Prussic Acid Toxicity

By Barry Whitworth

As the temperature rises and the ground gets drier, plants in the sorghum family may become toxic. A common scenario for a veterinarian is to receive a call from a frantic rancher who says that his cows are “dropping like flies” or that he has found several dead cows after turning his cattle on to new pasture. One question the veterinarian will probably ask is if the cattle have been grazing Johnson grass.

Hydrocyanic acid (HCN), which is also referred to as cyanide or prussic acid, is a toxin in these plants that causes problems. The toxin is created when the harmless hydrocyanic glycosides in plants are stressed and break down. Once the hydrocyanic glycosides in the plants are damaged, they quickly convert to prussic acid which can kill an animal in minutes when consumed. When cattle ingest the plants high in hydrocyanic glycoside and break them down by chewing, the prussic acid is released in the rumen and absorbed into the blood stream.

Once in the circulatory system, the toxin prevents cells in the body from taking up oxygen. The blood becomes saturated with oxygen, which is why venous blood appears bright red. The clinical signs are excitement, muscle tremors, increased respiration rate, excess salivation, staggering, convulsions, and collapse. The cattle actually die of asphyxiation.

Learn more in the August issue of OKFR!