Penny Fisher

Differential particle size ingestion : Promoting target-specific baiting of feral cats

Abstract Predation by feral cats (Felis catus) is believed to threaten the conservation of a range of terrestrial vertebrates in southeastern Australia, and new baiting techniques are sought for broad-scale control of feral cats. In southeastern Australia there are 34 native mammals that may consume meat baits, and ways to minimize their exposure to bait toxicants are needed. We determined the potential of cats to ingest larger particles relative to most nontarget species as a mechanism to increase baiting selectivity. Feral cats reliably ingested inert, spherical bearings up to 4.7 mm in diameter when the bearings were implanted within a specialized bait medium. Presence of bearings did not affect bait consumption relative to untreated baits. Repetitive ingestion was highly reliable in the first 9 days of a feeding trial and diminished only marginally in a consecutive trial. We presented captive plains rats (Pseudomys australis), fat-tailed dunnarts (Sminthopsis crassicaudata), eastern barred-bandicoots (Perameles gunnii), and northern quolls (Dasyurus hallucatus), with baits containing 4.7-mm-diameter coated pellets formulated with the marker dye rhodamine B (RB). Exposure to the marker dye for each species was not biased to individual or day of presentation. Exposure to RB in the pellet occurred in only 3.1–6.5% of presentations for each species, and the mean daily mass of the pellet in g kg−1 day−1 ingested was 0.078–0.01% of the mean bait mass in g kg−1 day−1 consumed. Pellet presentation greatly reduced (P ≤ 0.001) the exposure of wild native rodents to RB relative to directly injected baits. Exploiting differential particle size ingestion between feral cats and nontarget species could potentially reduce exposure of many nontarget mammals to bait toxicants and decrease the risk of baiting to nontarget species.

Oral toxicity of p-aminopropiophenone to ferrets

Ferrets (Mustela furo) are pests in New Zealand and new methods are being sought for their control. The hydrochloride form of p-aminopropiophenone (PAPP) was highly toxic to ferrets when delivered by gavage, with LD50 and LD99 values of 15.52 and 20.80 mg kg–1, respectively. Signs of toxicosis progressed through pallor and cyanosis, impaired coordination, prostration with reduced responses to stimuli, with death in ~2 h. In a preliminary assessment of effective doses of PAPP in bait, 19 of 20 ferrets consumed chick carcasses containing ~46 mg PAPP within 40 min. Ferrets that ate chick baits all showed signs of toxicosis, with 32% mortality. Three ferrets died within 151 min and the other three within the following 14 h. Ferrets that died ingested 29.9–79.3 mg PAPP per kilogram bodyweight. The low mortality was attributed to decreased bioavailability of PAPP delivered in food, and increased doses need to be tested to establish an effective lethal concentration of PAPP in baits for ferrets. Future development of bait formulations will also need to take into account the role of emesis in reducing efficacy. Bait formulations incorporating microencapsulated PAPP may improve bioavailability and the basis for a new option for the management of ferrets in New Zealand.

Acute oral toxicity of p‐aminopropiophenone to stoats (Mustela erminea)

Abstract No toxic baits are currently registered in New Zealand for stoat (Mustela erminea) control. PAPP (p‐aminopropiophenone) has potential application as a vertebrate pesticide for stoat control in New Zealand, and in the hydrochloride form was highly toxic (LD509.3 mg/kg) to stoats in pen trials. The first visible signs of toxicosis, cyanosis and salivation, generally appeared within 20 min of gavage administration of PAPP. These signs were generally followed by rapid respiration, lack of coordination and lethargy. Death generally followed within an hour of receiving a lethal dose. Stoats remained at least partly conscious until just prior to death, or may have had periods of intermittent consciousness in the later stages. PAPP toxicosis in stoats appeared relatively rapid (in comparison with other vertebrate pesticides in mustelids) and on preliminary assessment appears relatively humane. Further investigation and development of targeted bait delivery of PAPP for stoat control in New Zealand is recommended. This should be conducted alongside formal assessment of humaneness and the potential for unwanted effects such as non‐target mortality and environmental contamination.

