Abstract
Disease introduction into the New World during colonial expansion is well documented and had a major impact on indigenous populations; however, few diseases have been associated with early human migrations into North America. 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. Anthrax has clearly been shown to be dispersed by human commerce and trade in animal products contaminated with Bacillus anthracis spores. 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 ∼13,000 ybp. In this study, we have defined the evolutionary history of Western North American (WNA) anthrax using 2,850 single nucleotide polymorphisms (SNPs) and 285 geographically diverse B. anthracis isolates. Phylogeography of the major WNA B. anthracis clone reveals ancestral populations in northern Canada with progressively derived populations to the south; the most recent ancestor of this clonal lineage is in Eurasia. Our phylogeographic patterns are consistent with B. anthracis arriving with humans via the Bering Land Bridge. This northern-origin hypothesis is highly consistent with our phylogeographic patterns and rates of SNP accumulation observed in current day B. anthracis isolates. Continent-wide dispersal of WNA B. anthracis likely required movement by later European colonizers, but the continent's first inhabitants may have seeded the initial North American populations.
Highlights
The basic premises of disease tracking have changed little since John Snow first described the London cholera epidemic of 1854
The history of B. anthracis in North America has certainly been affected by recent trade, and livestock movement [13], here we present evidence that the introduction of this pathogen can be traced to much more ancient human migrations
One of these seven strains was the Western North American (WNA) strain (A0193) [14]. These single nucleotide polymorphisms (SNPs) were screened among 128 diverse isolates and identified WNA as a monophyletic group rooted in the Old World trans-Eurasian subpopulation (TEA) group
Summary
The basic premises of disease tracking have changed little since John Snow first described the London cholera epidemic of 1854. The dispersal of Bacillus anthracis, Yersinia pestis, and human RNA viruses often reflect short term human movement frequently associated with trading contaminated animal products or inadvertently transporting primary vectors or hosts [1,3,8,9,10,11,12] Such potentially frequent and long range dispersal of pathogens can obscure more ancient phylogeographic patterns. Many have been observed along the East Coast and are associated with trade and industrial processing of contaminated animal products, often wool in textile mills [17,18,19] These cases contribute to the overall genetic diversity of North American B. anthracis isolates, but generally represent small case clusters that do not become ecologically founded. The ecological dominance and disease importance of the WNA clade led us to examine its evolutionary history in greater detail using whole genome sequence analysis and highly accurate phylogenetic reconstructions
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