Anaplasmosis, 2006

Human anaplasmosis (HA) is the new nomenclature for the disease formerly known as human granulocytic ehrlichiosis. HA is caused by Anaplasma phagocytophilum, a rickettsial organism transmitted to humans by bites from Ixodes scapularis (the deer tick or blacklegged tick). The same tick vector also transmits the agents of Lyme disease and babesiosis. In 2006, 176 HA cases (3.4 cases per 100,000) were reported (Figure 1). This represents a 5% decrease from the record high of 186 cases in 2005 (3.6 per 100,000) but is markedly higher than the median number of cases reported annually from 1995 to 2004 (median, 58 cases; range, 5 to 139). One hundred twelve (64%) case-patients reported in 2006 were male. The median age of case-patients was 58 years (range, 5 to 91 years), nearly 20 years older than the median age of Lyme disease cases. Onsets of HA peaked in June (59 cases [33%]), earlier in the season than Lyme disease because of a shorter incubation period and more abrupt symptoms. In 2006, 41% of HA case-patients were hospitalized for their infection.

HA co-infections with Lyme disease and/or babesiosis can occur from the same tick bite. During 2006, six (3%) HA case-patients also had objective evidence of Lyme disease, and three (2%) 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, especially Aitkin, Cass, and Crow Wing Counties.

For a discussion of the recent increase in tick-borne disease in Minnesota and the distribution of ticks that transmit HA and other tick-borne diseases, see “Expansion of the Range of Vector-borne Disease in Minnesota” in the March/April 2006 issue (vol. 34, no. 2) of the DCN.

• For up to date information see>> Anaplasmosis
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2006