Elsevier

Veterinary Parasitology

Volume 167, Issues 2–4, 10 February 2010, Pages 108-122
Veterinary Parasitology

The natural history of Anaplasma phagocytophilum

https://doi.org/10.1016/j.vetpar.2009.09.013Get rights and content

Abstract

Anaplasma phagocytophilum is the recently designated name replacing three species of granulocytic bacteria, Ehrlichia phagocytophila, Ehrlichia equi and the agent of human granulocytic ehrlichiosis, after the recent reorganization of the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales.

Tick-borne fever (TBF), which is caused by the prototype of A. phagocytophilum, was first described in 1932 in Scotland. A similar disease caused by a related granulocytic agent was first described in horses in the USA in 1969; this was followed by the description of two distinct granulocytic agents causing similar diseases in dogs in the USA in 1971 and 1982. Until the discovery of human granulocytic anaplasmosis (HGA) in the USA in 1994, these organisms were thought to be distinct species of bacteria infecting specific domestic animals and free-living reservoirs. It is now widely accepted that the agents affecting different animal hosts are variants of the same Gram-negative obligatory intracellular bacterium, which is transmitted by hard ticks belonging to the Ixodes persulcatus complex. One of its fascinating features is that it infects and actively grows in neutrophils by employing an array of mechanisms to subvert their bactericidal activity. It is also able to survive within an apparently immune host by employing a complex mechanism of antigenic variation. Ruminants with TBF and humans with HGA develop severe febrile reaction, bacteraemia and leukopenia due to neutropenia, lymphocytopenia and thrombocytopenia within a week of exposure to a tick bite. Because of the severe haematological disorders lasting for several days and other adverse effects on the host's immune functions, infected animals and humans are more susceptible to other infections.

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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