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Australia's mouse plague highlights low-tech responses in high-tech era

Infestation brings despair to farmers but deadly chemical solution may be poisoned chalice

Australia's latest mouse plague is wreaking social and economic havoc in New South Wales, the country's most populous state. (Photo courtesy of CSIRO)

SYDNEY -- Perspective is everything in the great Australian mouse plague of 2021. For animal activists, mice are clever and interesting little creatures with a right to life; for farming families facing ruin as they watch their crops disappear under a constant rodent attack, mice are fast-breeding alien invaders who leave a trail of devastation and disease in their wake. For native predators such as eagles, owls, snakes and goannas (large lizards), the surging mouse population makes for a bountiful feast, but with a potentially deadly sting in the tail.

Certainly, the humble house mouse (or mus musculus) has a lot to answer for in Australia these days. Since arriving on the First Fleet with British settlers in 1788, the little rodent has established itself as "an invasive species par excellence," in the view of mouse expert Bill Bateman, an associate professor at Western Australia's Curtin University.

Given the right food and weather conditions, mice thrive in Australia's vast grain-growing regions, outbreeding any native species to eventually reach the sort of plague numbers that are now wreaking economic and social havoc in the country's most populous state, New South Wales.

Another mouse expert, Steve Henry of the national science agency CSIRO, reckons a mouse plague happens in Australia about once in a decade. "Mice can breed from about six weeks old and can reproduce every 19 to 21 days," he explained recently. "Their litters can have as many as 10 pups, and as soon as they have a litter, they breed again." Winter usually slows breeding activity, but no one knows yet how this year will pan out. The worst plagues before 2021 were in 1993-94 and in 1979.

For desperate farmers seeking to combat overwhelming mouse numbers, one response might be found in bromadiolone, a deadly chemical known in some quarters as "napalm for mice." Its potential use as a grain bait has triggered a bitter debate about the best way to handle this year's plague, with environmentalists and academics arguing its downsides outweigh its effectiveness.

A farmer unties rope holding a tarpaulin in place over hay destroyed by mice on his property Gilgandra, New South Wales, on May 31

Bromadiolone is a super strong anticoagulant rodenticide that condemns mice to a nasty death after just one feed. But it is deadly to other animals as well, and has prompted so much concern about safety in the food chain that Australia's independent regulator of animal pesticides, the Australian Pesticide and Veterinary Medicine Authority (APVMA), so far has held off giving permission for its widespread use.

Since November 2020, mouse numbers have soared as a consequence of bumper spring and summer harvests in grain-growing areas of Australia. For the past eight months, farmers in New South Wales and parts of the neighboring states of Victoria and Queensland have watched their livelihoods being destroyed by legions of mice munching into wheat fields and grain storage areas, chomping into electrical wires that have sparked power failures in machinery and houses, and sometimes biting people as they sleep in their beds.

A farming association, NSW Farmers, estimates the mouse plague will cut more than 1 billion Australian dollars from the farmgate value of the state's forthcoming winter grain crop. Only a harsh winter, widespread floods, or heavy-duty poisoning can break the mouse-breeding cycle.

The stresses of farm life weigh heavily on rural communities, with average suicide rates 50% higher than in major cities, according to the University of Newcastle's Center for Rural and Remote Mental Health.

Mice scurry as a farmer lifts a tarpaulin covering stored grain near Tottenham, New South Wales, on May 19.

With many farming households already on their knees after battling years of drought followed by bushfires, floods, the COVID-19 pandemic and now the mouse plague, the NSW government has sought emergency approval from the APVMA to use bromadiolone. That has prompted protests from animal rights activists and a mixture of disapproval and guarded agreement from the scientific community.

The animal rights organization People for the Ethical Treatment of Animals Australia, views mice as "clever, interesting individuals" that should be "rehomed" if possible. It said mice felt pain and fear, just as dogs, cats and farmers do. "If they are going to be killed, it is only ethical and right to do so as painlessly as possible, not with gut-wrenching poisons that cause slow, agonizing deaths to mice or other animals who may eat the poison or its victims," PETA said.

