Cyanide poisoning of a Cooper’s hawk ( Accipiter cooperii )

Abstract

A Cooper’s hawk (Accipiter cooperii) was found dead in a ditch leading from a heap leach pad at a gold mine in Nevada. Observations at autopsy included an absence of external lesions, traces of subcutaneous and coronary fat, no food in the upper gastrointestinal tract, and no lesions in the viscera. Cyanide concentrations (µg/g ww) were 5.04 in blood, 3.88 in liver, and 1.79 in brain. No bacteria or viruses were isolated from tissues, and brain cholinesterase activity was within the normal range for a Cooper’s hawk.

The gold mining industry started using cyanide over a century ago in the extraction of gold from ores in a process of bringing gold into solution, which can then be recovered by adsorption onto carbon. ⁵ Aqueous cyanide solutions are delivered to ore heaps (heap leach pads) or mixed with crushed ore, and animals exposed to cyanide-contaminated water at the base of heap leach pads or in mine tailings ponds are subject to poisoning.^2,6 More than 9,500 animals, primarily migratory birds, including 15 unnamed species of raptors, were reported killed by cyanide used in gold mining operations from the mid-1980s to the early 1990s in Nevada. ¹⁰ Raptors reported with cyanide poisoning include bald eagles (Haliaeetus leucocephalus) and golden eagles (Aquila chrysaetos), although the potential source of cyanide was not discussed.^12,14,15 A presumed case of cyanide poisoning in a California condor (Gymnogyps californianus) was linked to a cyanide delivery device used in predator control. ¹⁸ Various measures have been used to reduce the risk of cyanide poisoning of wildlife around mines, including treating water to reduce the cyanide concentration, habitat modification to reduce the attractiveness of the area to wildlife, and denying access, primarily of mammals, to cyanide-contaminated water. ⁵ Here, I report a case of cyanide poisoning, supported by autopsy and analysis of cyanide in tissues, in a Cooper’s hawk (Accipiter cooperii).

The Cooper’s hawk is a medium-sized hawk widely distributed in the United States, southern Canada, and northern Mexico and is listed as a year-round resident of Nevada. ³ Although prey items of the Cooper’s hawk vary geographically and among habitat types, they typically feed on birds and small mammals. ³ In September 1994, a Cooper’s hawk was found dead in a ditch leading from a heap leach pad at a gold mine in Nevada and submitted to the U.S. Geological Survey National Wildlife Health Center in Madison, Wisconsin, for diagnostic evaluation. Autopsy and diagnostic laboratory testing followed those previously described. ⁸ No external lesions were noted. There were traces of subcutaneous and coronary fat, food was absent in the upper gastrointestinal tract, and no lesions were seen in the viscera. The gonads were characteristic of a juvenile male. Free cyanide concentrations in heart blood, liver, and brain were determined by a method involving the absorption of cyanide in a solution of sodium hydroxide with conversion of the cyanide to cyanogen chloride, followed by a colorimetric reaction and spectrophotometric assay. ⁷ Briefly, a weighed sample of heart blood or tissue slurry (1:5 dilution in distilled water) was added to the outer compartment of a Conway diffusion cell ^a and 0.1 N sodium hydroxide was added to the center compartment to absorb the liberated cyanide. Finally, 10% sulfuric acid was added to the outer compartment of the diffusion cell to liberate the cyanide in the sample. After 3 h, the absorbing solution from the center chamber was treated to convert cyanide to cyanogen chloride and then subjected to a colorimetric reaction. Free cyanide was determined spectrophotometrically ^b (580 nm) by measuring the absorbance of the sample and determining the concentration by comparison with a calibration curve. ⁷ Recovery of cyanide from a spiked sample of Cooper’s hawk liver was 94.4%, the relative percent difference of duplicate analyses was 7.8%, and the method detection limit was 0.03 µg cyanide/g wet tissue. Free cyanide concentrations (µg/g ww) in tissues of the Cooper’s hawk were 5.04 in blood, 3.88 in liver, and 1.79 in brain. To screen for other potential causes of mortality, liver tissue for routine culture was inoculated onto 5% sheep red blood agar and eosin–methylene blue plates, ^c incubated at 37°C for 72 h, and bacterial growth was characterized by a biochemical panel. ^d Liver for virus detection and isolation was processed for inoculation on cell cultures and embryonating eggs, and brain cholinesterase activity was determined by ultraviolet–visible spectrophotometry.^4,9,16 No bacteria or viruses were isolated, and brain cholinesterase activity was within the normal range for a Cooper’s hawk. ¹¹

Cyanide concentrations have been measured in tissues of domestic chickens (Gallus domesticus), black vultures (Coragyps atratus), and turkey vultures (Cathartes aura) dosed with sodium cyanide (NaCN). ¹⁷ Chickens administered NaCN at 6, 12, 24, and 48 mg/kg of body mass had mean blood and liver cyanide concentrations (µg/g ww) of 0.95–1.2 and 0.04–0.26, respectively. ¹⁷ Cyanide was not detected in the liver of at least 1 chicken in each of the 4 dosages administered. ¹⁷ Mean cyanide concentrations in blood of black vultures (dosed with 7 and 36 mg NaCN/kg body mass) and turkey vultures (dosed with 36 mg NaCN/kg body mass) that died within 20 min after NaCN exposure were 2.0–2.8 µg/g ww and 2.3 µg/g ww, respectively. ¹⁷ These cyanide results for chickens and vultures were from blood and liver analyses initiated on the same day that the birds died.

There is some evidence that cyanide concentrations in blood increase after being stored frozen. For example, blood from a turkey vulture dosed with 36 mg NaCN/kg body mass and stored frozen for 19 d had a concentration of 8.9 µg/g ww cyanide, compared with 4.1 µg/g on the day of original analysis. ¹³ A blood sample from another vulture receiving the same dose that was stored frozen for 26 d contained 1.9 µg/g ww cyanide, compared with 0.9 µg/g in the original analysis. ¹³ The authors of that study suggested the increase in free cyanide concentration in blood stored frozen was caused by the conversion of thiocyanate to cyanide, catalyzed by free hemoglobin released by hemolysis as a result of the freeze–thaw process.^1,13 Thus, although free cyanide increased, bound cyanide concentrations in blood declined after freezing. ¹³ The carcass of the Cooper’s hawk had been frozen for 3 wk before it was autopsied. Assuming that the blood cyanide concentration had doubled during this time, to 5.04 µg/g ww, the original concentration would have been ~2.5 µg/g ww. This is similar to or greater than the blood cyanide concentrations reported in birds that died of cyanide poisoning. ¹⁷ I conclude that the Cooper’s hawk died of cyanide poisoning. This is based on the history of being found dead at a gold mine near a heap leach pad, the concentrations of cyanide found in blood, liver, and brain, and the absence of other diagnostic findings.