Cyanide (also referred to as prussic acid and hydrocyanic acid) is a highly potent, rapidly acting poison. Cyanide poisoning can occur as acute or chronic in horses, however acute cases are usually seen more in cattle then in horses. During acute poisoning, signs develop within 10 minutes to an hour.
Several common plants have the capability of accumulating large quantities of cyanogenic glycosides, which are found in the epidermal cells (outer tissue) of the plant. These plants also contain certain enzymes within the mesophyll cells (inner tissue) that upon rupture, mix with cyanogenic glycosides, produce cyanide. These plant cells are ruptured by wilting, drought, crushing, chewing, chopping, trampling, freezing, and cutting into the plant.
Upon ingestion of plant parts containing varying levels of cyanide, the toxin is quickly absorbed from the gastrointestinal tract into the bloodstream. Once in the blood it travels all over the body and inhibits blood cells from delivering oxygen to tissue cells. The blood becomes supersaturated with oxygen and appears bright red. Cyanide essentially inhibits the use of oxygen by the cells in the horse's body, causing them to eventually suffocate to death.
Plants capable of producing high levels of cyanide:
An increase in toxicity in plants containing cyanogenic glycosides can be caused by a number of factors which include:
- Stage of plant growth: Generally, cyanide toxin levels are highest when the plant is young and/or rapidly growing.
- Plant part: Certain plant parts tend to accumulate higher cyanide levels than others. For example, the leaves of cyanide-accumulating forage grasses were found to produce 25 times more cyanide than the stems.
- Wet leaves, such as from dew or rain
- Environmental stresses: Such as drought, frost, hail, and flooding.
- A high nitrogen to phosphorus ratio in the soil
- ome herbicides can increase both the toxicity and the palatability of the plants
Any stressful condition that inhibits the growth of the plant can cause higher amounts of cyanide to develop.