Does the blood-sucking nematode Ashworthius sidemi (Trichostrongylidae) cause deterioration of blood parameters in European bison (Bison bonasus)?

European bison (Bison bonasus), after extinction in the wild at the beginning of twentieth century, were reintroduced in over 30 free-ranging populations in central and eastern Europe. Major threats to this species include disease and parasites. In the last decades, the highly pathogenic blood-sucking nematode Ashworthius sidemi (Trichostrongylidae) has been found in high infection intensity and prevalence within the abomasa of free-living European bison. We investigated the impact of this invasion on blood parameters associated with the red blood cell system in 64 bison (53 free-living and 11 captive) which were culled in two neighbouring populations in north-eastern Poland between 2008 and 2015. We observed a significant decrease in red blood cell count, haemoglobin and haematocrit relative to the intensity of A. sidemi infection. Furthermore, the same parameters were significantly lower, and the percentage of reticulocytes was increased in highly infected free-living bison compared to captive individuals. Studies indicate the urgent need to change the conservation management of herds because traditional management that includes supplementary feeding causes a large concentration of bison in winter feeding areas. This causes an increase in parasitic load which ultimately impacts on their condition. The reservoir of this parasite in wildlife may also adversely affect livestock grazing on pastures used by bison.