In April 1979, a mysterious biological weapon escaped from a lab in the Soviet Union, killing at least 66 people and an unknown number of animals in what is now central Russia.

Almost 40 years later, researchers have finally sequenced the genome of the bacteria behind the deaths, confirming that it was a strain of anthrax-causing bacterium, Bacillus anthracisthat had been modified by Soviet scientists.

Although the Cold War is over, the threat of biological warfare still looms, and now that the researchers have the genetic sequence of this anthrax strain, they'll be able to identify if the deadly bacteria - or any of its relatives - happens to be released again.

And hopefully they'll be able to figure out how to stop it.

The research also provides fascinating insight into the Soviet Union's strategy and capabilities when it came to biological warfare.

At the time of the accident, this deadly strain of anthrax was being stored in a top-secret lab called Compound 19, near the Soviet city of Sverdlovsk, which is now known as Ekaterinburg.

But on 2 April 1979, the modified bacteria escaped as a result of air filters being replaced and not properly reinstalled. Within a week, at least 66 people - and according to some reports, up to 105 - were dead.

Soviet officials at the time blamed the outbreak on tainted meat sold on the black market, but in 1994, a Harvard University investigation showed that the real cause was a plume of anthrax spores released from Compound 19.

"It was one of the largest inhalation anthrax outbreaks in history - and one of the few traces of the Soviet's secret dabbling in bioweapons research - all thanks to botched air filter maintenance," writes Beth Mole for ArsTechnica.

The event is sometimes referred to as "biological Chernobyl".

In fact, if the wind had been blowing towards the centre of town that day, thousands, or even hundreds of thousands, could have been killed. 

But even though the 1994 investigation revealed that anthrax was to blame for this devastating event, until now, scientists hadn't been able to sequence the bioweapons' DNA, and had no idea how the Soviets had modified the bacteria.

They also had no way of knowing if the deadly bug had ever been released again - or would be in future. 

Anthrax is caused by the bacteria Bacillus anthracis, and even in its unmodified, natural state, it's pretty terrifying. 

It's usually pretty rare in humans, and is spread through animals. But the bacteria are also capable of forming spores, which are pretty much indestructible and undetectable, and can lie dormant for decades.

Once inhaled, eaten, or introduced into our bodies some other way, the bacteria produces toxicity, and an infection that can kill people in days. 

The good news is regular anthrax can be treated by antibiotics, and we even have a vaccine against it. 

But, during the Cold War, the Soviets were working on a strain of anthrax that would be resistant to antibiotics, and would even be able to evade a vaccine.

To figure out what this final version looked like, a team from North Arizona University took samples of the deadly bacteria from preserved tissue belonging to victims of the 1979 attack.

These samples had been preserved in formaldehyde, so the bacteria's DNA had been damaged, but the team figured out how to piece together the damaged bits to form a full genome's worth of DNA code.

When they compared this new strain to the family tree of other anthrax strains, they found that it was a very close relative of the original vaccine strain of the bacteria.

In fact, there were just a few genetic differences between them.

This could mean one of two things. First, it's possible that the vaccine strain was so nasty, the researchers didn't have to do much to it to make it highly virulent.

"All of this is highly suggestive of a weapons program that identified a suitable strain, maintained master cell stocks to avoid extensive passage, and per- formed minimal manipulations in order to maintain virulence," the team concluded.

"This strategy must have been used to produce large quantities of highly virulent material, as evidenced by the anthrax deaths in 1979."

But the results also suggest that the Soviet researchers might have struggled to make their anthrax strain more deadly than the original. Although this was just one strain that escaped from Compound 19, so we have no way of knowing what else was in there.

There are rumours the Soviets did similar tinkering with plague bacteria (Yersinia pestis); Francisella tularensis, the bacteria that causes tularemia; and Legionella bacteria, which causes Legionnaire's disease.

That's a problem, because there's evidence that the Soviet Union's bioweapons are still lurking in labs in Russia today.

In theory, facilities such as Compound 19 should have been shut down in 1975, after the international Biological Weapons Convention. But, as the Sverdlovsk accident shows, that wasn't the case.

And there are rumours that the biowarfare research continued even after that. 

"In fact, reports from Soviet defectors in the 1990s tell of a vast, highly funded program involving tens of thousands of researchers working on biological weapons to lob at America and other targets," writes Mole.

Since then, Russian officials have refused outside access to any remaining labs and facilities. So while we don't have any evidence that these bioweapons still exist, we also don't have any evidence that they don't.

At least now we have some idea what we might be dealing with, if we ever face the same foe again. 

The research has been published in mBio.

H/t: ArsTechnica