New World Anthrax

Stop me if you’ve heard this one before.  Back in 1492, Columbus sailed the ocean blue bringing along a host of deadly diseases to the American continents’ “virgin soil”.  These “virgin soil epidemics” then proceeded to devastate the Native American peoples, paving the way for European colonization of the Americas.

As with most things in life, it’s not quite true.

Although, yes, Europeans did bring over some diseases that had never been present in the Americas, like smallpox and measles, there were several diseases that were already present at the time of European contact whose introduction have been attributed to European explorers/settlers.  One of these diseases is anthrax.

Anthrax is a zoonotic disease, caused by the B. anthracis bacterium, that primarily infects bovines and is found across the globe.  Although humans aren’t the pathogen’s primary target, they can be infected and are the primary vehicle for the spread of anthrax via contaminated animal products (such as hides).  One interesting characteristic of anthrax is that it has a very low mutation rate, resulting in a slow accumulation of nucleotide polymorphisms.  This makes it relatively easy to assess the relatedness of different clades of B. anthracis.

There are two clades of B. anthracis that are endemic to North America; the Ames clade, found in Texas, and the Western North American (WNA) clade, found throughout north and central western North America.  Although the Ames has been determined to originate from Asia, it contains very few small nucleotide polymorphisms (SNPs) that separate it from it’s closest relative, indicating that it’s a rather recent arrival.  The WNA clade is where things start to get interesting.

The WNA clade is separated from its closest relatives by 106 different SNPs, which is a large amount for the B. anthracis bacterium.  It’s also been found that there’s six different sub-clades  within the WNA clade that are, phylogenetically, arranged in a north to south pattern, which indicates that the ancestral bacterium invaded in the Northwestern portion of North America and moved South.  An eerily similar pattern to the models of human migration into the Americas.  What’s more, given a conservative estimate of SNP accumulation and B. anthracis generation time, the estimate for the divergence of the WNA clade from its closest relatives is approximately 13,000 ybp, which falls conveniently close to a migration event of early humans into the Americas across the Beringia land bridge (16,000 to 13,000 ybp).

Phylogeography of the WNA B. anthracis
This data indicates that B. anthracis likely first arrived in the Americas via early human migration across Beringia and then spread south through human migration and trade routes.  This means that, although European contact was likely necessary for larger scale and faster spread of the bacterium, it was already endemic well before Europeans were even considering attempting to find a sea route to Asia.

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By:  Taylor Hellmann

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