The agents of scrub typhus

The vast majority of cases of scrub typhus are caused by infection with the obligate intracellular bacterium Orientia tsutsugamushi, a member of the family Rickettsiaceae.   The agent of scrub typhus was named Rickettsia orientalis as late as the 1940’s and subsequently classified as Rickettsia tsutsugamushi, a member of the genus Rickettsia.   Although it remains most closely related genetically to the members of the genus Rickettsia, the introduction of DNA sequence analysis indicated that O. tsutsugamushi was sufficiently divergent from other members of the genus to warrant classification into a new sister genus Orientia (Tamura et al., 1995).

Until recently, O. tsutsugamushi was thought to be the only member of the genus.   In 2010, a report on a case of scrub typhus acquired in Dubai identified a related bacterium, named O. chuto, to be an agent of infection (Izzard et al., 2010).    Molecular analysis of an isolate of O. chuto showed that it was beyond the known genetic similarities found among the known isolates of O. tsutsugamushi, but formed a complex with O. tsutsugamushi, confirming the genus as divergent from any of the other taxa within the Rickettsiaceae.   Analyses of genomic sequences suggest that the genera Orientia and Rickettsia share a more recent common ancestor with each other than with other rickettsiacea such as Anaplasma, Ehrlichia, Neorickettsia or Wolbachia.

There are indications that additional members of the genus Orientia may exist in divergent geographic areas such as South America or Africa (Balcells, et al., 2011; Horton et. al., 2016 ; Cosson et al., 2015) .