hours of cyanide ingestion. Signs occur rapidly, and the affected horse is often found dead. Horses that survive the first hour of the poisoning have a more favorable recovery. Prevent your horse from becoming poisoned using forage management strategies, such as:
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