Wild Boars as Hosts of Human-Pathogenic Anaplasma phagocytophilum Variants

Jerzy Michalik

, Joanna Stańczak, Stella Cieniuch, Maria Racewicz, Bożena Sikora, and Mirosława Dabert

Author affiliations: Adam Mickiewicz University, Poznań, Poland (J. Michalik, B. Sikora, M. Dabert); Medical University of Gdańsk, Gdańsk, Poland (J. Stańczak, S. Cieniuch, M. Racewicz)

Abstract

To investigate the potential of wild boars to host Anaplasma phagocytophilum, we analyzed bacterial 16S rRNA and ank genes. DNA sequencing identified several A. phagocytophilum variants, including a predominance of strains known to cause human disease. Boars are thus hosts for A. phagocytophilum, notably, strains associated with human granulocytic anaplasmosis.

The enzootic cycle(s) of Anaplasma phagocytophilum, a tick-transmitted bacterium that causes granulocytic anaplasmosis (GA) in humans (HGA) and certain domesticated animals is driven by the distribution of its vector ticks and wild mammal reservoirs (1). Molecular and phylogenetic analyses of A. phagocytophilum sequences from ticks and hosts provide evidence that this bacterium comprises a complex of closely related strains that differ in their host preferences and pathogenicity (2–4). Although 16S rRNA, groESL, and ank gene variants from horses with GA in Europe, and less frequently from infected dogs, are identical to sequences from most HGA patients (4–7), the wild reservoir hosts for strains causing human anaplasmosis (AP-ha) in Europe are poorly understood.

In contrast to the eastern United States, where white-footed mice are a primary reservoir for strains that infect humans, rodents in Europe have not been found to display high zoonotic potential (8). Moreover, cervids have been found to propagate mostly A. phagocytophilum variants that have not been detected in humans (9,10). An exception to this finding is that red deer seem to maintain strains that induce HGA (4). In Slovenia, identical A. phagocytophilum groESL sequences have been identified in patients and wild boars (Sus scrofa), which suggests that boars may represent a potential reservoir for AP-ha variants (10,11).

Although several clinical cases of HGA have been reported in Poland (12), no data are available concerning A. phagocytophilum infections in boars, even though they are the most abundant big game animals (≈200,000 animals are hunted and killed annually) and host all 3 parasitic stages of the tick vector Ixodes ricinus. Thus, we sought to determine the frequency of A. phagocytophilum in populations of wild boars and in host-derived ticks to clarify the role of boars in the ecology and epidemiology of GA.