Anaplasma phagocytophilum

Classification

Taxonomy

• Phylum : Proteobacteria.
• Order : Rickettsiales. 
• Family : Anaplasmataceae. 
• Genus : Anaplasma .
• Species : phagocytophilum .
• Several host adapted variants exist.
• Distinct host species correlate with Anaplasma phagocytophilum ankA gene clusters.
• Variants may behave differently from one another in their interaction/pathogenicity within the host.
• For example, in ruminants different variants may exist within the same herd and even simultaneously in the same animal; while different horse variants exist it is yet unknown whether they can occur simultaneously in the same animal.

Etymology

• Formerly known as Ehrlichia equi and Ehrlichia phagocytophila    .
• Gk. an-, without; plasma-, anything formed or molded; phagein-, to eat up, devour; phylum-, friend, loving. Literally: A thing (a bacterium) without form, attractive to phagocytes.

Distribution

• Worldwide.
• The most widespread tick-borne infection in animals in Europe.
• Has been known in sheep and cattle for over 200 years; initially described in the horse in the late 1960s.
• America: mainly in the north eastern states; Minnesota, Wisconsin, and California in the United States.
• Scandinavia: an emerging disease on continental Europe, the European horse variants appear to be identical to the human variant in North America.
• It is also prevalent in regions with tropical to sub-tropical climates.
• There are minor but potentially clinically important genetic differences in variants obtained from various geographic regions.

Significance

• Causes equine granulocytic anaplasmosis    .
• Zoonotic: causing Human granulocytic anaplasmosis, although horses are considered to be dead end hosts and do not provide a direct source of the infection to humans.
• The causative agent of a tick-borne fever of ruminants and canine and equine granulocytic anaplasmosis 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.
• Identified as causing disease in sheep, goat, cattle, horse, dog, cat, roe deer, reindeer and humans.
• Seropositive and PCR-positive individuals have been detected in several other species without clinical signs of disease.

Sources

Publications

• Recent references from PubMed.
• Granquist E G et al (2010) A morphological and molecular study of Anaplasma phagocytophilum transmission events at the time of Ixodes ricinus tick bite. Acta Vet Scand 52, 43 PubMed.
• Franzén P, Aspan A, Egenvall A, Gunnarsson A, Karlstam E & Pringle J (2009) Molecular evidence for persistence of Anaplasma phagocytophilum in the absence of clinical abnormalities in horses after recovery from acute experimental infection. J Vet Intern Med 23 (3), 636-642 PubMed.
• Franzén P, Aspan A, Egenvall A, Gunnarsson A, Aberg L & Pringle J (2005) Acute clinical, hematologic, serologic, and polymerase chain reaction findings in horses experimentally infected with a European strain of Anaplasma phagocytophilum. J Vet Intern Med 19 (2), 232-239 PubMed.
• Lepidi H, Bunnell J E, Martin M E, Madigan J E, Stuen S & Dumler J S (2000) Comparative pathology, and immunohistology associated with clinical illness after Ehrlichia phagocytophila-group infections. Am J Trop Med Hyg 62 (1), 29-37 PubMed.
• Pusterla N et al (1998) Experimental infection of four horses with Ehrlichia phagocytophila. Vet Rec 143 (11), 303-305 PubMed. 
• Reubel G H, Kimsey R B, Barlough J E & Madigan J E (1998) Experimental transmission of Ehrlichia equi to horses through naturally infected ticks (Ixodes pacificus) from Northern California. J Clin Microbiol 36 (7), 2131-2134 PubMed.
• Korbutiak E & Schneiders D (1994) Equine granulocytic ehrlichiosis in the UK. Vet Rec 135 (16), 387-388 PubMed.