Increasing Incidence of Ehrlichia chaffeensis and Anaplasma phagocytophilum in the United States, 2000–2007

TLDR

Ehrlichia chaffeensis and Anaplasma phagocytophilum are tick‑borne rickettsial pathogens that can cause severe, sometimes fatal, human disease, and improved recognition and surveillance are expected to sustain rising reporting. From 2000 to 2007, reported incidence of both diseases rose from 0.8 to 3.0 and 1.4 to 3.0 cases per million per year, respectively, with case‑fatality rates of 1.9 % for ehrlichiosis and 0.6 % for anaplasmosis, hospitalization rates of 49 % and 36 %, lower rates in females (rate ratios 0.68 and 0.70), and 80 % of cases meeting only a probable definition while PCR confirmation increased.

Abstract

Ehrlichia chaffeensis causes human monocytic ehrlichiosis, and Anaplasma phagocytophilum causes human granulocytic anaplasmosis. These related tick-borne rickettsial organisms can cause severe and fatal illness. During 2000–2007, the reported incidence rate of E. chaffeensis increased from 0.80 to 3.0 cases/million persons/year. The case-fatality rate was 1.9%, and the hospitalization rate was 49%. During 2000–2007, the reported incidence of A. phagocytophilum increased from 1.4 to 3.0 cases/million persons/year. The case-fatality rate was 0.6%, and the hospitalization rate was 36%. Rates among female patients were lower than among male patients for ehrlichiosis (rate ratio = 0.68) and anaplasmosis (rate ratio = 0.70). Most (80%) ehrlichiosis and anaplasmosis cases met only a probable case definition, although, use of a polymerase chain reaction to confirm infections increased during 2000–2007. Heightened reporting of these diseases will likely continue with improving recognition, changing surveillance practices, and appropriate application of diagnostic assays.