[Equine granulocytic anaplasmosis (EGA): Case description and overview of the epidemiological situation with focus on...

Equine granulocytic anaplasmosis (EGA) is a tick-borne disease caused by Anaplasma (A.) phagocytophilum. In Central, Northern, Western, and Eastern Europe, ticks of the Ixodes (I.) ricinus/I. persulcatus complex, in Germany mainly I. ricinus, are considered as vectors. Ixodes ricinus ticks show peaks of activity during the spring and summer months, particularly April to July. Changing climatic conditions, however, have meanwhile led to a year-round risk of tick exposure and thus infections with A. phagocytophilum. The presented case report underlines the seasonal peak phase of the infection risk in that the presentation of the horse to the veterinarian occurred in June. Additionally, clinical signs - including fever - are mostly unspecific and thrombocytopenia represents the most significant hematological abnormality. Direct and indirect detection methods are available for the diagnosis of an infection or contact with the pathogen. A positive PCR confirms an acute infection, as in the presented case, while positive antibody levels indicate contact with the pathogen in the (recent) past. Additionally, inclusion bodies, so called morulae, may be detected rapidly and inexpensively in smears derived from the peripheral blood and are predominantly found in neutrophilic granulocytes, less frequently in eosinophilic granulocytes. However, microscopy has a lower sensitivity compared to PCR-testing. In the presented case, morulae could not be detected in EDTA blood despite positive PCR. Antibiotics with intracellular activity, such as oxytetracycline and/or doxycycline are the treatment of choice and, as in the described case, often result in rapid clinical improvement in horses suffering from EGA. The elimination of the pathogen should be confirmed by PCR-testing 5-8 days after the completion of the antibiotic therapy. Whether and how long antibodies against A. phagocytophilum could possibly be protective against reinfection remains unknown. In humans, A. phagocytophilum is classified as an emerging pathogen of increasing clinical importance in the northern hemisphere.