Collateral diseases: Aquaculture impacts on wildlife infections

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Abstract
Aquaculture is a promising source of fish and other aquatic organisms to ensure human food security but it comes at the price of diverse environmental impacts. Among others, these include diseases which often thrive under the conditions in aquaculture settings and can cause high economic losses. These diseases may also affect wildlife, however, the impacts of aquaculture on disease dynamics in wild species in surrounding ecosystems are poorly understood. In this Review, we provide a conceptual framework for studying the effects of aquaculture on wildlife diseases, and illustrate the different mechanisms identified with examples from the literature. In addition, we highlight further research needs and provide recommendations for management and policy. We identified five potential means by which farmed populations may alter wildlife disease dynamics: (a) farmed species may co‐introduce parasites to the new environment, which infect wild conspecifics without infecting other species (intraspecific parasite spillover); (b) these co‐introduced parasites from farmed species may infect other wild host species potentially leading to emerging diseases (interspecific parasite spillover); (c) parasites from other wild host species may infect farmed species, amplifying parasite numbers and increasing parasite infections when spilling back to wild hosts (interspecific parasite spillback); (d) farmed species may acquire parasites from wild conspecifics, increasing parasite population size and subsequently raising infection loads in the wild host population (intraspecific parasite spillback); and (e) farmed species may be neither hosts nor parasites, but affect the transmission of parasites between wild host species (transmission interference). Although these mechanisms can alter wildlife disease dynamics, we found large knowledge gaps regarding collateral disease impacts and strong biases in terms of production countries, aquaculture practices and host taxa. Synthesis and applications. The strong potential for aquaculture to affect the dynamics of diseases in wildlife populations calls for the consideration of collateral disease impacts in risk assessments and biosecurity protocols regarding aquaculture. In particular, comprehensive parasite inventories of both farmed and wild hosts as well as disease monitoring in wildlife surrounding farms will be necessary to increase our knowledge on aquaculture impacts on wildlife disease and to develop adequate prevention and mitigation measures.

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