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Other bacterial diseasesAnaplasmosis, ehrlichiosis and neorickettsiosis 

Other bacterial diseasesAnaplasmosis, ehrlichiosis and neorickettsiosis
Other bacterial diseasesAnaplasmosis, ehrlichiosis and neorickettsiosis

Richard Birtles

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date: 26 January 2022

In 2001, taxonomic reorganization of the bacterial genera Anaplasma, Ehrlichia, Cowdria and Neorickettsia resulted in the transfer of numerous species between these taxa, and the renaming of the transferred species to reflect their new taxonomic position (Dumler et al. 2001). Among the members of these genera, there are four species of established zoonotic importance, which are therefore the subject of this chapter. Two of these species were affected by the changes outlined above.

Although these four species possess markedly different ecologies, they share the fundamental biological character of being obligate intracellular bacteria that reside within vacuoles of eukaryotic cells. This lifestyle underlies their fastidious nature in the laboratory and hence our limited knowledge of their biology and pathogenicity. Nonetheless, despite this shortfall, all four are associated with diseases of established or emerging importance: E. chaffeensis provokes human monocytic ehrlichiosis (HME), E. ewingii causes human ewingii ehrlichiosis (HEE), A. phagocytophilum causes human granulocytic anaplasmosis (HGA), N. sennetsu is the agent of sennetsu neorickettsiosis.

The first three pathogens are transmitted by hard (ixodid) ticks and are encountered across the temperate zones of the northern hemisphere (and maybe beyond), although the vast majority of human infections caused by them are currently reported in the USA. There, HME and HGA are second only to Lyme disease (caused by Borrelia burgdorferi) in terms of public health significance. Furthermore, given that there is evidence of increasing population sizes and changing distributions for ixodid species (Scharlemann et al. 2008), it is not unreasonable to predict that the infections they transmit will present an increased medical burden in the future. N. sennetsu remains an enigmatic pathogen; case reports remain scarce, but serological surveys suggest high levels of exposure. The widespread consumption of raw fish across east Asia presents specific infection risks to this region, and an increased awareness that sennetsu neorickettsiosis is among the infections that can be acquired from this source is required before its public health importance can be accurately assessed.

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