There is no doubt about that - the different strains have different names and a well-identified regional spreading.
But that remains to be seen. Arguably, the American strains (more so the South and Central American ones) are capable of causing Hanta Pulmonary Syndrome, which does not seem to occur elsewhere. But something in the order of ten deaths per year in a population of 300 million does not really make it "highly virulent" as long as we must assume the uneventful infection rate in the US to be high as elsewhere.
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