The natural history of Anaplasma phagocytophilum
Section snippets
Historical background
Anaplasma phagocytophilum is the recently emended name replacing three species of granulocytic bacteria, Ehrlichia phagocytophila, Ehrlichia equi and the agent of human granulocytic ehrlichiosis, after a reorganization of the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales (Dumler et al., 2001). The prototype of A. phagocytophilum, the causative agent of tick-borne fever (TBF) in sheep, cattle and goats, was first described in 1940 (Gordon et al., 1940), 8 years after the
Bacteriology
The prototype of A. phagocytophilum, which causes TBF in sheep and cattle, was initially given the name Rickettsia phagocytophila (Foggie, 1949). It was then renamed as Cytoecetes phagocytophila (Foggie, 1962) to reflect its predilection to granulocytes and its morphological similarity to Cytoecetes microti (Tyzzer, 1938). It was subsequently included in the tribe Ehrlichieae of the order Rickettsiales, as a separate species, E. phagocytophila (Ristic and Huxsoll, 1984), but workers in the UK
Host range and reservoirs of infection
The host range of A. phagocytophilum appears to vary according to geographical regions. The incidence and severity of the disease in a particular host also appear to vary from one region of the world to the other. This variation is largely dictated by the strain or variant of A. phagocytophilum the reservoir, the incidental and reservoir hosts to which it has adapted and the capacity of the vectors present in a particular region or area (Tuomi, 1967c, Ogden et al., 2002b, Taglas and Foley, 2006
Sites of primary multiplication
The early stages of pathogenesis of A. phagocytophilum in its various mammalian hosts remain to be clearly elucidated. For example, it remains to be established where the bacteria replicate after entering the dermis following tick bite before the development of bacteraemia 4–7 days later. Even when susceptible animals are intravenously inoculated with infected blood, bacteraemia is not detected for up to 72–96 h. It appears therefore that the organism remains at undetectable levels in the blood
Conclusions
Although A. phagocytophilum has been known as a veterinary pathogen for over 70 years, it was only recognized as a cause of human disease in the USA and parts of Europe since 1994, creating a renewed interest in this fascinating bacterium. The recognition that it is uniquely capable of evading the bactericidal effects of granulocytes has also attracted interest from scientists to use it as a tool to investigate the mechanisms of bacterial killing by neutrophils. This has contributed to
Conflict of interest
None declared.
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