Secondary poisoning risk for encapsulated sodium nitrite, a new tool for possum control

Brushtail possums (Trichosurus vulpecula) present an ongoing threat to New Zealand’s environment and economy. Research into additional control techniques is vital to ensure that a variety of efficient tools are available to help achieve population suppression. Encapsulated sodium nitrite (NaNO2) has been developed in New Zealand as a new toxin for possum and feral pig (Sus scrofa) control. Its toxic effects at high doses are mediated through the induction of methaemoglobinaemia, a condition in which the carrying capacity of oxygen in red blood cells is reduced. This study investigated the potential secondary poisoning risks associated with NaNO2. Secondary poisoning risks were assessed for dogs, cats and chickens in small-scale trials. Trial groups for each species consisted of two treatment groups with four individuals per group and one non-treatment group with two individuals. For 6 consecutive days, the treatment groups of dogs, cats and chickens were fed entire or partial carcasses from possums lethally poisoned with paste bait containing encapsulated NaNO2. Individuals in each group were observed continuously for 3 hours following each daily feeding and blood samples were taken from dogs and cats. Individuals were observed for obvious physiological signs of NaNO2 poisoning and symptoms of methaemoglobinaemia specific to dogs, cats and chickens. None of the dogs, cats or chickens displayed any obvious physiological signs of poisoning or symptoms of methaemoglobinaemia. Blood chemistry and haematology parameters measured for dogs and cats were either within the range considered normal or when outside this range comparable levels were also recorded in the control group. No changes in histology relating to NaNO2 intoxication were observed in dogs or cats after being fed carcasses, minced meat, vital organs or stomachs of possums poisoned with NaNO2. Therefore, the secondary poisoning risk appears to be minimal.