International Health (2009) 1, 17—25 available at www.sciencedirect.com journal homepage: http://www.elsevier.com/locate/inhe REVIEW Old and new tick-borne rickettsioses Aurélie Renvoisé, Oleg Mediannikov, Didier Raoult * Unité des Rickettsies, CNRS-IRD UMR6236-198, Université de la Méditerranée, Faculté de Médecine, 27 bd Jean Moulin, 13385 Marseille cedex 5, France Received 30 January 2009; received in revised form 10 March 2009; accepted 18 March 2009 KEYWORDS Arthropods; Ticks; Emerging infectious disease; Inoculation; Eschar; Rickettsioses Summary The field of rickettsiology is rapidly evolving. Rickettsiae are small Gram-negative bacteria that can be transmitted to humans by arthropods. In most cases they are transmitted transovarially in the arthropod; human beings are incidental hosts. In recent years the use of cell culture and molecular biology has profoundly changed our knowledge of rickettsiae and has led to the description of several new species. New rickettsial diseases have been found in three main situations: firstly, in places where no new species have been identified, typical rickettsial symptoms have been observed (Japan, China); secondly, typical rickettsioses have been found to be caused by different organisms — in such cases a new Rickettsia species has been misdiagnosed as a previously identified bacterium (for example, R. parkeri was confused with R. rickettsii); thirdly, atypical clinical symptoms have been found to be caused by rickettsial organisms such as R. slovaca. These findings challenge the old dogma that only one tick-borne rickettsiosis is prevalent in one geographical area. Many Rickettsia spp. have been identified in ticks, but have not yet been implicated in human pathology. These rickettsiae should be considered as potential pathogens. All known or suspected rickettsial diseases should be treated (including in children) with doxycycline. © 2009 Published by Elsevier Ltd on behalf of Royal Society of Tropical Medicine and Hygiene. 1. Introduction Among the well-known group of diseases named rickettsioses are found both well-characterized diseases 1 and emerging pathologies that have only recently been described. 2 The latter group represents diseases that have been discovered during the last 15—20 years, concurrently with advances in molecular biology and cell culture techniques. New species * Corresponding author. Tel.: +33 491 385 517; fax: +33 491 387 772. E-mail addresses: aurelie.renvoise@gmail.com (A. Renvoisé), olegusss1@gmail.com (O. Mediannikov), didier.raoult@gmail.com (D. Raoult). in the Rickettsia genus are regularly described, 3 and some of them have been shown to play a role in human pathology. 2 Rickettsiae are small obligate intracellular bacteria that are strongly associated with eukaryotic cells. They are often found in arthropods (ticks, mites and other insects, includ- ing lice, fleas, beetles and homopterans), 4 amoebae 5 and leeches. 6 Only blood-sucking arthropods may transmit the disease to humans, via transdermal inoculation with the arthropod’s saliva. 4 Rickettsia akari and Orientia tsutsug- amushi are transmitted by mites, Candidatus R. felis and R. typhi are transmitted by fleas, and R. prowazekii is trans- mitted by body lice; the other human rickettsioses, which are reviewed here, are transmitted by ticks. Tick-borne rickettsioses are diseases of marked endemi- city. Their prevalence is strongly linked with vector and 1876-3413/$ — see front matter © 2009 Published by Elsevier Ltd on behalf of Royal Society of Tropical Medicine and Hygiene. doi:10.1016/j.inhe.2009.03.003