Anaplasmosis in horses

Anaplasma (A.) phagocytophilum is the causative agent of anaplasmosis, a bacterial disease which is transmitted through bites from ticks of the genus Ixodes. A. phagocytophilum is a gram-negative, obligate intra-cellular bacterium which attacks mostly neutrophilic granulocytes, but also, in rare cases, eosinophilic granulocytes. Anaplasmosis occurs worldwide, its prevalence depending on the distribution area of the transmitting vectors.

A. phagocytophilum infection in horses is known under various names: granulocytic ehrlichiosis (obsolete), equine granulocytic anaplasmosis, and in most cases also: anaplasmosis. The clinical symptoms of equine anaplasmosis are a reduced general condition with fever, anorexia, lethargy, oedema of the limbs, petechia, icterus, reluctance towards physical activity, and ataxia. Older horses show clearer symptoms than younger ones.

Antibodies against A. phagocytophilum occur in the serum of infected animals after seven to fourteen days. Different techniques are used for the serological detection of antibodies, such as ELISA or indirect immunofluorescence test (IIFT). It needs to be taken into consideration that many dogs and horses show specific antibodies against A. phagocytophilum but are not clinically conspicuous. For diagnosis, it is hence necessary to investigate two blood samples at a time interval. A twofold titer increase or a seroconversion are here diagnostically relevant. If the first blood sample tests negative, a second sample should be examined after two weeks in cases of suspected anaplasmosis since dogs and horses do not produce antibodies in the early phase of infection. Borreliosis should be excluded by differential diagnosis. As of yet, no vaccines are available for dogs or horses.

• Anti-Anaplasma phagocytophilum ELISA Horse (IgG)