Babesiosis is a malaria-like illness caused by the protozoan Babesia microti or other Babesia organisms. B. microti is transmitted to humans by bites from Ixodes scapularis (the blacklegged tick or deer tick), the same vector that transmits the agents of Lyme disease (Borrelia burgdorferi), human anaplasmosis (Anaplasma phagocytophilum), and a strain of Powassan virus. Babesia parasites can also be transmitted by blood transfusion.
In 2010, a record number of 56 confirmed and probable babesiosis cases (1.1 per 100,000 population) were reported (Figure 1). The previous record was 31 cases in 2009. The median annual number of babesiosis cases from 2004 through 2010 (median, 24 cases, range, 9 to 56) is notably higher than the median number of annual cases from 1996 to 2003 (median, 2 cases; range, 0 to 7). Thirty-three (59%) babesiosis cases reported in 2010 were male. The median age of cases was 61 years (range, 5 to 84 years). Onsets of illness were elevated from June through August and peaked in July (40% of cases). In 2010, 26 (46%) cases were hospitalized for their infection, for a median duration of 4 days (range, 2 days to >1 month). One reported case died from complications of babesiosis in 2010.
Babesia co-infections with the etiologic agents of Lyme disease or anaplasmosis can occur from the same tick bite, although many Babesia infections are asymptomatic. During 2010, 8 (14%) babesiosis case-patients were also confirmed cases of Lyme disease, and a different 8 (14%) were confirmed or probable cases of anaplasmosis.
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