Human anaplasmosis (formerly known as human granulocytic ehrlichiosis) is caused by Anaplasma phagocytophilum, a rickettsial organism transmitted to humans by bites from Ixodes scapularis (the blacklegged tick or deer tick). In Minnesota, the same tick vector also transmits the etiologic agents of Lyme disease, babesiosis, one form of human ehrlichiosis, and a strain of Powassan virus. A. phagocytophilum can also be transmitted by blood transfusion.
In 2011, a record number of 782 confi rmed or probable anaplasmosis cases (14.7 cases per 100,000 population) were reported (Figure 1). The median number of 298 cases (range, 139 to 782 cases) reported from 2004 through 2011 is also considerably higher than the median number of cases reported annually from 1996 to 2003 (median, 56 cases; range, 14 to 149). Five hundred three (64%) cases reported in 2011 were male. The median age of cases was 58 years (range, 2 to 92 years),18 years older than the median age of Lyme disease cases. Onsets of illness were elevated from May through July and peaked in June (32% of cases). In 2011, 27% of anaplasmosis cases (210 of 778 cases with known information) were hospitalized for their infection, for a median duration of 4 days (range, 1 to 17 days).
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