How a plague of 'mutating' super rats are taking over Sydney and becoming resistant to poison: 'Impossible to kill'
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Black rats in Australia's biggest cities have developed a genetic mutation that increases their resistance to one of the most widely used poisons.
New research led by Edith Cowan University PhD student and environmental toxicologist Alicia Gorbould found the mutation in over half the black rats tested in Brisbane, Melbourne, Perth and Sydney between 2021 and 2024.
The mutation suggests the rats have developed a resistance to second-generation anticoagulant rodenticides, also known as SGARs, but other animals continue to be inadvertently killed by these types of rat poison.
SGARs are deadly to secondary predators, such as tawny frogmouths, Australian boobooks and eastern barn owls, that feed on rodents.
Black rats are the most common introduced rat in Australia and experts warn their increase in resistance to poison could pose a serious risk to the country's wildlife.
Ms Gorbould also expressed fears of a public health crisis as Australians may be using larger quantities of poison in an attempt to rid rats from their home.
However, instead of working effectively, this would serve to introduce more poison into the food chain, and into waterways.
Animals that consume the poison, which prevents blood clotting, die from internal bleeding.
Research carried out by Ms Gorbould and her team found genetic mutation Tyr25Phe, which is associated with resistance to anticoagulant rodenticides, in 53.7 per cent of the 108 black rats they tested.
Alarmingly, the gene was found in over 80 per cent of black rats tested in Perth, over 45 per cent in Sydney, 39 per cent in Melbourne.
Tyr25Phe was found in none of the 10 rats tested in Brisbane.
'If you're using one of the baits that don't work … people will probably try to use more, and more, and more,' she told Daily Mail Australia.
'And so it's feeding back into that cycle of increasing the rates of resistance in the population, increasing those non-targeted poisonings, and then we're ending up essentially with a public health issue because we've got these rats that can't be controlled.'
Second-generation rodenticides are so potent they are banned in the United States, Canada and the European Union.
The poison makes it way up the food chain and kills other animals in a huge risk to Australia's biodiversity.
'Second-generation anticoagulant rodenticides don’t break down quickly in the body of a rat or mouse, even once it has died,' Ms Gorbould said.
'If a predator eats a mouse or rat that has been feeding on a second-generation rodenticide, that predator takes the poison into their bodies.
'These poisons take time to take effect (kill an animal) so rats and mice can re-visit baits and eat more so they end up with very high concentrations of these poisons in their bodies in excess of what it takes to kill a rodent.
'Essentially, they can become walking baits themselves.'
Scientists at Deakin University previously found rat poison was to blame for killing powerful owls.
In 2022, in a study looking at the prey of powerful owls, the team dissected 160 possums.
They found rat poison in 91 per cent of brushtail possums and 40 per cent of ringtail possums tested.
Ms Gorbould urged Australians to consider against using rat poisons and instead look to non-poison alternatives.
'We don’t know the full extent of resistance in our rats now (more research needs to be done here) so we need to tread carefully,' she said.
'Using other methods (physical methods) is important because the type of resistance we found in black rats can lead to resistance that can be stubborn to control.
'We also know that a lot of Australians attempt rodent control in their homes themselves. Until we have a co-ordinated and strategic approach to managing rodents in Australia, it’s important to try to change our behaviours when it comes to dealing with rodents.
'This means preventing rodents in the first place (sealing up entry points to the home, making sure compost is sealed and no pet food is left out and available) and using physical traps such as snap traps, electric traps, or even natural products (such as corn gluten meal and salt-based products).
'Using physical methods to control rats and mice does not feed into the resistance cycle and cannot be passed through our native food chains.'
