Anaplasmosis is caused by several bacterial species of the genus Anaplasma. From their reservoir hosts (e.g. mice, deer, possibly birds) the bacteria are transmitted by ixodid ticks like the Castor Bean tick (Ixodes ricinus), the Deer tick (Ixodes scapularis), the Western black-legged tick (Ixodes pacificus) and the Brown Dog tick (Rhipicephalus sanguineus). In general, anaplasmosis leads to milder disease than monocytic ehrlichiosis caused by Ehrlichia canis and appears to be largely a self-limiting infection in dogs.

Anaplasma phagocytophilum is the most important representative and has been detected in blood samples from a wide range of wild and domestic animals. It can cause an acute febrile illness e.g., granulocytic anaplasmosis in dogs, cats and horses. In humans, the pathogen is responsible for human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis (HGE).


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. 


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.


Important veterinary and human species of the family Anaplasmataceae

SpeciesCommon name of diseases(s)Common natural host(s)Cells most commonly infectedPrimary vector(s)Distribution
Anaplasma bovisBovine ehrlichiosisCattleMonocytes, macrophages, erythrocytesHaemaphysalis spp.,
Ixodes spp.,
Hyalomma spp.,
Rhipicephalus (Boophilus) spp.
Asia, Africa, South America
Anaplasma phagocytophilum
(formerly Ehrlichia phagocytophila, Ehrlichia equi and HGE-agent)

Canine anaplasmosis

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

Human granulocytic anaplasmosis (HGA)

Equine ehrlichiosis


Cattle, goats, sheep, wild ruminants  




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 the Northern hemisphere (Europe and America), Asia, Africa
Anaplasma platys
(formerly Ehrlichia platys)
Canine cyclic thrombocytopeniaDogsPlateletsRhipicephalus sanguineus, (Dermacentor spp.)Southern USA, Australia, Southern Europe (Mediterranean), South America, Asia, Middle East,  Africa
Ehrlichia canisCanine 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
Ehrlichia chaffeensisHuman monocytic ehrlichiosis (HME)Humans, dogs, deer (horses, rodents)Monocytes, macrophages, neutrophils, lymphocytesDermacentor variabilisUSA, Europe, Africa, South and Central America
Ehrlichia ewingiiCanine granulocytic ehrlichiosis (CGE) (mild form), human granulocytic ehrlichiosis (HGE)Dogs (humans)Primarily neutrophils and eosinophilsAmblyomma americanum,
Dermacentor variabilis,
Rhipicephalus sangineus,
(putative vector: Otobius megnini)
Ehrlichia murisNot currently associated with diseaseRodents, humansMononuclear cellsHaemaphysalis spp.Japan
Ehrlichia ondiriOndiri disease, bovine petechial feverCattle, sheepGranulocytesUnknownAfrica
Ehrlichia (Cowdria) ruminantiumHeartwater diseaseRuminantsEndothelial cellsAmblyomma spp.Africa, Caribbean
Neorickettsia helminthoecaSalmon poisoning diseaseDogs, foxes, coyotesMacrophages, monocytesInfected trematodes (Nanophyetus salmincola) in salmonsUSA
Neorickettsia risticiiPotomac horse fever, equine monocytic ehrlichiosis;
atypical syndrome of monocytic ehrlichiosis in dogs
Horses (dogs, cats, coyotes, pigs, goats)Monocytes, mast cells, enterocytesInfected trematodes in snails and aquatic insectsUSA, Canada, (France, India)
Neorickettsia sennetsuSennetsu fever, glandular feverHumansMonocytes, macrophagesPresumably infected trematodes in fishJapan, Malaysia



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 mentioned in the table above).




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