Thermal Tolerance is linked with Virulence in a Fish Pathogen

Abstract

Although increase in temperatures may boost the number of pathogens, a complex process involving the interaction of a susceptible host, a virulent strain, and environmental factors would influence disease virulence in unpredictable ways. Here we explored if the virulence of an environmentally growing opportunistic fish pathogen, Flavobacterium columnare, would be malleable to evolutionary changes via correlated selection on thermal tolerance. Virulence among the strains increased over years, but tolerance to higher temperatures was associated with reduced virulence. Our results suggest that observed increase in frequency of columnaris epidemics over the last decade is most likely associated with increased length of growing season, or other time dependent change in environment, rather than increased regional average temperatures. Our results also indicate that most virulent bacteria had weaker ability to tolerate outside host environments, which suggest trade-off between more obligate pathogen behaviour and ability to grow outside host.