Bait acceptance by house mice: an island field trial

Predation by introduced house mice Mus musculus on islands is one cause of decline in native birds and has deleterious impacts on other ecological aspects. Eradication of rats (Rattus spp.) from islands of up to >10000 ha has been achieved, but for mice scale is still an issue with the largest island cleared being only 710 ha. The feasibility of eradicating mice from larger islands is being considered, and to support these assessments, we undertook a field study on Gough Island (6400 ha) to determine whether all mice would be likely to accept toxic bait. We replicated a toxic bait operation as closely as possible, in timing, probable bait density and distribution, using a bait formulation used commonly in rodent eradication operations. Baits lacked toxin but were coated with the fluorescent dye rhodamine B. Mice trapped in and around the baited areas were inspected under ultraviolet light for fluorescent marking indicative of bait consumption. Of 434 mice, 97% tested positive, including mice trapped on assessment lines up to 90 m from the closest bait. There was no difference in the proportions of unstained mice from assessment lines outside baited sites compared with mice trapped in the core baited sites, suggesting large-scale foraging movements over relatively large distances into the baited sites from surrounding, non-baited habitat. Despite the high bait densities (15.7 kg ha−1 at initial application and 7.9 kg ha−1 at second application), bait consumption rates of ~4 kg ha−1 day−1 occurred after both applications. This was much higher than expected (probably the result of large-scale movements) and meant that all baits were consumed before trapping began. Thus the 13 unstained mice trapped in the core of the baited area may have moved there after bait was consumed. Further trials are required to assess whether all unmarked mice were false negatives (not exposed to bait) or if any were true negatives (rejected bait). A separate experiment found that all 11 mice trapped in a cave had eaten bait applied aboveground around the cave’s entrances, suggesting that caves do not serve as refugia for mice and are thus unlikely to compromise an eradication attempt.

Why 0.02%? A review of the basis for current practice in aerial 1080 baiting for rabbits in New Zealand

Abstract. Sodium fluoroacetate (1080) has been used as an aerially distributed toxin against mammalian pests in New Zealand since the 1950s. Although its use for rabbit (Oryctolagus cuniculus) control ceased temporarily after the illegal release of rabbit haemorrhagic disease virus (RHDV) in 1997, there has been a recent resurgence in the use of aerial baiting with 1080 to control rabbits as the efficacy of RHDV has fallen. Current practices for rabbit control using 1080 have changed little since the 1980s, with high sowing rates and low toxin loadings commonplace. The lack of ongoing development in baiting practices for rabbit control contrasts sharply with continued improvements in the aerial 1080 baiting practices for brushtail possums (Trichosurus vulpecula) in New Zealand, such as a shift to a comparatively high 1080 loading and using much lower application rates of prefeed and toxic bait. These modifications have resulted in an overall reduction in the amount of toxin used for possum control. The disparity in these two approaches prompted a formal review of the rationale on which the current 1080 baiting practices for rabbits are based. Two issues emerged strongly. First, the current low toxin loading used (0.02–0.04% 1080 in bait) is not based on experimental optimisation in New Zealand but, rather, on research conducted several decades earlier in Australia. Second, despite long-standing concerns about the quality of carrot bait used in New Zealand, current bait manufacturing and distribution practices still produce large numbers of small sublethal fragments. Thus, the current New Zealand practice of multiple prefeeds and very high sowing rates of bait with a low 1080 loading used against rabbits seems to have resulted from the need to compensate for the low toxic loading and poor quality control of the bait (carrots). We, therefore, suggest that there is considerable potential to improve current aerial 1080 baiting practices for controlling rabbits in New Zealand. More generally, these findings also help illustrate that ‘best’ pest-management practice may sometimes be based on pragmatic solutions aimed at overcoming unrecognised internal constraints that are in fact avoidable. Refining and modernising vertebrate pest-control programs, so that they better meet efficacy requirements and contemporary public expectations, therefore requires understanding not just that a solution works, but also how it works.

A systematic approach to evaluating and ranking the relative animal welfare impacts of wildlife control methods: poisons used for lethal control of brushtail possums (Trichosurus vulpecula) in New Zealand

