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Anaplasmosis

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Pathogens

Anaplasma phagocytophilum is an obligate, intracellular, gram-negative bacterium with a size of 0.2-2.0 µm and of coccoid shape. It is the cause for the widespread granulocytic form of canine anaplasmosis in temperate zones of the world. Former synonyms for this disease have been “tick-borne fever” or “pasture fever”. In Europe, the predominant vector is the Castor Bean tick (Ixodes ricinus), while the Deer tick (Ixodes scapularis) and the Western black-legged tick (Ixodes pacificus) are the main transmitting vectors in North America. Due to a spread of ixodid ticks, the geographical distribution of A. phagocytophilum is expanding to northern regions, like South Scandinavia. Besides dogs, A. phagocytophilum can be detected in a wide range of mammals, including cats, horses, sheep, goats, cattle, wild animals and humans.

Anaplasma platys (former Ehrlichia platys) causes canine cyclic thrombocytopenia in tropical and warm regions of the world, like the Mediterranean, Asia, Middle East, Africa, Australia, and the USA. The Brown Dog tick (Rhipicephalus sanguineus) and Dermacentor spp. are thought to transmit the pathogen. A. platys are the only rickettsia known to infect platelets. The organisms appear as round, oval or bean shaped blue cell inclusions in platelets and range from 0.35 to 1.25 µm in diameter.

Co-infection with other tick-transmitted pathogens is not uncommon and may change the clinical course. A. phagocytophilum has been detected together with Borrelia spp., as both share the same tick vectors, like the Castor Bean tick, Ixodes ricinus. The same applies to A. platys and Ehrlichia canis, which share the Brown Dog tick (Rhipicephalus sanguineus) as a vector.

Classification

Anaplasma species are closely related to the genus Ehrlichia. Both genera belong nowadays to the family Anaplasmataceae, in the order Rickettsiales. Species of the genus Anaplasma are implicated as pathogens of dogs, cats, ruminants, horses and humans.

Species

Common name of disease(s)

Common natural host(s)

Cells most commonly infected

Primary vector(s)

Distribution

Anaplasma bovis

Bovine ehrlichiosis

Cattle

Monocytes, macrophages,

erythrocytes

Haemaphysalis spp.,

Ixodes spp.,

Hyalomma spp.,

Rhipicephalus (Boophilus) spp.

Asia, Africa,

South America

A. phagocytophilum

Canine anaplasmosis

Tick-borne fever, 'pasture disease', benign ovine rickettsiosis

Human granulocytic anaplasmosis (HGA)

Equine ehrlichiosis

Dogs

Cattle, goats, sheep, wild ruminants  

Humans

Horses

Neutrophils (eosinophils, monocytes)

Ixodes ricinus (Europe),
I. scapularis, I. pacificus (North America),

Dermacentor silvarum, I. persulcatus (Asia, Russia),

I. trianguliceps, I. hexagonus, I. ventalloi (Europe),

Hyalomma longicornis

moderate and temperate areas/ many countries of northern hemisphere (Europe and America), Asia, Africa

A. platys

Canine cyclic thrombocytopenia

Dogs

Platelets

Rhipicephalus sanguineus

(Dermacentor spp.)

Southern USA, Australia, Southern Europe (Mediterranean), South America, Asia, Middle East,  Africa

Ehrlichia canis

Canine monocytic ehrlichiosis (CME)

Dogs, wolves, jackals (members of the family Canidae) (humans)

Primarily mononuclear cells (monocytes)

Rhipicephalus sanguineus, (Dermacentor variabilis)

Worldwide, primarily tropical and temperate climates

E. chaffeensis

Human monocytic ehrlichiosis (HME)

Humans, dogs, deer (horses, rodents)

Monocytes, macrophages, neutrophils, lymphocytes

Dermacentor variabilis

USA, Europe, Africa, South and Central America

E. ewingii

Canine granulocytic ehrlichiosis (CGE) (mild form), human granulocytic ehrlichiosis (HGE)

Dogs (humans)

Primarily neutrophils and eosinophils

Amblyomma americanum,

Dermacentor variabilis,

Rhipicephalus sangineus,

(putative vector: Otobius megnini)

USA

E. muris

Not currently associated with disease

Rodents, humans

Mononuclear cells

Haemaphysalis spp.

Japan

E. ondiri

Ondiri disease, bovine petechial fever

Cattle, sheep

Granulocytes

Unknown

Africa

E. (Cowdria) ruminantium

Heartwater disease

Ruminants

Endothelial cells

Amblyomma spp.

Africa, Caribbean

Neorickettsia helminthoeca

Salmon poisoning disease

Dogs, foxes, coyotes

Macrophages, monocytes

Infected trematodes (Nanophyetus salmincola) in salmons

USA

N. risticii

Potomac horse fever, equine monocytic ehrlichiosis;
atypical syndrome of monocytic ehrlichiosis in dogs

Horses (dogs, cats, coyotes, pigs, goats)

Monocytes, mast cells, enterocytes

Infected trematodes in snails and aquatic insects

USA, Canada, (France, India)

N. sennetsu

Sennetsu fever, glandular fever

Humans

Monocytes, macrophages

Presumably infected trematodes in fish

Japan, Malaysia

The family Anaplasmataceae comprises the medically relevant genera Anaplasma, Ehrlichia, Neorickettsia and Aegyptianella . Medically relevant pathogens within the genus Anaplasma are A. phagocytophilum, A. platys and A. bovis . A. bovis belongs to the monocytotropic geno-/senogroup II, A. phagocytophilum to the granulocytotropic geno-/senogroup II, and A. platys to the thrombocytotropic geno-/senogroup II. Due to phylogenetic and phenotypic characteristics, the pathogens Ehrlichia phagocytophila, E. equi and the HGE-agent (the causative agent of human granulocytic ehrlichiosis/anaplasmosis) were re-classified into the species A. phagocytophilum in 2001, which actually is the most relevant representative. The former Ehrlichia platys species was renamed Anaplasma platys

Figure 1: Neighbour-joining phylogenetic tree of 16S rRNA gene sequences of species formerly known as Ehrlichia. GenBank database accession numbers are indicated in parenthesis. The scale bar indicates the number of substitutions/site. (With permission of E.B. Breitschwerdt.)

The closely related genus Ehrlichia was initially grouped according to the type of blood cells most commonly infected (granulocyte, lymphocyte, monocyte, platelet), and disease classes have been termed "granulocytic (or granulocytotropic) ehrlichiosis" or "monocytic (or monocytotropic) ehrlichiosis". However, this way of classification was misleading because some of the Ehrlichia species have been found in cells other than their main target cell type. In addition, more than one species may be responsible for the broad category of "monocytic" or "granulocytic" ehrlichiosis. Thus, the former classification was changed as described above.

Further information

  • Carrade DD, Foley JE, Borjesson DL, et al.: Canine granulocytic anaplasmosis: A review. J Vet Intern Med. 2009, 23, 1129–41

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