Human intervention in freshwater ecosystems drives disease emergence

P>1. We present a causal web to map some important anthropogenic drivers (long-distance movement of live aquatic animals, large-scale movements of animal products and farming practices) via proximate causes to different types of emerging disease. 2. The long-distance movement of animals and their pathogens and parasites will directly lead to increased geographical distribution. In addition, the introduction of non-native species for aquaculture or as ornamental animals may promote host-switching through the mixing of pathogens and parasites with new hosts, driving the emergence of new diseases and known diseases in new hosts. The movement of animal products may drive disease emergence by the same processes, but the risks are lower. 3. The high stocking density found in many aquaculture systems and the relative genetic homogeneity of farm animals can increase disease prevalence. Generally, the overspill of pathogens into the environment is not prevented and in addition, farm or ornamental animals may escape or be released. Thus, wild aquatic animal populations may be exposed to pathogens introduced via the movement of animals for farming. In addition, high stocking density (and short lifespans) favours the selection of virulent pathogen strains. Suboptimal farm conditions (poor water quality and co-infections) can lead to decreased host resistance, leading to more severe signs of disease and higher prevalence. 4. Many emerging diseases have already seriously affected populations of wild aquatic animals. Future disease emergence presents a threat to freshwater animal populations. Early detection and prompt action, however, can minimise the impact of emerging diseases. Aquaculture and the ornamental trade pose significant threats to wild populations by acting as ultimate drivers for disease emergence via pathways summarised in our causal web. An improved understanding of the influence of human activities will allow us to evaluate the risks associated with current practices, identify measures to mitigate the likelihood and impact of disease emergence.