Babesiosis is most often caused by the tiny parasites Babesia microti and transmitted to humans in warmer months by deer ticks — the same ones that spread Lyme disease. It is rarely passed through blood transfusion.
Most U.S. cases occur in the Northeast and Upper Midwest, but babesiosis does appear elsewhere, according to the Centers for Disease Control and Prevention. Peter Krause, a senior research scientist at Yale School of Public Health and Yale School of Medicine, said that babesiosis is “increasing in frequency and geographic range.”
A recently published 12-year study of babesiosis among U.S. Medicare beneficiaries reported “substantially increasing babesiosis diagnosis trends” particularly in endemic states and “expansion of babesiosis infections in other states.”
In 2011, when the CDC began collecting data on babesiosis, 1,126 cases were reported. Today, over 2,000 cases of babesiosis are reported in the United States each year, although Krause said he believes the actual number is many times higher. He explained: Lyme is underreported by more than tenfold, both diseases are spread by the same tick, and babesiosis is harder to diagnose. While 30,000 cases of Lyme are reported each year, the CDC estimates the actual number to be closer to 476,000.
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