Cyanide kills tissues by lowering their ability to use oxygen. (Also see Poisoning: Sorghum Poisoning (Sudan Grass Poisoning).) Cyanides are found in plants, fumigants (such as disinfectants), soil sterilizers, fertilizers, and rodent poisons (rodenticides). Poisoning can result from improper or malicious use, but another frequent cause is ingestion of plants that contain cyanogenic glycosides. This is most common in livestock. Eucalyptus species, kept as ornamental houseplants, have been implicated in deaths of dogs and cats.

Signs can begin within 15 to 20 minutes to a few hours after animals consume toxic plants. The animals become excited and breathe rapidly with a rapid heartbeat. Drooling, watery eyes, vomiting, and voiding of urine and feces may occur. Muscle spasms are common. Mucous membranes are bright red at first but then become a bluish color. Death usually occurs in 30 to 45 minutes during severe convulsions. Animals that live 2 hours or more after signs begin may recover, unless cyanide continues to be absorbed from the gastrointestinal tract.

The history, signs, and finding hydrocyanic acid in diagnostic specimens support a diagnosis of cyanide poisoning. The suspected source of poisoning (plant or otherwise), stomach contents, blood, liver, and muscle may all be tested for cyanide. If cyanide poisoning is suspected, it is important that specimens for testing are collected as soon as possible after death, preferably within 4 hours.

Immediate treatment is extremely important. Sodium nitrite and sodium thiosulfate are used as an antidote. Oxygen may also be helpful, especially in dogs and cats.

Pasture grasses (for example, Sudan grass and sorghum-Sudan grass hybrids) should not be grazed until they are 15 to 18 inches tall, and forage sorghums should be several feet tall. Animals should be fed before first turning out to pasture. Free-choice salt and mineral with added sulfur may help protect against toxicity. Grazing should be monitored closely during periods of environmental stress, for example, drought or frost. Abundant regrowth of sorghum can be dangerous. These shoots should be frozen and wilted before grazing. Although the process of curing sorghum hay and silage usually decreases the potential for cyanide toxicity, hazardous concentrations can still be present. Feeds should be analyzed before use if cyanide is suspected.

Last full review/revision July 2011 by Barry R. Blakley, DVM, PhD; Cheryl L. Waldner, DVM, PhD; Rob Bildfell, DVM, MSc, DACVP; William D. Black, MSc, DVM, PhD; Herman J. Boermans, DVM, MSc, PhD; Cecil F. Brownie, DVM, PhD, DABVT, DABT, DABFE, DABFM, FACFEI; Raymond Cahill-Morasco, MS, DVM; Keith A. Clark, DVM, PhD; Gregory F. Grauer, DVM, MS, DACVIM; Sharon M. Gwaltney-Brant, DVM, PhD, DABVT, DABT; Larry G. Hansen, PhD; Safdar A. Khan, DVM, MS, PhD, DABVT; Garrick C. M. Latch, MASc, PhD; Gavin L. Meerdink, DVM, DABVT; Lisa A. Murphy, VMD; Frederick W. Oehme, DVM, PhD; Gary D. Osweiler, DVM, MS, PhD, DABVT; Mary M. Schell, DVM; David G. Schmitz, DVM, MS, DACVIM; Norman R. Schneider, DVM, MSc, DABVT