That prompted a stinging response from Australia's Deputy Prime Minister Michael McCormack, who is leader of the rural-based National Party in the federal coalition government. In a recent television interview, McCormack declared: "The only good mouse is a dead mouse." He said for organizations to even contemplate that mice should be rehomed or otherwise saved was ridiculous. "It is absurd in the extreme," he added.

If the APVMA grants approval, 20 towns in New South Wales will host grain treatment sites to distribute bromadiolone, with NSW Agriculture Minister Adam Marshall saying the chemical will be used for baiting crop perimeters. Farmers will receive the poison free of charge, but the chemical will only be handled by Local Land Services staff to ensure no other wildlife is harmed.

A grain farmer disposes of mice caught in traps on his property in Gilgandra on May 30.   © Reuters

Since January, the APVMA has given emergency permission to use double-strength versions of the lower-level poison, zinc phosphide, which goes under various brand names such as MouseOff, DeadMouse, SureFire and Last Supper.

The APVMA warns that even this poison is toxic to native wildlife and birds and very toxic to aquatic life. "Fields must be closely monitored for nontarget species during a pre-baiting period," it says.

In the view of NSW Farmers Vice-President Xavier Martin, the clock is ticking and without a concerted baiting effort in June, the mouse plague potentially could run for two years.

"Each day we delay in taking effective action to control these mice will increase economic losses and the likelihood we will still be battling mice come Christmastime," he said.

Dr. Ian Musgrave, a senior lecturer in the faculty of medicine at the University of Adelaide, said the sheer scale of the plague meant that traps and nonlethal approaches had "no hope" of controlling mouse numbers. He said that both zinc phosphide and bromadiolone were toxic to humans, but with the recommended safe handling procedures, "farmers should have negligible risk."

Mice in the Darling Downs farming region of Queensland. (Photo by Grant Singleton, courtesy of CSIRO)

Dr. Peter Brown, who leads the rodent management team at CSIRO Health & Biosecurity, said bromadiolone was a second-generation anticoagulant that was highly potent and worked with a single feed. "Because there is a delay in the onset of symptoms, rodents cannot associate eating the rodenticide with becoming sick. They normally die three to seven days after eating the baits."

According to the Royal Society for the Prevention of Cruelty to Animals Australia, the body of a poisoned rodent presents a "significant risk" if eaten by other animals, including native wildlife. It says a recent Australian study showed that more than 70% of dead and dying boobook owls sampled had been exposed to rodent anticoagulants.

Bromadiolone's lethality for other animals and potentially, humans as well, prompted a group of prominent wildlife academics to declare it a "bad idea" and warn of the dangers to humans who eat animals or animal products that have ingested poisoned mice.

"While bromadiolone effectively kills mice, it also travels up the food chain to poison predators who eat the mice, and other species," they said. "These predators, from wedge-tailed eagles to goannas, are coming out in droves to feast on their abundant prey."

Academics Robert Davis, a senior lecturer in wildlife ecology at Edith Cowan University in Perth; Bill Bateman, associate professor, and Damian Lettoof, Ph.D. candidate, both of Curtin University; Maggie J. Watson, lecturer in ornithology, ecology, conservation and parasitology at Charles Sturt University; and Michael Lohr, adjunct lecturer at Edith Cowan University, said humans risked exposure from eating eggs laid by chickens that feed on poisoned mice, or more directly from eating other animals that may have ingested poisoned mice.

"Rolling out double-strength zinc phosphide may be the lesser of the evils in causing secondary poisoning, but only if used very carefully," they said, noting that mouse plagues were a regular cycle in Australia. "Second-generation rodenticides will only destroy and weaken the predator populations we need to help us combat the next plague."

Curtin University's Bateman says that while rodenticides probably still have a role, the front-line defense against the mouse plague rests with maintaining farmland habitat that supports natural predators such as birds of prey, native carnivores, snakes and large lizards.

UPDATE -- On June 23, the Australian Pesticides and Veterinary Medicines Authority issued the New South Wales Department of Primary Industries with a notice of intention to refuse its emergency permit application for the use of bromadiolone. The department has 28 days to respond to the proposed decision to refuse.

The APVMA said its primary concern was environmental safety, "particularly in relation to animals that eat mice."

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