Tracing anthrax's American roots

— -- Anthrax, the bioterror scourge and cattle killer, has a surprisingly ancient North American pedigree, report genetic researchers.

Best-known as the lethal bacteria mailed in the 2001 bioterror attacks that killed five people, anthrax is found naturally in the soil worldwide. Cows and goats — grazing animals — most often suffer from anthrax, with veterinary cases reported every year.

"With genomic analysis, we can really ask interesting questions about the origins of something like anthrax," says Talima Pearson of Northern Arizona University in Flagstaff, an author of a PloS ONE journal study that sought the hardy bug's origins. The introduction of cattle by Spaniards, and trade with Europe that started in the 1600s have often been pointed to as the start of the disease in North America, part of a wave of Old World diseases, such as the smallpox thought to have killed millions in the colonial era.

Call it a case of archaeology by biology. In the study, Pearson and colleagues looked at 285 anthrax-laced soil samples from throughout Canada and the United States. As an outgrowth of the 2001 terror attacks, the NAU lab has expanded its "molecular genotyping" capabilities for the bacteria, and looked at 2,850 gene markers common to all those samples to see how closely related each one was, and how those links changed with geography.

The researchers developed a "molecular clock" to estimate the age of the bacteria samples. Anthrax reproduces by cloning itself, so changes to its genes are relatively rare, says NAU study senior author Paul Keim, which suggested to the researchers that they should estimate its age based on the frequency of those changes.

Along the lines of the conquistador theory, the team expected to see the oldest varieties of Western North American anthrax residing in the south and diversifying as they moved northwards. Instead, says the study, the analysis found the oldest, "ancestral" bacteria populations in northern Canada, with newer ones further south."The pattern just jumped out for anthrax coinciding with the peopling of the New World," says geologist James Mead of East Tennessee State University in Johnson City, a study co-author, after the molecular clock roughly traced the oldest, northernmost, samples to around 13,000 years ago.

The team speculates that anthrax arrived in North America over the now-vanished Bering Land Bridge that connected Asia and Alaska during a past Ice Age. Rather than bison slowly carrying the bacteria south in their travels, the wide dispersal of the samples points to human hunters, the earliest immigrants to the continent, as the culprits who carried tainted furs and meat with them in their travels.

"The idea is speculative at this point," Mead says.

Related work by the same group, described by Keim at a recent biodefense meeting in Baltimore, finds a more recent origin for the well-known Ames strain of anthrax, best known as the type of anthrax used in the 2001 mailings. The strain was traced to soil samples in Jim Hogg County in south Texas in a 2008 Emerging Infectious Diseases journal study. Ames hides in the dirt amid the Western North American anthrax varieties in Texas and Oklahoma but not much farther north, Keim said at the meeting.

That Ames strain most closely, although still distantly, resembles strains found today in China, he added, suggesting it travelled by trade within the last 300 years. These Chinese strains transition toward European varieties along the path of Asia's ancient Silk Road trading routes, suggesting another area for historical anthrax investigation.

"I would like to compliment Keim and collaborators for their remarkable discovery that represents a real scientific breakthrough in being able to trace back the spread of a bacterial pathogen over a period of more than 10 millennia," says anthrax researcher Joachim Frey of Switzerland's Institute of Veterinary Bacteriology in Berne, who was not part of the study.

"It has been pretty clear for some time that Ames, as we know it, had originated in China and arrived somewhere in (New England) as a result of imported contaminated hair or hides, probably hair," says Martin Hugh-Jones of Louisiana State University in Baton Rouge, who was also not part of the study. "From the New England mills it made its way in the washing waters into local cattle downstream, and from their carcasses, into bone meals which were fed, eventually, to cattle in western and southern Texas."

But Hugh-Jones has doubts about the Asian-origins theory when applied to the more common Western North American strain.

Anthrax is thought to have arisen in Africa, which means it would have had to travel through Asia in prehistory "at a fair clip", he says, to reach Alaska by 13,000 years ago. "A simpler hypothesis is that it did come over with the early Spanish settlers in the 16th Century," says Hugh-Jones, which would mean that the oldest bacterial populations are instead in the south.