When pathogens arrived in the Americas is important for understanding the demographic history and biogeography of humans, animals and microbes of the Western Hemisphere. There have been two major periods of human migration to this hemisphere: across the Bering Land Bridge from Asia during the last Ice Age and the arrival of Christopher Columbus in 1492 began a major European and African migration. Although there were probably several minor maritime contacts between the closing of the Bering Land Bridge and 1492, most of these hypotheses are controversial and no pathogens have been linked to those contacts that I know of. So the question often becomes is the pathogen pre-Columbian (pre-1492) or post-Columbian (after 1492)?
Anthrax is a major pathogens of cattle/bison and secondarily humans. We know that it reached epidemic levels on Hispaniola in 1770, as I wrote about earlier, the island where Columbus landed in 1492. The question becomes did it come to the Americas with Columbus (or later Spaniards) and spread out of the Caribbean or had it been in North America before the arrival of the Spanish?
Phylogenetic analysis of isolates collected from all over North America show a clonal distribution pattern consistent with a northern origin (1). Bacillus anthracis is a young species with relatively few clonal lineages and few identified SNPs. Anthrax’s transmission method by spores means that there are relatively few generations for its overall age.
The Western North American (WNA) clade of Bacillus anthracis is the dominant lineage in the western hemisphere. It in turn is derived from the Trans Eurasian (TEA) clade of the A group of B. anthracis; group A represents about 90% of all strains. Polymorphisms separating the WNA clade from TEA clade suggest a genetic bottleneck early in the founding of the North American population (1). Commerce has caused local outbreaks and in a few cases the establishment of a local strain from other clades (such as the Ames strain in Texas) (1, 2). The failure of these recent outbreaks and local strains to spread points toward the importance of a native herd animal to spread and maintain it in the environment.
The transmission cycle is accomplished via spores with a slow dispersal method. Northern Canadian outbreaks in the last 50 years suggest that the natural disease cycle produces only 0.28 generations per year (1). In nature, these spores must wait for the carcase of the dead animal to decay or be preyed upon for the spores to be scattered in the soil or transmitted to scavengers. Humans have been among the most important scavengers of sick and dead animals. Early humans took hides, bones, and even meat from the infected animals. Commerce and use of hides has been the primary means of long distance transport of anthrax throughout the world (2). Anthrax kills its host within about a week, too short of a time period for the sick animal to carry it far from where it was contracted. It seems likely that anthrax was carried to North American over the Bering Straight land bridge (Beringian steppe ecosystem) in hides and other goods by Asians migrants.
“Oh give me a home where the buffalo roam and the deer and antelope play…”
Anthrax can infect a wide variety of animals, but in the American bison it has found a nearly perfect host. It is a large herd animal that ranges over a wide region and yet returns to favorite areas (wallows and salt licks). Anthrax spores are kicked up in the dust by hoofed animals and the bison head is angled down toward the ground where it can inhale the spores. (Consider the difference between the orientation of a bison nose vs. a deer or elk.) Perhaps even more importantly bison wallow in shallow depressions, called unsurprisingly buffalo wallows, where they rub their hide in the dirt probably to remove parasites and sooth irritated skin. It is thought that these wallows are key sites in the spread of anthrax. As you can see in the picture to the right, a wallowing bison stirs up a lot of dust probably rubbing cutaneous anthrax into the soil and bison who die in the wallows deposit large numbers of spores there.
Bison once roamed nearly the length of the North American continent. The ancestral Holocene bison is represented by the lightest tan area in the diagram to the right, with its descendents the wood bison (medium brown) and plains bison (dark brown). Kenefic et al (2009) predict that anthrax wasn’t introduced until the glaciers withdrew enough to open up a corridor of grasslands into the mid-continent about 13,000 years ago. The movement of humans rather than bison is more critical to the dispersal of the anthrax. As Keneifc et al (2009) point out the directionality of the evolution of lineages of WNA is distinctly north to south. The bison spread much earlier and more widely than the anthrax areas. I am left wondering about the role of Native American tribes that highly utilized and some followed herds of bison.
Bison areas became prime cattle grazing territory with the arrival of Europeans. Cattle and other domestic animals contract anthrax perpetuating the anthrax as the bison were nearly hunted to extinction. The movement of cattle around the continent further spread the anthrax and introduced some of the smaller regional lineages like the Ames strain in Texas, whose closest match is to strains in China.
Concentration of animals in an area by extensive cattle husbandry amplifies anthrax in an environment. Colonial Haiti would be a prime example of a confined area that was over exploited by cattle ranching and therefore amplified anthrax. On Hispaniola, anthrax is still considered endemic today and its the WNA clade of anthrax (1). We don’t know what it was in 1770 but it seems likely that it originated from contact with the North American continent rather than Europe.
[1] Kenefic LJ, Pearson T, Okinaka RT, Schupp JM, Wagner DM, Hoffmaster AR, Trim CB, Chung WK, Beaudry JA, Jiang L, Gajer P, Foster JT, Mead JI, Ravel J, & Keim P (2009). Pre-Columbian origins for North American anthrax. PloS one, 4 (3) PMID: 19283072
[2] Keim PS, & Wagner DM (2009). Humans and evolutionary and ecological forces shaped the phylogeography of recently emerged diseases. Nature reviews. Microbiology, 7 (11), 813-21 PMID: 19820723
See the previous post: Famine and Epidemic Anthrax, Saint-Dominque (Haiti), 1770
Contagions 