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."
The Ames strain used in the 2001 mailings, which naturally occurs only in Texas, accumulated only eight mutations since its introduction from Asia, revealing itself as a recent immigrant.
On the other hand, the Western North American version exhibited greater genetic diversity and diverged by 106 mutations from its nearest Old World relatives, suggesting a more ancient introduction into North America.
"Our phylogeographic patterns are consistent with B. anthracis arriving with humans via the Bering Land Bridge," the researchers concluded.
"Historically, an animal that died of anthrax was scavenged by people for its hair, hide, bones and even consumed as food, facilitating the dispersal of spores away from a carcass," they said.
Coming from the Greek word for coal -- the disease leaves a black scab on the skin -- anthrax is at its most lethal when inhaled. Humans can also contract it by ingestion or absorption through the skin.
"Keim's genetic discovery is important and absolves Christopher Columbus and the conquistadors of inadvertently introducing the deadly bacteria in newly conquered lands," Adrienne Mayor, a visiting scholar at Stanford University's Department of Classics and History of Science, told Discovery News. "But historically, anthrax has indeed traveled with invading armies and was associated in ancient literature with war hostilities."
According to Mayor, who has authored the book "Greek Fire, Poison, Arrows and Scorpion Bombs," two plagues described in the Old Testament -- the fifth and sixth plagues called down by Moses on the Egyptians (about 1300 B.C.) -- and a similar plague during the legendary Trojan War (also set in about 1300 B.C.) appear to have been anthrax.
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