ABSTRACT
INTRODUCTION Infections with Anaplasmataceae pathogens were previously known only as diseases important to veterinary medicine. However, over the past two decades, several new species or strains have been isolated and characterized as the causative organisms of emerging or reemerging infectious diseases. Anaplasmataceae are now known to be major vector-borne pathogens of both humans and animals. Two major Anaplasmataceae that infect humans are Anaplasma phagocytophilum and Ehrlichia chaffeensis. Both of these are known to be tick-borne pathogens. A. phagocytophilum is transmitted by Ixodes persulcatus complex ticks, including I. scapularis, I. pacificas, and I. ricinus, whereas E. chaffeensis is transmitted by Amblyomma americanum. I. persulcatus complex ticks have a worldwide distribution, but A. americanum has a strong geographic specificity. This may explain why E. chaffeensis has been found only in the United States. As ticks are seasonally active, tick-borne anaplasmosis and ehrlichiosis occur seasonally. For example, A. phagocytophilum infection transmitted by I. scapularis in the United States occurs from April to October.
