Ticks: Emerging and emergent tick-borne infections

INTRODUCTION Human activities continue to change the landscape vastly, altering faunal associations and thereby contact with arthropod vectors, producing circumstances that serve as the basis for the emergence of a vector-borne infection. However, few ‘emerging’ tick-borne infections are novel. Many (ehrlichiosis, babesiosis) have long been recognized as veterinary health problems. Some rickettsioses may be due to agents that were once thought to be tick endosymbionts. Others, such as the agents of bartonellosis, may form paratenic or dead-end associations with ticks. Some recently identified agents (deer tick virus, Borrelia lonestari ) are ‘in search of an emerging disease’. Emergent epidemiological associations (Masters' disease) are in search of an agent. Finally, apparently well-characterized tick-borne infections, such as Rocky Mountain spotted fever, tularaemia and tick-borne encephalitis, remain neglected by researchers but retain the potential for resurgence. We briefly review the diversity of these infectious agents, identify aetiological enigmas that remain to be solved, and provide a reminder about ‘old friends’ that should not be forgotten in our pursuit of novelty. We suggest that newly recognized agents or tick–pathogen associations receive careful scrutiny before being declared as potential public health burdens. REDISCOVERED, BETTER CHARACTERIZED, OR NEW? Modern approaches to identifying and characterizing infectious agents, using nucleic acid amplification and molecular phylogenetic algorithms, are very powerful (Relman, 2002). However, there are fallacious assumptions that: (1) data accumulated by older (‘classical’) methods are not as precise and thus not to be trusted; and (2) a DNA or RNA sequence represents something novel if it does not match one that is already present in GenBank or other genetic information databases.