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 also transmits the etiologic agents of Lyme disease (Borrelia burgdorferi), babesiosis (Babesia microti), and a strain of Powassan virus. A. phagocytophilum can also be transmitted by blood transfusion.
In 2010, a record number of 720 confirmed or probable anaplasmosis cases (16.9 cases per 100,000 population) were reported (Figure 1), more than twice the 317 cases reported in 2009. The median number of 322 cases (range, 139 to 322 cases) reported from 2004 through 2010 is also considerably higher than the median number of cases reported annually from 1996 to 2003 (median, 56 cases; range, 14 to 149). Four hundred twenty-seven (59%) cases reported in 2010 were male. The median age of cases was 57 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 (36% of cases). In 2010, 27% of anaplasmosis cases (194 of 719 cases with known information) were hospitalized for their infection, for a median duration of 4 days (range, 1 to 42 days). One reported case died from complications of anaplasmosis in 2010.
A. phagocytophilum co-infections with the agents of Lyme disease and/or babesiosis can occur from the same tick bite. During 2010, 33 (5%) anaplasmosis cases were also confirmed cases of Lyme disease, and 8 (1%) were confirmed or probable cases of babesiosis. Because of under-detection, these numbers may underestimate the true frequency of co-infections.
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