On the first Monday in April 1979, a wind blew south through a Siberian city called Sverdlovsk.

A few days later, people and sheep began to die.

Government officials said the victims had eaten meat contaminated with anthrax or come into contact with animals sickened by the deadly bacteria. Or perhaps, officials hinted, it was a plot by the American government.

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By the time the outbreak ended two months later, 64 people had died.

It would take nearly four decades for western scientists to figure out what had happened.

They shared their latest conclusions on Tuesday in the journal mBio: The Soviets had been mass-producing deadly anthrax spores as a bioweapon, but they weren’t manipulating the bacteria’s genes. There was no need. It was deadly enough without bioengineering.

“I guess I was a little relieved,” said senior author Paul Keim, a professor of biology and director of the Center for Microbial Genetics and Genomics at Northern Arizona University. “It would have been really dastardly if they had been genetically engineering and weaponizing it.”

The new research was made possible by a 1992 trip to Sverdlovsk, by then renamed Yekaterinburg, by American researchers determined to piece together what had really happened in the spring of 1979.

Team members brought back samples of the victims’ lungs and other tissue, preserved in paraffin and formaldehyde. They wrote several scientific articles and books about their research. But they didn’t have the tools to sequence the genes of the anthrax strain until quite recently.

The new genetic analysis shows that the deadly strain was nearly genetically identical to a wild strain found in farm animals earlier in the year.

The new genetic analysis doesn’t just solve a historical mystery. It can also be used as a fingerprint to identify the Soviet strain if it ever turns up again, Keim said.

Already, expertise developed from researching the 1979 anthrax outbreak has come in handy.

A week after the 9/11 attacks, someone began sending anthrax-laced letters to news organizations and political leaders. Five people were killed over the next six weeks. Keim said his lab quickly determined that the letters hadn’t come from Iraqi anthrax supplies. The strains just didn’t match.

A lethal spore, drifting on the wind

The American team that traveled to Siberia in the early ’90s was led by Matthew Meselson, a professor at Harvard, and his wife, Jeanne Guillemin, an expert in medical sociology who now works at MIT. They toured a local cemetery and conducted interviews with family members, survivors, and hospital workers, according to a paper they published in Science in 1994.

They checked meteorological data that revealed a cold front had brought temperatures to well below freezing on that early April day. A brisk wind, of about 10 mph, blew from 4 a.m. to 7 p.m.

At some point that day, spores of anthrax weighing something less than a paperclip were apparently released into the wind from a nearby military biological factory called Compound 19.

Later, one official suggested that workers had temporarily forgotten to replace a filter in an exhaust system, triggering the release.

Anthrax spores can lie dormant in soil for decades. They don’t multiply unless they gain access to the lungs, stomach, or skin of a living creature. It takes a few thousand spores inhaled deep into the lungs to cause disease in people. The Sverdlovsk spores killed sheep — which are more susceptible to anthrax — as far as 30 miles downwind from the Soviet facility, while human deaths were seen for about 2.5 miles.

The spores, which tend to clump, have to be just the right size to cause inhalational anthrax. Too many spores in a clump and they will get trapped by the mucus of the nose or digested by the stomach. The Soviet weapons makers were focusing on keeping the spores in clumps that measured just a few microns across, too small to get trapped in mucus, Keim said.

Once inside the body, spores are carried by the lymphatic system into the sternum, where they germinate and begin to reproduce, said Raymond Zilinskas, who directs the chemical and biological nonproliferation program at the Middlebury Institute of International Studies at Monterey, in California. Then, the activated bacteria begin pumping out toxins. Left untreated, these can kill within a few days.

Anthrax is easily defeated by common antibiotics, including penicillin and Cipro, Zilinskas said. There is an effective vaccine, but it has to be given before exposure.

The bacteria are considered well-suited to biological warfare. Anthrax spores can remain stable for years. Small quantities can wreak havoc.

Haunting questions about the fate of the anthrax

The Soviets manufactured enough anthrax, Keim believes, “that if dispersed in the right way, it would have killed a large fraction of the human population.”

During the Cold War, they were losing the weapons race to the US, and they couldn’t afford to keep building nuclear weapons, he said. “So, they went to this cheaper weapon,” Keim said. “Anthrax has been called ‘the poor man’s nuclear weapon.’”

David Walker, executive director of the Center for Biodefense and Emerging Infectious Diseases at the University of Texas Medical Branch in Galveston said he thinks anthrax is likely to remain the biological weapon of choice among terrorists.

So, does he stay up at night, brooding?  “I don’t worry about anything,” the 73-year-old said, noting that Americans are far more likely to be injured on their drive to work than in a terrorist attack.

The Soviets reportedly destroyed their anthrax stockpiles in the 1990s, by inactivating them with Clorox and burying them on “Resurrection Island” in the Aral Sea — a giant lake at the border between Kazakhstan and Uzbekistan. Water levels in the sea have since dropped, diverted to irrigate cotton fields, and the island is now an easily accessible peninsula, Keim said.

No one knows if all the stocks actually made it to the island, whether or where they were inactivated — or whether any spores might still survive.

“I wonder whether all the stocks were destroyed or not,” Walker said. “That to me was the worry. We haven’t seen them.”

Correction: An earlier version of this story misstated the number of people killed in the anthrax outbreak.

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