An evaluation of two potential risk factors, MHC diversity and host density, for infection by an invasive nematode Ashworthius sidemi in endangered European bison (Bison bonasus)

Parasites are thought to increase the risk of host extinction, but their dynamics in endangered species have not been well investigated. The free-living European bison population in Białowieża Forest has recently been massively invaded by a blood-sucking nematode, Ashworthius sidemi. This nematode originated in Asia and was probably transmitted to Europe with the introduction of the sika deer. Here, we investigate the impacts of genetic factors (the Major Histocompatibility Complex class II genes responsible for extracellular parasite recognition) and management practices (supplementary feeding affecting winter population density) on the intensity of A. sidemi infections in bison. All but two out of 110 animals investigated between 2005 and 2009 were infected with A. sidemi, and the intensity of infection increased significantly with time. Due to a severe population bottleneck experienced by the bison, only four class II DRB alleles are retained in the Białowieża population. We found that despite high sequence divergence, neither any of the alleles nor DRB heterozygosity was significantly associated with infection intensity. We did find, however, that winter density of bison herds was positively associated with infection intensity. Winter bison population densities were in turn predicted by the intensity of supplementary feeding.