In 1950, Australia released the Myxoma virus as biological warfare against its invasive rabbit population. The virus killed 99.8% of infected rabbits within two years

Australia once faced a rabbit invasion so severe that conventional methods such as trapping, poisoning, fencing and destroying warrens could barely slow it down. So scientists tried something extraordinary: they turned a deadly virus into a biological weapon against the country’s exploding rabbit population.

In 1950, Australia released the myxoma virus into wild rabbit populations. The results were astonishing. Myxomatosis swept through rabbit colonies, killing more than 99% of infected European rabbits in some areas and causing the national population to plunge from an estimated 600 million to around 100 million within two years.

But the story did not end there. The rabbits began to fight back through natural selection, while the virus itself evolved. What followed became one of the most remarkable examples of biological control ever attempted.

How did rabbits become such a huge problem in Australia?

The story began long before the virus was released.

A small number of domesticated rabbits arrived in Australia with the First Fleet in 1788, partly as a potential food source. But the population explosion was linked to a later introduction.

In 1859, wealthy Victorian grazier Thomas Austin imported 24 European wild rabbits for hunting on his property. The animals found Australia's conditions remarkably favourable. With abundant food, few effective natural controls and their extraordinary ability to reproduce, rabbit numbers grew rapidly.

The animals soon spread across large parts of the country.

Rabbits were not simply eating vegetation. They also consumed plant seeds, preventing new vegetation from regenerating. Their grazing contributed to the degradation of pastures, exposed soil and erosion, while competition for food placed additional pressure on livestock and native wildlife.

For farmers and pastoralists, the rabbit invasion became an economic problem as well as an environmental one.

Australia tried fences, poison and traps—but rabbits kept spreading

Authorities tried almost everything they could think of.

Rabbits were trapped and poisoned. Their warrens were dug up and destroyed. In Western Australia, the government began constructing enormous fences from 1902 in an attempt to prevent rabbits from moving farther west.

The famous rabbit-proof fences became some of the world's longest fences, but they could not provide a permanent solution.

Scientists eventually began investigating whether biology itself could be used against the rabbits.

One promising candidate was myxomatosis, a disease caused by the myxoma virus.

The virus naturally affects some species of South American rabbits, but European rabbits are extremely vulnerable to it. In severe cases, infected rabbits develop swelling and skin lesions followed by a systemic infection that can be fatal.

Scientists tested the virus years before the 1950 release

The idea of using myxomatosis to control Australia's rabbits was not new.

Scientists had investigated the possibility as early as the 1920s. Interest intensified during the 1930s, with researchers including Dr Lionel Bull working on the disease and its potential as a biological control.

But there was a major problem.

It was one thing to infect rabbits in controlled conditions. It was another to make the virus spread naturally through enormous populations of wild rabbits.

In early field experiments, scientists could introduce infected rabbits into individual warrens and cause deaths. But the disease often failed to travel effectively from one rabbit colony to another.

That meant myxomatosis was potentially lethal—but not necessarily useful on a continental scale.

The strange 1937 experiment on Wardang Island

In 1937, scientists conducted an important field experiment on Wardang Island, off South Australia.

Researchers released rabbits infected with myxoma virus into an enclosure containing hundreds of other rabbits. They then observed what happened as the disease moved through the population.

The experiment helped scientists understand a critical part of the problem: transmission.

Myxoma virus can be spread by blood-feeding insects such as mosquitoes and fleas. This meant environmental conditions, insect populations and the movement of the virus between rabbits could determine whether an outbreak remained local or became widespread.

The Wardang Island trials did not immediately solve Australia's rabbit crisis. But they provided scientists with valuable information about why earlier attempts had struggled.

Then came 1950—and the rabbits were hit by a viral catastrophe

More than a decade after the Wardang Island experiments, scientists returned to myxomatosis.

By then, Australia's rabbit problem remained enormous, and conventional control measures had failed to eliminate it. Researchers had also learned more about how the virus could spread.

In 1950, myxoma virus was released into wild rabbit populations.

This time, the results were dramatic.

The disease spread rapidly through rabbit populations, and the mortality rate among susceptible European rabbits was extraordinarily high. In some reports, more than 99% of infected rabbits died.

Australia's estimated rabbit population fell from roughly 600 million to about 100 million in only two years.

The impact was felt beyond the rabbit population. With fewer rabbits competing for pasture, agricultural production recovered, with contemporary estimates putting the economic benefit to wool and meat production at tens of millions of dollars.

For a moment, it appeared that Australia had finally defeated its rabbit problem.

But evolution had other plans.

The rabbits began evolving resistance

The spectacular effectiveness of myxomatosis did not last forever.

As the virus spread, rabbits with greater natural resistance were more likely to survive. Those survivors reproduced, passing some of their resistance to subsequent generations.

The virus changed too.

Extremely lethal strains could kill rabbits so quickly that the virus had fewer opportunities to spread. Less virulent strains, by contrast, could keep infected animals alive for longer, allowing mosquitoes and other vectors more time to transmit the disease.

Over generations, the relationship between the rabbits and the virus changed.

The virus remained a powerful source of mortality, but it was no longer capable of producing the extraordinary devastation seen during the first major outbreaks.

In other words, the rabbits and the virus began adapting to each other.

Australia eventually tried another virus

Myxomatosis never completely eliminated Australia's rabbit population. Instead, it became one component of long-term rabbit management.

Decades later, scientists developed another biological-control strategy.

In the 1990s, researchers successfully used Rabbit Calicivirus Disease, also known as RCD or rabbit haemorrhagic disease, to reduce rabbit numbers.

The history of Australia's rabbit invasion therefore became much more complicated than a simple story of humans releasing a virus and winning.

It was a story about invasive species, biological control and evolution.

Why the 1937 experiment still matters

The Wardang Island experiment is particularly important because it shows that scientific breakthroughs rarely happen in a single dramatic moment.

The infected rabbits released in 1937 did not suddenly solve Australia's rabbit crisis. Instead, the experiment exposed the challenges involved in making a disease spread naturally through wild populations.

Those lessons helped scientists understand what was required when myxomatosis was deployed on a much larger scale in 1950.

The episode also demonstrated something scientists would see repeatedly: nature does not simply accept a biological intervention without responding to it.

Australia's rabbits adapted. The virus evolved. And the balance between the two changed.

What began as an attempt to stop one of the country's most destructive invasive species ultimately became a remarkable real-world demonstration of evolution in action.

The rabbit invasion was never completely erased. But in 1950, Australia showed just how dramatically a virus could transform an ecosystem when conventional pest-control methods had failed.