What are the risks to native wildlife with using poison to kill rodents?

Poisons, such as anticoagulant rat bait (known as anticoagulant rodenticides), are commonly used to kill rodents in residential, commercial and agricultural settings, but they pose a significant risk to native wildlife, either by direct ingestion of baits or ingestion of poisoned rodents. This secondary exposure to these baits intended to kill rodents can cause organ failure, bleeding and subsequent death in native animals and have population-wide impacts on ecosystems. Avoiding the purchase and use of rodent poisons and instead using more humane methods of rodent control, can help to protect native wildlife.

What kind of poisons are being used and how do they work?

Poisons are regularly used in residential, commercial and agricultural settings to kill animals often considered ‘pests’ such as rats and mice. While other methods of rodent control, such as live traps and snap traps, are available, toxic baits are commonly used as they are generally considered more cost effective and less labour intensive, particularly when controlling rodent infestations. These toxic baits are available in various forms, including blocks, pellets, and treated grains.

At present, two main categories of rodenticides; namely anticoagulant rodenticides and non-anticoagulant rodenticides, are approved for use in Australia. Anticoagulant rodenticides work by preventing blood from clotting, causing death mainly due to extensive internal bleeding. Anticoagulant rodenticides are further divided into first-generation (e.g. warfarin, coumatetralyl, diphacinone) and second-generation rodenticides (e.g. brodifacoum, bromadiolone, difethialone, difenacoum, flocoumafen) depending on their mode of action and potency.

In terms of risks to native wildlife, the primary concern is the use of second-generation anticoagulant rodenticides as they are much more potent, require only a single feeding to kill a rodent, and can remain in the rodent carcass for days after death. As further described below, native animals feeding off poisoned/sick animals or poisoned rodent carcasses, can lead to ‘secondary poisoning’ of those native animals. However, first-generation anticoagulant rodenticides, require multiple feedings to take effect and break down more quickly meaning there is less risk of sufficient poison being present in the rodent carcass before or after death to endanger native animals.

After consumption of an anticoagulant rodenticide, it can take up to seven days before death occurs. Studies have shown that the poison causes internal bleeding into joint spaces, body cavities and organs and can result in significant pain, respiratory distress and disability [2]. Due to the prolonged pain, suffering and distress caused by anticoagulant poisons, the RSPCA does not consider these to be humane. Non-anticoagulant rodenticides (e.g. zinc phosphide, a nerve toxin mainly used in mouse plagues in agricultural settings) cause the rodent to die quickly and have little residual effect, thus minimising the risk of secondary poisoning to native wildlife [1]. Zinc phosphide is not considered humane as it causes significant pain and suffering.

Impacts on native wildlife

When a small native mammal, like a brushtail possum, directly ingests rat bait, this is referred to as primary poisoning. However, it is far more common for carnivorous native animals to scavenge on poisoned rodents or other species, thereby risking ‘secondary poisoning’. Poisoning can cause significant impacts in native wildlife, similar to effects in rodents, including lethargy, blood loss, and organ failure resulting in death. Sub-lethal doses can also alter behaviour and cause growth abnormalities, making poisoned animals more susceptible to injuries as well as predation by other animals [4]. The other major risk to native animals due to secondary poisoning is that the poison enters the food chain and can accumulate in larger predators, potentially affecting ecosystems long after the initial poisoning event [3].

A recent study has, for the first time, detected the presence of anticoagulant rodenticides in Australian scavenging mammals, highlighting the previously undocumented risk of secondary poisoning in these animals [5]. Affected animals include Tasmanian devils and quolls, which are both listed as threatened species. This finding is consistent with previous studies that have confirmed rodenticide exposure in many other species including possums, reptiles, owls, eagles and other predatory birds [1,6,7,8].

Over time, the potential for toxins to affect the animal’s ability to produce and raise offspring (reproductive success), their survival rates, and overall population health could have detrimental impacts on genetic and biological diversity. Rodenticides, by killing rodents, can reduce the total abundance of prey available to native carnivores, further compromising reproductive success in these predator species that rely on small mammals as their primary food source. Many larger predators have a relatively lower number of offspring over their lifetime compared to smaller predators and so can take longer to recover from population losses. In other words, the combined effects of poisoning and having less food available to eat, could cause lower breeding success, fewer offspring, and lower survival rates of newborns in native wildlife [9]. For threatened wildlife species, this becomes even more critical, as the additional stress of rodenticide exposure could accelerate their decline and affect conservation outcomes.

What you can do to minimise the risks to native wildlife

Unfortunately, in Australia, rodent poisons are readily available in supermarkets and hardware stores, with no restrictions put in place on their use in areas with wildlife populations. This contrasts with the United States, Canada, the European Union and the United Kingdom, where stringent regulatory policies restrict their sale to only licensed professionals [1]. Even then, specific rodenticides only used by pest control professionals have been detected in wildlife, meaning that commercial activity may contribute to secondary poisoning and raises concerns whether tighter regulations will be sufficient to curb the risks to native wildlife [8].

The public can play a key role in helping to protect our native wildlife by avoiding the purchase of rodent poisons and instead using more humane methods of rodent control. There are many things that can be done to deter rodent infestations including ‘proofing’ buildings and removing access to feed. Where baiting is necessary as a last resort, only first-generation anticoagulant rodenticides should be used and label instructions strictly followed. A ‘pre-baiting period’ without using poison is recommended to help detect if there is active feeding by non-target species. If native wildlife is sighted, baits should not be used. If anticoagulants are used, dead rodents should be safely and promptly disposed of to minimise risk of secondary poisoning of native wildlife. Where large-scale control of rodent infestations is needed, RSPCA urges government agencies, community landholder groups and pest control operators to use more humane alternatives to anticoagulants.