Differential effects of two prevalent environmental pollutants on host-pathogen dynamics

Chemical pollutants are a major factor implicated in freshwater habitat degradation and species loss. Microplastics and glyphosate-based herbicides are prevalent pollutants with known detrimental effects on animal welfare but our understanding of their impacts on infection dynamics are limited. Within freshwater vertebrates, glyphosate formulations reduce fish tolerance to infections, but the effects of microplastic consumption on disease tolerance have thus far not been assessed. Here, we investigated how microplastic (polypropylene) and the commercial glyphosate-based herbicide, Roundup (R), impact fish tolerance to infectious disease and mortality utilising a model fish host-pathogen system. For uninfected fish, microplastic and Roundup had contrasting impacts on mortality as individual stressors, with microplastic increasing and Roundup decreasing mortality compared with control fish not exposed to pollutants. Concerningly, microplastic and Roundup combined had a strong interactive reversal effect by significantly increasing host mortality for uninfected fish (73% mortality). For infected fish, the individual stressors also had contrasting effects on mortality, with microplastic consumption not significantly affecting mortality and Roundup increasing mortality to 55%. When combined, these two pollutants had a moderate interactive synergistic effect on mortality levels of infected fish (53% mortality). Both microplastic and Roundup individually had significant and contrasting impacts on pathogen metrics with microplastic consumption resulting in fish maintaining infections for significantly longer and Roundup significantly reducing pathogen burdens. When combined, the two pollutants had a largely additive effect in reducing pathogen burdens. This study is the first to reveal that microplastic and Roundup individually and interactively impact host-pathogen dynamics and can prove fatal to fish.