The influence of infective dose, nutrient availability and coinfection on virulence of Flavobacterium columnare : implications of intensive aquaculture on opportunistic infections

Abstract

Ecological factors are known to affect disease dynamics and even lead to disease emergence. Especially in opportunistic, environmentally transmitted pathogens, the environment may significantly contribute to pathogen virulence. Intensive farming, including aquaculture, has been suggested to create conditions favouring development of highly virulent pathogens. At Finnish fish farms, epidemics caused by opportunistic Flavobacterium columnare have been constantly increasing in their prevalence and severity since the 1980’s. Yet, factors behind the increased virulence of the pathogen and their mechanisms of action have largely remained unsolved. In this thesis, I explore the effects of infection dose, outside-host nutrients and coinfection with conspecific strains on virulence and disease dynamics of F. columnare. Virulence of bacterial strains isolated from fish farms was tested in experimental fish infections, where bacterial dose, water nutrient concentration and number of infecting strains were manipulated. Two fish species, zebra fish (Danio rerio) and rainbow trout (Oncorhynchus mykiss), were used as hosts to confirm the suitability of zebra fish for further infection experiments. Finally, strains isolated during 2003-2010 were studied for temporal and local changes in their virulence, growth and competitive ability. Increase in infection dose was found to increase bacterial virulence in both hosts. High nutrient level increased virulence via enhanced bacterial growth and virulence factor activation. I also found that coinfection can increase F. columnare virulence, but the disease outcome depends on the strain characteristics. Finally, it is shown that the aquaculture environment may select for strains with high virulence and enhanced competitiveness. Taken together, the results indicate that all the studied factors can contribute to opportunistic disease outbreaks in aquaculture.