Natural enemies of human infectious diseases in the environment.

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Many pathogens and parasites spend a significant amount of their life cycle in the environment, where encounters with natural enemies (predators, pathogens, and ecological competitors) can directly impact their survival and, consequently, human infection risk. However, the ecosystem services that natural enemies provide for human health remain underappreciated.

We discuss three key reasons why natural enemies deserve more attention as potential tools to mitigate environmentally mediated infections. First, natural enemies of free-living stages of parasites and pathogens, or of their vector or animal hosts, are abundant in nature. There is an untapped potential, largely unexplored, to leverage natural enemies to protect human health.

Second, natural enemy solutions could enhance disease control when conventional intervention strategies, such as vaccination, drug treatment, and insecticide-based vector control, are limited. Currently, there are no vaccines for most neglected tropical diseases and emerging infectious diseases. Drug treatment is essential to alleviate disease and reduce transmission, but does not address the environmental sources of many infections. And, a worrying rise in insecticide-resistance demands new tools to reduce vector populations in the environment. Natural enemies could complement conventional strategies to control infections at their source.

Third, some natural enemy solutions may have important co-benefits for people and nature. Where conservation or restoration of specific species also provides protection against infections, goals for human health and biodiversity conservation may align. In other cases, some natural enemies of disease vectors and animal hosts, such as freshwater fish and crustaceans, may be important food sources for people in addition to disease control agents. Identifying natural enemy solutions that provide multiple benefits could incentivize more research on natural enemy impacts and sustainable natural enemy tools.

We are optimistic that natural enemies can play an important role in sustainable infection control. However, evidence on their impact and feasibility is lacking, and past experiences show that some natural enemy solutions are not without risk for unintended consequences. We hope that natural enemy discovery, impact evaluation, and risk assessment will be priorities that align researchers from diverse fields to generate novel solutions that benefit people and the planet.