Shared vector-borne zoonoses of the Old World and New World: home grown ortranslocated?

Humans inhabiting the Old World and New World share a wide variety of patho gens. Processes that result in the disjunct biogeographic distribution of p athogens with common vertebrate reservoirs or vectors are more difficult to unravel than those influencing the distribution of infections spread only through human-to-human transmission. The origins of species and complexes o f tick-borne bacteria are unclear. The agent of Lyme borreliosis may have s peciated in the New World following geographical isolation of ticks harbori ng ancestral spirochetes; the subsequent spread to Europe of B. burgdorferi sensu stricto may have occurred within historical times. Other tick-borne agents, such as the ehrlichiae causing human granulocytic ehrlichiosis, are genetically very similar in the Old World and New World. As the taxonomic distinctions among these related agents of human and veterinary importance appear increasingly blurred, the processes leading to the current discontin uous geographic distributions will also become the source of continuing spe culation. Accumulating data suggest an Old World origin for a group of bact eria that include B. elizabethae, a human pathogen first identified from th e New World. The potential public health significance of these newly descri bed organisms is undefined, but of international interest as their vertebra te reservoir has been introduced throughout the world.