Persistent Blood-borne Infections and Complex Disease Expression

It is important for the clinician to remember that vector-transmitted bacteria and protozoa have a very long evolutionary history, which spans millions of years. Many vector-borne organisms, such as Anaplasma, Babesia, Bartonella, Ehrlichia, Leishmania and Rickettsia species have an intracellular and intravascular life style, which for certain organisms are obligate in nature. Obviously, this means that the organism must reside within the cell of an arthropod, such as a louse, flea or tick, or within the cell of a mammalian host. As an example, Rickettsia has lost several critically important genes that are required for extracellular survival. In the absence of these genes, Rickettsia species have developed mechanisms that allow intracellular survival by manipulating selected biochemical pathways within the mammalian cell. For the clinician, it is critical to recognize evolutionary adaptation has facilitated complex interactions between arthropod vectors, bacteria and protozoa and mammalian reservoir hosts.