Detection of Anaplasma phagocytophilum in horses from Germany by molecular and serological testing (2008-2021)

Equine granulocytic anaplasmosis (EGA) is a tick-borne disease caused by Anaplasma (A.) phagocytophilum. In Germany, this pathogen is transmitted primarily by Ixodes ricinus. There is limited knowledge about its prevalence in horses in Germany. The aim of this retrospective study was to analyze the results of serological and molecular testing for A. phagocytophilum in horses which were done in a commercial laboratory in Germany over fourteen years. Additionally, risk factors were evaluated, and hematological abnormalities were addressed in horses with positive PCR results.

This retrospective study examined results of direct (Polymerase chain reaction [PCR]) and indirect (immunofluorescence antibody test [IFAT]) detection methods for A. phagocytophilum in horses on samples provided by German veterinarians and processed by the commercial laboratory LABOKLIN from 2008 to 2021. In horses with positive test results, a Complete Blood Count (CBC) and Serum Amyloid A (SAA) were also analyzed where possible.

In total, 1217/4834 horses tested positive (PCR: 190/1246 horses, 15.2%; IFAT: 1036/3849 horses, 26.9%). Seasonality and location, as classified by federal state, had a statistically significant impact on PCR results (P < 0.001 for both). In horses with positive PCR results, hematological abnormalities were detected in 112/118 horses (95%), with thrombocytopenia (86%) and anemia (52%) representing the most common findings. The remaining 6/118 horses (5%) showed no hematological abnormalities on CBC. SAA was measured in 35 horses with positive PCR results, which exclusively showed marked elevation.

The seasonality of A. phagocytophilum infections confirmed by PCR testing was consistent with known peaks in vector activity in Germany. The high rate of horses with positive PCR results when compared to dogs and cats may be due to a lack of ectoparasite prophylaxis. Infections with A. phagocytophilum should be considered as a differential diagnosis in horses with cytopenia on CBC and SAA elevation, especially in the summer and after any possible tick exposure.