A simple and highly sensitive spectrophotometric method for the determination of cyanide in equine blood

An epidemiological association among black cherry trees (Prunus serotina), eastern tent caterpillars (Malacosoma americana), and the spring 2001 episode of mare reproductive loss syndrome in central Kentucky focused attention on the potential role of environmental cyanogens in the causes of this syndrome. To evaluate the role of cyanide (CN^-) in this syndrome, a simple, rapid, and highly sensitive method for determination of low parts per billion concentrations of CN^- in equine blood and other biological fluids was developed. The analytical method is an adaptation of methods commonly in use and involves the evolution and trapping of gaseous hydrogen cyanide followed by spectrophotometric determination by autoanalyzer. The limit of quantitation of this method is 2 ng/mL in equine blood, and the standard curve shows a linear relationship between CN^- concentration and absorbance (r > .99). The method throughput is high, up to 100 samples per day. Normal blood CN^- concentrations in horses at pasture in Kentucky in October 2001 ranged from 3-18 ng/mL, whereas hayfed horses showed blood CN^- levels of 2-7 ng/mL in January 2002. Blood samples from a small number of cattle at pasture showed broadly similar blood CN^- concentrations. Intravenous administration of sodium cyanide and oral administration of mandelonitrile and amygdalin yielded readily detectable increases in blood CN^- concentrations. This method is sufficiently sensitive and specific to allow the determination of normal blood CN^- levels in horses, as well as the seasonal and pasture-dependent variations. The method should also be suitable for investigation of the toxicokinetics and disposition of subacutely toxic doses of CN^- and its precursor cyanogens in the horse as well as in other species.