Anaplasmosis, 2007

Human anaplasmosis (HA) (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.

In 2007, a record number of 322 HA cases (6.2 per 100,000 population) were reported (Figure 1). This represents an 83% increase from the 176 cases in 2006 (3.4 per 100,000) and a 73% increase from the previous record of 186 cases in 2005 (3.6 per 100,000). It is also is markedly higher than the median number of cases reported annually from 1996 to 2004 (median, 76 cases; range, 14 to 139). Two hundred four (63%) case-patients reported in 2007 were male. The median age of case-patients was 58 years (range, 4 to 92 years), 17 years older than the median age of Lyme disease cases. Onsets of illness peaked in June (33% of cases), earlier in the season than Lyme disease. In 2007, 40% of HA case-patients were hospitalized for their infection, for a median duration of 4 days (range, 1 to 26 days). One case-patient died from complications of HA in 2007.

HA co-infections with Lyme disease and/or babesiosis can occur from the same tick bite. During 2007, 18 (6%) HA case-patients also had objective evidence of Lyme disease, and three (1%) had evidence of babesiosis. Because of under-detection, these numbers may underestimate the true frequency of co-infections.

The risk for HA is highest in many of the same Minnesota counties where the risk of Lyme disease is greatest. In 2007, 140 (59%) of 237 case-patients with a single known county of exposure in Minnesota were exposed in Aitkin, Cass, or Crow Wing counties. About half of anaplasmosis case-patients in 2007 (133 [51%] of 262 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, 2007