Anaplasmosis, 2008

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 2008, 278 anaplasmosis cases (5.3 cases per 100,000 population) were reported (Figure 1). This represents a 14% decrease from the record number of 322 anaplasmosis cases (6.2 per 100,000 population) reported in 2007 but a 54% increase from the median number of 181 cases (range, 139 to 322 cases) reported from 2004 through 2007. 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 eighty-one (65%) case-patients reported in 2008 were male. The median age of case-patients was 58 years (range, 3 to 91 years), 18 years older than the median age of Lyme disease cases. Onsets of illness were elevated from June through August and peaked in July (26% of cases). In 2008, 40% of anaplasmosis case-patients were hospitalized for their infection, for a median duration of 4 days (range, 1 to 51 days).

Anaplasmosis co-infections with Lyme disease and/or babesiosis can occur from the same tick bite. During 2008, nine (3%) anaplasmosis case-patients were also confirmed cases of Lyme disease, and four (1%) 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 Minnesota counties where the risk of Lyme disease is greatest. In 2008, 93 (61%) of 153 case-patients with a single known county of exposure in Minnesota were exposed in Aitkin, Beltrami, Cass, Crow Wing, or Hubbard Counties. Nearly two-thirds of anaplasmosis case-patients in 2008 (142 [63%] of 226 cases with a known activity) were most likely exposed to I. scapularis ticks at their home property.

• For up to date information see>> Anaplasmosis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2008