As parts of Australia swelter, trees in many towns and cities are rustling – not just with the sound of dry leaves, but also with bats fanning themselves with their wings to keep cool.

Some areas have recorded temperatures above 48C, and bat deaths have been reported on a “biblical scale”.

The record-breaking heatwave has seen temperatures remain at 39C even at midnight.

For some, the relentless heat has been too much. Temperatures above 42C can kill flying foxes, and thousands have dropped dead from the trees in Adelaide, South Australia.

Back in November, amid another heatwave, more than 23,000 spectacled flying foxes died in just two days in the northern city of Cairns. Residents were forced to move out of their homes due to the smell of rotting carcases, the ABC reported.

The figure represents a third of Australia’s spectacled flying foxes.

“This sort of event has not happened in Australia this far north since human settlement,” said Dr Justin Welbergen.

“It’s clear from climate change projections that this is set to escalate in the future,” he told the BBC, adding the deaths were “the canary in the coal mine for climate change”.

Growing numbers of bats are roosting in urban areas, according to the Australian government – a trend that has made the deaths conspicuous.

But ecologists have warned the impact of the heatwave could be taking a heavy toll on unseen populations of wild animals.

In New South Wales, officials have issued a “bat danger alert” for the state’s Hunter region, where the heat has affected bat behaviour. A spate of bites and scratches to humans has been reported.

Seven people have been bitten over the past two weeks, including two by bats infected with an incurable virus similar to rabies.

Bats can carry a large number of dangerous diseases. Australian bat lyssavirus is related to rabies – though can have a longer incubation period in humans – and in known infection cases has always been fatal.

Trained bat handlers wear protective clothing and are required to have vaccinations.

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“Two of the bats submitted for testing have actually had lyssavirus infection, so it is a real concern for us and for those people who have been exposed,” New South Wales public health physician David Durrheim told the ABC.

“A few of these were bat carers but there were a number of members of the public, and they are the people we are really concerned about because they haven’t been vaccinated,” he said.

Wildlife carer Rebecca Koller told the ABC last month almost 850 bats were rescued during the November heatwave and she was looking after about 200 at her own home.

“None of our carers were prepared for the numbers we would have. We already had 500 orphans in care prior to this event,” she said.

The highest temperature recorded in recent days was in Tarcoola in the state of South Australia, where it reached 49C on Tuesday afternoon.

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