Rickettsiales and rickettsial diseases in Australia - Murdoch ...

1.3. Interactions of Rickettsiales and arthropods
The route of transmission of all species of Anaplasma, Ehrlichia and Rickettsia
is via an arthropod vector 248 . The hard ticks (Ixodidae) are the primary vectors
for Anaplasma, Ehrlichia and the spotted fever and transitional group rickettsia,
with the exception of R. akeri and R. felis, which are transmitted by mites and
fleas respectively 2, 113, 248 . Typhus group rickettsiae are transmitted by fleas
while scrub typhus group rickettsiae are transmitted by trombiculid mites 220, 225 .
Infection in vertebrates is typically via a bite from an infected arthropod 248 .
Only selected animals produce rickettsemias of sufficient extent and duration to
allow uninfected ticks to become infected, for example, Anaplasma
phagocytophilum in the white tailed deer 65, 246 . For the majority of rickettsial
species, the tick vector is also the reservoir for the organism and these are
maintained by inheritance of the agent, specifically the tick’s progeny are born
infected due to transovarial transmission 248 . Uninfected ticks however are
believed to primarily acquire rickettsiales by co‐feeding, when a number of ticks
feed within close proximity resulting in direct spread 248 .
1.4. Genomics of the order Rickettsiales
Genetically, the species Anaplasma, Ehrlichia, Rickettsia and Orientia have one
major trait in common, and that is the small size of their genomes (between 1.1
to 1.5Mb) 167 with G+C content of approximately 30% 14 . Even with such a small
genome, as much as 24% of the DNA of some Rickettsiales species is non-
coding DNA or pseudogenes 5 . This is believed to be a result of genome
degradation and reduction accompanying adaptation to a parasitic intracellular
lifestyle
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22 . Many biosynthetic pathways present in free-living bacteria are