Abstract Context. Control of unwanted wildlife (‘pest’ animals) is undertaken for conservation and economic reasons, and when such animals are considered a nuisance. Such control should be undertaken using approaches that minimise, as far as possible, detrimental impacts on the welfare of the animals. Using a scientific framework based on the Five Domains model, the relative welfare impacts of pest control methods can be compared across methods and pest species. Aims. We demonstrate the application of a modified version of this framework to evaluate the relative impacts of seven Vertebrate Toxic Agents (VTAs) used to control brushtail possums (Trichosurus vulpecula) in New Zealand. The evaluation is used to produce a ranking of the seven VTAs based on their relative impacts on possum welfare. Methods. Scientific literature describing mode of toxic action, specific effects in possums or other animals and reports from human poisonings was collated as reference material for a panel of six experts. The panel produced a median welfare impact score (‘none’ to ‘extreme’) for each of the Five Domains. The ‘Overall Grade’ (1 to 8) reflected the intensity and duration of all impacts of a VTA on possums. Key results. All VTAs evaluated have at least moderate impacts on possum welfare, lasting for at least minutes. Cyanide was assessed as having the lowest welfare impacts (median grade 4), and cholecalciferol and the anticoagulants the highest impacts (7.5 to 8). Zinc phosphide was assigned an intermediate grade (6) with high confidence. While the overall impacts of sodium fluoroacetate (1080) and phosphorus were also assessed as intermediate (6), the panel’s confidence in these scores was low. Conclusions. From an animal welfare perspective, anticoagulant poisons and cholecalciferol should be the least preferred options for controlling possums in New Zealand, as VTAs with less severe welfare impacts are available. Implications. The results of such assessments allow animal welfare impacts to be integrated with other factors in wildlife management decision-making and policy development, and are thus useful for managers, researchers, regulators and operators. Evaluation of welfare impacts aligns with the goals and mandates of ethical wildlife control and may also be valuable in wider wildlife research and management activities.

Pindone residues in rabbit tissues: implications for secondary hazard and risk to non-target wildlife

Abstract Context. Pindone is used to control rabbits in Australia and New Zealand, but the secondary non-target risks presented by pindone-poisoned rabbits are poorly known. Aims. We aimed to generate new data on residual concentrations of pindone in poisoned rabbits for use in a review of the secondary poisoning risks posed to non-target animals in New Zealand. Methods. Laboratory rabbits were offered pellet bait containing 0.25 g kg–1 pindone in three trials to simulate a range of bait uptake scenarios: single exposure and low or high chronic exposure. Residual pindone concentrations measured in body tissues of laboratory rabbits that had ingested known exposures of pindone were compared with those in wild rabbit carcasses collected after three pindone-baiting operations. Residues in the faeces of some laboratory rabbits were also measured. Key results. Highest concentrations of pindone residues were in the liver and fat tissue of poisoned rabbits, with consistently lower concentrations in muscle tissue. A dose–response relationship between pindone exposure and liver residue concentrations was found only at the highest chronic exposures. Rabbit carcasses collected after field-baiting operations had generally higher pindone residue concentrations than did laboratory rabbits that had ingested known lethal amounts of bait. Unmetabolised pindone was excreted in rabbit faeces. Conclusions. The occurrence of the highest residual pindone concentrations in rabbit liver was consistent with the known tissue distribution of anticoagulants in mammals; however, the co-occurrence of similar-range pindone concentrations in rabbit fat has not been previously described. Re-ingestion of soft faecal pellets (caecotrophy) in rabbits that have eaten pindone bait may function as a secondary exposure to increase or prolong their oral exposure to pindone. Some rabbits poisoned following field pindone-baiting operations are likely to have consumed well in excess of a lethal amount of bait. Implications. Concentrations of residual pindone in fat and liver of poisoned rabbits suggest that secondary poisoning hazard to some non-target predators and scavengers is high. The lack of field-based assessments of the non-target impacts of pindone is a marked information gap that needs to be addressed.

Degradation rate of sodium fluoroacetate in three New Zealand soils.

The degradation rate of sodium fluoroacetate (SFA) was assessed in a laboratory microcosm study incorporating 3 New Zealand soil types under different temperature (5 °C, 10 °C, or 20 °C) and soil moisture (35% or 60% water holding capacity) conditions using guideline 307 from the Organisation for Economic Co-operation and Development. A combination of nonlabeled and radiolabeled (14) C-SFA was added to soil microcosms, with sampling and analysis protocols for soil, soil extracts, and evolved CO(2) established using liquid scintillation counting and liquid chromatography-mass spectrometry. Degradation products of SFA and their rates of formation were similar in the 3 soil types. The major degradation pathway for SFA was through microbial degradation to the hydroxyl metabolite, hydroxyacetic acid, and microbial mineralization to CO(2), which constituted the major transformation product. Temperature, rather than soil type or moisture content, was the dominant factor affecting the rate of degradation. Soil treatments incubated at 20 °C displayed a more rapid loss of (14)C-SFA residues than lower temperature treatments. The transformation half-life (DT50) of SFA in the 3 soils increased with decreasing temperature, varying from 6 d to 8 d at 20 °C, 10 d to 21 d at 10 °C, and 22 d to 43 d at 5 °C.

