Tick-borne Agents: Can You Predict Whether a New Rickettsia Is a Human Pathogen?

Spotted fever group rickettsiae are obligately intracellular bacteria that are primarily transmitted by ixodid ticks. The advent of molecular techniques and their wide application for routine tick surveillance has resulted in a growing list of new genetic types of Rickettsia; these are found within or outside geographic locations that are typically associated with previously known human pathogens such as Rickettsia rickettsii, the cause of Rocky Mountain spotted fever in the Americas or R. conorii, the agent of Mediterranean spotted fever in Eurasia. Although the pathogenicity of new arthropod-associated Rickettsia spp. is often unknown, their presence can affect perceptions about the epidemiology, ecology and distribution of classic disease agents, especially those conclusions based on serology in animals or humans. This review summarizes recent investigations of tick-borne Rickettsia found in California and discusses how these findings may influence trends and observations drawn from surveillance of spotted fever group rickettsioses in sentinel animals and humans.