Columbus: Not Guilty in Spreading Anthrax

March 31, 2009 -- Best known for the 2001 bioterror attacks that killed five people, the gram-positive, spore-forming Bacillus anthracis was believed to be an Old World disease that was introduced in North America by Christopher Columbus and other conquistadors in the late 15th century.

But at least for anthrax, Columbus was not guilty.

When the explorer set foot in the Americas, the cattle-killer was already present in North American grasslands, according to new genetic research that traced the introduction of the deadly spores to early human migrations.

The new study published in the journal PloS One, finds that North Americans started dying from anthrax some 13,000 years ago. The deadly anthrax spores were originally transported by early humans and their herds, as they migrated out of Africa, across Europe and Asia.

"During the late Pleistocene epoch, Asia and North America were joined by the Beringian Steppe ecosystem, which allowed animals and humans to freely cross what would become a water barrier in the Holocene," Paul Keim and colleagues at the Northern Arizona University wrote.

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"Humans appear to have brought B. anthracis to this area from Asia and then moved it further south as an ice-free corridor opened in central Canada about 13,000 years ago," the researchers reported.

Anthrax can live as a spore in the soil for decades until the right environment sparks it, infecting grazing animals.

Keim, the anthrax expert who led the genetic investigation of the 2001 attacks, analyzed 285 soil samples from the United States and Canada and looked at 2,850 gene markers in those samples to trace the evolutionary history of Western North American anthrax.

To estimate the age of the bacteria samples, Keim and colleagues examined the frequency of gene mutations. Since anthrax reproduces by cloning itself, changes to its genes are somewhat rare.

According to the conquistador theory, anthrax first appeared in the southern United States or Mexico and then spread northward to Canada. Along this line, the researchers expected to identify the oldest varieties of Western North American anthrax in the south.

On the contrary, the analysis found the oldest, "ancestral" bacteria populations in northern Canada, "with progressively derived populations to the south."