Anaplasmosis, 2009

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). The same tick also transmits the agents of Lyme disease and babesiosis. A. phagocytophilum can also be transmitted by blood transfusion.

In 2009, 317 anaplasmosis cases (6.1 cases per 100,000 population) were reported (Figure 1), nearly as high as the record 322 cases reported in 2007. The 317 cases in 2009 represent a 14% increase from the 278 anaplasmosis cases (5.3 per 100,000 population) reported in 2008 and a 70% increase from the median number of 186 cases (range, 139 to 322 cases) reported from 2004 through 2008. It is also markedly higher than the median number of cases reported annually from 1996 to 2003 (median, 56 cases; range, 14 to 149). One hundred ninety-eight (62%) cases reported in 2009 were male. The median age of cases was 58 years (range, 5 to 96 years), 19 years older than the median age of Lyme disease cases. Onsets of illness were elevated from June through August and peaked in July (37% of cases). In 2009, 29% of anaplasmosis cases (90 of 313 cases with known information) were hospitalized for their infection, for a median duration of 4 days (range, 1 to 18 days). One case died from complications of anaplasmosis in 2009.

A. phagocytophilum co-infections with the agents of Lyme disease and/or babesiosis can occur from the same tick bite. During 2009, 9 (3%) anaplasmosis cases were also confirmed cases of Lyme disease, and 7 (2%) were confirmed cases of babesiosis. Because of under-detection, these numbers may underestimate the true frequency of co-infections.

The risk for anaplasmosis is highest in many of the same areas where the risk of Lyme disease is greatest. In 2009, approximately two-thirds of anaplasmosis cases described I. scapularis exposures in Aitkin, Beltrami, Cass, Crow Wing, Hubbard, Itasca, or Pine Counties. The remainder occurred in other counties of eastern, northern, and southeastern Minnesota, or in Wisconsin.

• For up to date information see>> Anaplasmosis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2009