Proceedings of the Vertebrate Pest Conference

Current evaluations of the anticoagulant diphacinone for field control of introduced rodents in New Zealand include risk assessment with respect to non-target mortality, secondary poisoning, and environmental contamination. As non-target wildlife of rodent control operations, feral pigs may be exposed to vertebrate pesticides if they access toxic baits or scavenge the carcasses of poisoned animals. Feral pigs are also a significant resource for recreational hunters who harvest wild pork for food, hence the need to assess the risks of human exposure to diphacinone residues through feral pigs. Pen trials with domestic pigs were used to evaluate the potential for pig mortality through oral exposure to diphacinone and the residual persistence of diphacinone in pig tissues following a sublethal exposure. In the first trial, pigs were offered palatable food containing diphacinone doses of 12.5 mg/kg, 0.25 mg/kg/day for 3 days, or 0.5 mg/kg/day for 5 days. Significant elevations in coagulation time tests were measured 2 days after dosing, but these had returned to baseline values by 7 days. Two of the dosed pigs were euthanased due to lameness, which was accompanied by elevated prothrombin times and severe hemorrhage in a leg joint. In the second trial, pigs were dosed with 12.5 mg/kg diphacinone and then euthanased in groups for tissue sampling at Days 1, 4, and 10. Liver, muscle, and fat samples were analysed for diphacinone residues using an HPLC method. Data fitted to an exponential decay model estimated the hepatic elimination half-life as between 5.43 days (overall) and 14.12 days (terminal phase only). Half-lives of diphacinone in pig muscle and fat were 4.48 and 2.29 days respectively. A conservative withholding period of approximately 160 days is suggested before feral pigs should be taken for human consumption from areas where diphacinone baits have been used. This would minimize the likelihood of detectable diphacinone residues (≥0.02 µg/g) occurring in wild pork.