Potential drivers of virulence evolution in aquaculture

Abstract

Infectious diseases are economically detrimental to aquaculture, and with continued expansion and intensification of aquaculture, the importance of managing infectious diseases will likely increase in the future. Here, we use evolution of virulence theory, along with examples, to identify aquaculture practices that might lead to the evolution of increased pathogen virulence. We identify eight practices common in aquaculture that theory predicts may favor evolution toward higher pathogen virulence. Four are related to intensive aquaculture operations, and four others are related specifically to infectious disease control. Our intention is to make aquaculture managers aware of these risks, such that with increased vigilance, they might be able to detect and prevent the emergence and spread of increasingly troublesome pathogen strains in the future.

Keywords: aquaculture; evolution of virulence; infectious diseases.

Figure 1

Illustration of a posited tradeoff between virulence and transmission. Virulence induced host mortality shortens the duration of an infection (top), while simultaneously increasing the instantaneous transmissibility of infection (middle). The tradeoff in these two components of pathogen fitness can generate situations where pathogen fitness is maximized at intermediate levels of virulence (bottom). Understanding how a management practice alters these curves is key to understanding how it might affect evolution of virulence, although other factors must also be considered. Mean infection duration above was calculated as the inverse of the sum of natural host mortality rate (μ), host recovery rate (γ), and virulence rate (ν). Instantaneous transmission rate was assumed to be ν/(1 + ν). New infections per susceptible host were calculated as the product of the mean infection duration and the instantaneous transmission rate. Above μ = 0.01 and γ = 0.1.