AETIOLOGY

Rickettsia, Anaplasma and Ehrlichia species are obligate intracellular bacteria that are transmitted by arthropods, primarily ticks. Infections caused by members of all three genera have been detected in numerous species of Euro­pean wildlife, but these infections are subclinical, reflect­ing the probable role of these animals as reservoir hosts.

The taxonomic order Rickettsiales comprises of several genera of bacteria, three of which are recognized as para­sites or pathogens of European mammals. These three genera are Rickettsia, Anaplasma and Ehrlichia. The genus Rickettsia currently contains 27 valid species, although numerous other partially characterized strains have also been described. Species within the genus are classically delineated into two groups, the ‘spotted fever group’ (SFG) and the ‘typhus group’ (TG). This delineation, although originally merely based on the clinical presentation of human infections, reflects phylogenetic divergence within the genus. The majority of Rickettsia species belong to the SFG, although this group now includes a number of species that have not been associated with human disease. Of these species, at least 11 are known to be extant in Europe, including R. aeschlimannii, R. akari, R. conorii, R. felis, R. helvetica, R. hoogstraalii, R. massiliae, R. raoultii, R. sibirica and R. slovaca. The TG comprises of only two species, R. prowazekii, which causes epidemic, or louse-borne typhus, and R. typhi, the agent of flea-borne, or murine, typhus. Only the second of these species is associated with non­human (wildlife) hosts. The genus Rickettsia also includes one species, R. bellii that has diverged from both the SFG and the TG. This species together with some of the SFG are considered to be endosymbiotes of ticks, and ‘rickettsia- like’ endosymbiotes are considered to be extremely common in many species of arthropod. The genus Anaplasma con­tains seven valid species. Of these, there is evidence that five, namely A. centrale, A. marginale, A. ovis, A. phagocy- tophilum and A. platys, are encountered in European wild­life. The genus Ehrlichia contains five species, of which only two, E. canis and E. muris are found in Europe.

All members of the order Rickettsiales are obligate intra­cellular bacteria, and all members of the genera Rickettsia,

Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.

Anaplasma and Ehrlichia are transmitted by arthropods. The vast majority of Rickettsia species and all Anaplasma and Ehrlichia species are transmitted by ticks. The remain­ing Rickettsia species are transmitted by fleas, lice or mites.