ANAPLASMOSIS

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. 

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.

 

Important veterinary and human species of the family Anaplasmataceae

Species	Common name of diseases(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
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	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 the Northern hemisphere (Europe and America), Asia, Africa
Anaplasma platys (formerly Ehrlichia 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
Ehrlichia chaffeensis	Human monocytic ehrlichiosis (HME)	Humans, dogs, deer (horses, rodents)	Monocytes, macrophages, neutrophils, lymphocytes	Dermacentor variabilis	USA, Europe, Africa, South and Central America
Ehrlichia 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
Ehrlichia muris	Not currently associated with disease	Rodents, humans	Mononuclear cells	Haemaphysalis spp.	Japan
Ehrlichia ondiri	Ondiri disease, bovine petechial fever	Cattle, sheep	Granulocytes	Unknown	Africa
Ehrlichia (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
Neorickettsia 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)
Neorickettsia sennetsu	Sennetsu fever, glandular fever	Humans	Monocytes, macrophages	Presumably infected trematodes in fish	Japan, 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).