Patterns of infection in a native and an invasive crayfish across the UK.

Invasive crayfish and the introduction of non-native diseases pose a significant risk for the conservation of endangered, white-clawed crayfish (Austropotamobius pallipes). Continued pollution of waterways is also of concern for native species and may be linked with crayfish disease dynamics. We explore whether crayfish species or environmental quality are predictors of infection presence and prevalence in native A. pallipes and invasive signal crayfish (Pacifastacus leniusculus). We use a seven-year dataset of histology records, and a field survey comparing the presence and prevalence of infectious agents in three isolated A. pallipes populations; three isolated P. leniusculus populations, and three populations where the two species had overlapped in the past. We note a lower diversity of parasites (Simpson's Index) in P. leniusculus ('Pacifastacus leniusculus Bacilliform Virus' - PlBV) (n = 1 parasite) relative to native A. pallipes (n = 4 parasites), which host Thelohania contejeani, 'Austropotamobius pallipes bacilliform virus' (ApBV), Psorospermium haeckeli and Branchiobdella astaci, at the sites studied. The infectious group present in both species was an intranuclear bacilliform virus of the hepatopancreas. The prevalence of A. astaci in A. pallipes populations was higher in more polluted water bodies, which may reflect an effect of water quality, or may be due to increased chance of transmission from nearby P. leniusculus, a species commonly found in poor quality habitats.