Anaplasma phagocytophilum in Horses… An Emerging Disease or Underdiagnosed?

Introduction 

Equine granulocytic anaplasmosis (EGA) is a tick-borne disease, caused by the obligate intra-cellular bacterium Anaplasma phagocytophilum, which can elicit febrile disease in animals and humans. The disease has been referred to as equine granulocytic ehrlichiosis in the past, and is transmitted in Europe by Ixodes ricinus ticks. Ticks of the Ixodes ricinus complex also act as vectors to spread Borrelia burgdorferi from one animal to another, and co-infections of A. phagocytophilum and Borrelia burgdorferi have been confirmed in horses (Chang and others 2000).

EGA was first described in the United States in 1969 (Gribble 1969) and has since also been reported in Europe (Artursson and others 1999; Bermann and others 2002; Korbutiak and Schneiders 1994; Von Loewenich and others 2003; Butler et al 2008). Following an incubation period of around 10 days (Gribble 1969; Pusterla and others 2002; Pusterla and others 1999b), infected horses may experience sub-clinical disease or develop overt signs that include fever, depression, in appetence, reluctance to move and distal limb oedema. The disease can be self-limiting when untreated, and clinical signs usually last from 7 to 14 days (Gribble 1969). [...]