Explainer: Which baits are used in the mouse plague?

The NSW government has released a $50 million package to assist with the harrowing mouse plague affecting the west of the state. Controversially, part of the plan is to ask for approval from the Australian Pesticides and Veterinary Medicines Authority (AVPMA) to use the bait bromadiolone – a rodenticide that has drawn criticism from some experts.

So what is bromadiolone, and why has it become so controversial?

What is bromadiolone?

“Bromadiolone is an anticoagulant rodenticide,” says Ian Musgrave, a senior lecturer in pharmacology at the University of Adelaide.

“These work by interfering with production of vitamin K, essential for blood clotting, so the animals die from uncontrolled bleeding. Bromadiolone is one of the second-generation anticoagulants, which only require one exposure to a bait to kill rodents, while the first generation ones needed animals to consume bait over more than one day.”

Its use is currently restricted, but not entirely outlawed. “Bromadiolone is approved to use in bait blocks and bait stations; it is currently not approved as a treatment for grain to use as a broadcast bait, although it has been in the past,” explains Musgrave.

Where does it pose a risk?

While the risk to humans is very low, Rob Davis, a senior lecturer in vertebrate biology at Edith Cowan University, says bromadiolone may poison native animals as well as mice. He has undertaken research on the effect of second-generation anticoagulant rodenticides (SGARs), like bromadiolone, on native fauna.

“Although they are lethal after a single feed, they take a long time to break down in the liver of mice and rats and this means that any predators that eat affected mice or rats (or their carcasses) also accumulate the poison,” says Davis.

“Our research has found lethal levels of these SGARs in 31 species of bird including most diurnal birds of prey and owls.”

They’ve also spotted effects in snakes, lizards, and native mammals.

“The broadscale use of bromadiolone cannot be recommended or supported by science and we are not aware of it being approved for use in such a broadscale way in other developed countries.”

Are there alternative baits to bromadiolone?

The “broadcast bait” the government is currently distributing for free as part of the package is zinc phosphide, which is chemically and toxicologically very different to bromadiolone, although its overall risk to people is similar. (“Both zinc phosphide and bromadiolone are toxic to humans, but with the recommended safe handling procedures farmers should have negligible risk,” says Musgrave.)

Bromadiolone’s toxicity comes from interfering with the body’s enzymes. It also hangs around in an animal’s body after death, meaning other animals that eat mice killed by bromadiolone may also become sick or die.

In contrast, zinc phosphide (Zn₃P₂) kills by producing phosphine gas upon contact with the stomach.

“It generates something like nerve gas in the stomach,” says Musgrave. “Once it kills the animal, there’s very little residue left over.”

This begs the question: why not just use zinc phosphide?

“The downside is that it’s not as effective,” says Musgrave. A grain treated with commercial zinc phosphide couldn’t kill a mouse with a single dose – meaning it takes more to get rid of mice, and mice are more likely to learn to avoid it.

However, a newer, higher-strength zinc phosphide has just been approved by the AVPMA. This bait can be used in a single dose.

So is the approval of bromadiolone entirely necessary?

“I suspect the government wants to be seen to be doing something and is keen to use these baits,” says Davis.

But he notes that zinc phosphide also “can be very harmful to most other species if not carefully utilised”.

Is there any other way to deal with the mouse plague?

Davis points out that prior to the introduction of the house mouse, native rodent populations fluctuated in similar ways to the European mouse – although “human modification of the landscape has altered the balance in favour of mice who now have vast amount of grain to eat in mostly disturbed landscapes with few natural predators”.

“Better land management practices including retaining bush certainly help as do investing in mouse-proofing farming infrastructure, particularly grain storage facilities,” he says.

There may be solutions in the pipeline for future plagues.

“Broadscale solutions in the future may lie with gene drive technologies, novel pathogens and other similar control opportunities,” says Davis.

“However as Australia has a large number of endemic rodent species – many already extinct and threatened – any control mechanisms must ensure they don’t harm native species.”

“If the authorisation goes ahead, then bromadiolone will probably replace the current zinc phosphide as a grain treatment for broadcast bait,” says Musgrave. “The sheer scale of the mouse plague means that traps and non-lethal approaches have no hope of controlling the mouse numbers.”

Davis has a slightly different perspective. “Unfortunately, at the end of the day, no amount of poisoning – even with more friendly first-generation baits – will likely have much impact on mouse plagues,” he says.

“Ultimately they will wane and disappear once cold and wet conditions set in.”

Cold comfort for those currently overrun by mice.

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