The city of Sverdlovsk, now renamed YekaterinburgAlexei Vladykin / AP

It was, they said, contaminated meat. In April of 1979, people in the Soviet city of Sverdlovsk began falling mysteriously ill: fevers, coughs, vomiting. At least 66 people died. When officials came 1,000 miles from Moscow to investigate, they concluded the victims had fallen ill from eating cattle infected with anthrax.

It was not contaminated meat.

It was an accident at a clandestine biological weapons lab that allowed deadly anthrax spores to contaminate Sverdlovsk’s air, as evidence unearthed later would show. Over the years, as DNA sequencing technology has improved, scientists have been piecing together more and more information about the anthrax strain. This year, as Ars Technica recently noted, U.S. scientists finally sequenced the strain—using decades-old autopsy samples Soviet pathologists had saved in secret.

Not too long after the accident, German and British newspapers began reporting on rumors of anthrax leaking from a military installation at Sverdlovsk. The Soviets denied it—vehemently. In 1988, Soviet officials came to U.S. to give a three-hour talk at the National Academy of Sciences, presenting facts and figures and even slides of gut tissue from autopsies. “Sverdlovsk’s ‘mystery epidemic’ of 1979 lost much of its mystery this month,” began a Science news article at the time, “when a group of Soviet doctors came to the United States and met with scientists and reporters to give a firsthand account of what happened.”

Not everyone believed the explanation. Anthrax spores do naturally live in the soil and occasionally sicken people—but never before on this scale. The U.S. intelligence community had already gathered information, classified of course, pointing to an accident at a bioweapons lab. “I knew they were lying,” says Philip Russell, a now-retired Army infectious disease researcher, “and I knew they were lying because I had been briefed on Defense Intelligence Agency evidence.” For the public and for academic scientists, irrefutable evidence of a bioweapons facility at Sverdlovsk didn’t come until 1992, after the fall of the Soviet Union.

That year, Harvard biologist Matthew Meselson led a team of American scientists to Russia to investigate. Meselson was the one who urged the Soviet officials to give the National Academy of Sciences presentation—and the team was inclined to believe the Soviet explanation. But in Sverdlovsk, now renamed Yekaterinburg, they found evidence that flat-out contradicted the official line.

That evidence was the bodies of the victims. The pathologists who performed the autopsies back in 1979, Faina Abramova and Lev Grinberg had hidden the tissue samples, preserved all this time with formaldehyde and embedded in paraffin wax. In her book Anthrax: The Investigation of a Deadly Outbreak, Jeanne Guillemin, Meselson’s wife and a member of the U.S. team, describes how Abramova saved the autopsy samples:

In 1979, she took responsibility for hiding her materials when she knew the KGB wanted the autopsy records. She has told us she placed the jars of gross organs preserved in formaldehyde in the hospital’s pathology museum, on shelves among other such jars, like so many purloined letters. She hid the tissue samples in an innocuous corner cabinet near the autopsy room. When I asked her why she did this, she replied proudly, “It was my work!”

When David Walker, a pathologist at the University of Texas Medical Branch and another member of the team, got a look at the photos from the Soviet pathologists, he knew immediately. “From the very first picture they showed me,” he says, “I knew it was inhalation anthrax,” says Walker. Anthrax spores, when inhaled, germinate in lymph nodes in the chest, causing them to swell with blood and fluid. That’s exactly what Walker saw. The victims clearly did not die of intestinal anthrax from eating contaminated meat; they had died of inhalation anthrax, after breathing in spores.

Other evidence piled up. The U.S. team interviewed family members of the patients, and a map of their locations showed them all downwind of the bioweapons facility. And in 1992, Ken Alibeck, a Soviet scientist who worked on the bioweapons program, defected to the U.S. He would go on to co-write Biohazard: The Chilling True Story of the Largest Covert Biological Weapons Program in the World—Told from Inside by the Man Who Ran It, which said the anthrax got out when an air filter at the Sverdlovsk plant was not properly replaced.

Eventually, Grinberg, one of the pathologists, came to Texas to work with Walker. He brought the preserved organs and slides with him. When he returned to Russia months later, he left several samples that ended up at Los Alamos National Laboratory in New Mexico.

Microscopes could only tell scientists so much about the anthrax at this point. To learn more about the strain itself, they had to get down to the molecular level—to sequence its DNA. Sequencing could shed light on whether the Soviets were working on antibiotic- or vaccine-resistant strains that could make anthrax an even deadlier biological weapon.

So in 1996, Paul Keim, a microbiologist at Northern Arizona University, set about figuring how to do that. “The tools in 1996 were pretty primitive by today’s standards, but we thought we were pretty hot,” he says. The problem is that formaldehyde, which preserves the structure of tissues, shreds DNA into pieces. They did sequence some parts, but the damage to the DNA only revealed so much.

Today, however, the first step of next-generation sequencing is in fact chopping up DNA into pieces, which are then reassembled using computers. “It literally was we just had to wait for this technology to catch up,” says Keim. His team took two preserved tissue samples still at Los Alamos and shaved off a corner to put through the sequencing machine. The Soviet anthrax strain, it turns out, closely matched natural strains. The lab at Sverdlovsk was probably not working on genetically engineering anthrax.

Almost forty years later, scientists now have a nearly complete picture of the strain that killed so many people in Sverdlovsk—still the deadliest outbreak of inhalation anthrax in history. It was, more definitively than ever, not contaminated meat.

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