Pharmacokinetics of Anticoagulant Rodenticides in Target and Non-target Organisms

The physiological and toxicological effects of xenobiotics are influenced by numerous factors including their chemical properties, route of exposure, absorption, distribution, metabolism, receptor binding, and excretion. Pharmacokinetic and toxicokinetic properties are often used to predict tissue concentrations and adverse effects, and therefore inform risk assessments. While anticoagulants rodenticides have been used to control pest rodents for decades, extant pharmacokinetic and toxicokinetic data for these compounds has principally been generated by studies using laboratory-bred or captive mammals and is scarce in non-target wildlife species, especially birds. Warfarin is by far the most studied anticoagulant rodenticide, likely because of its human therapeutic use. In general, anticoagulants are rapidly and principally absorbed in the intestine following ingestion. Metabolism is mediated by cytochrome P450 isozymes and ring hydroxylation also appears to be an important biotransformation step. Hydroxylated metabolites can further undergo conjugation with glucuronic acid prior to entering the systemic circulation, with potential enterohepatic recirculation. While the metabolite suite remains poorly described for most anticoagulant rodenticides, both the parent compounds and known metabolites appear to be largely bound to albumin while circulating. Hepatic metabolism is generally biphasic with a rapid initial phase and more prolonged terminal phase. Binding to vitamin K epoxide reductase sites, especially in liver tissue, exerts the common toxic effect of ‘anticoagulation’. The affinity of different anticoagulant compounds for the binding sites, and different biotransformation pathways for isomeric forms or second-generation compounds containing bromine, sulfur or fluorine likely contributes to some notable differences in the toxicity, metabolism and excretion of first versus second-generation compounds and coumarin versus indandione compounds. In general, second-generation anticoagulant rodenticides are more toxic with hepatic half-lives approximating 100–300 days, in contrast to first-generation compounds with hepatic half-lives in the range of 60 days or less. The indandiones tend to have the shortest hepatic persistence. Excretion of second-generation compounds tends to occur mostly through feces, while first-generation compound excretion is largely in urine. Additional data on transplacental and in ovo transfer, metabolism and elimination of anticoagulant rodenticides in non-target species would enhance hazard and risk evaluations.

Effect of pre-feeding and sodium fluoroacetate (1080) concentration on bait acceptance by house mice

Context. In New Zealand, the aerial application of toxic baits containing sodium fluoroacetate (1080) can consistently achieve significant reductions in populations of multiple vertebrate pest species including brushtail possums (Trichosurus vulpecula), ship rats (Rattus rattus) and stoats (Mustela erminea). Reductions in house mouse (Mus musculus) populations by 1080 baiting appear less consistent, possibly due to low acceptance of 1080 bait by mice in field conditions. Aims. We tested the effect of pre-feeding and 1080 concentration on the acceptance of pellet food by mice. Methods. Wild-caught mice were individually housed and presented with a series of two-choice laboratory feeding tests, using estimates of the daily amount eaten to indicate relative acceptance of different types of pellet food. Key results. Pre-feeding mice on non-toxic food did not increase their subsequent acceptance of the same food containing 0.15% 1080. Mice showed low acceptance of food containing 0.08 and 0.15% 1080 (by weight), with similar mortality (25%). Acceptance of food containing 1.5% 1080 was also very low in comparison with non-toxic food, although mortality in mice was higher (~66%). In comparison with other concentrations, mice ate comparatively more of food containing 0.001% 1080 with no mortality, although the non-toxic food was still significantly favoured. Presentation of a choice between non-toxic food and food containing 0.08, 0.15 or 1.5% 1080 to mice was followed by a significant decrease in average total daily food intake over the following 2 days. In surviving mice this ‘drop feed’ effect was followed by an increase in average daily intake of non-toxic food over the next 3 days until normal daily intake levels were again reached. Conclusions. We suggest that wild mice can rapidly identify food containing 1080 and subsequently will avoid it. Implications. This feeding response partly explains the variable success of 1080 baiting operations against wild mouse populations (M. musculus) in New Zealand.