Human activities have increased the intensity and frequency of natural stressors and created novel stressors, altering host-pathogen interactions, and changing the risk of emerging infectious diseases. Despite the ubiquity of such anthropogenic impacts, predicting the directionality of outcomes has proven challenging. Here, we conduct a review and meta-analysis to determine the primary mechanisms through which stressors affect host-pathogen interactions and to evaluate the impacts stress has on host fitness (survival and fecundity) and pathogen infectivity (prevalence and intensity). We assessed 891 effect sizes from 71 host species (representing seven taxonomic groups) and 78 parasite taxa from 98 studies. We found that infected and uninfected hosts had similar sensitivity to stressors and that responses varied according to stressor type. Specifically, limited resources compromised host fecundity and decreased pathogen intensity, while abiotic environmental stressors (e.g., temperature and salinity) decreased host survivorship and increased pathogen intensity, and pollution increased mortality but decreased pathogen prevalence. We then used our meta-analysis results to develop Susceptible-Infected theoretical models to illustrate scenarios where infection rates are expected to increase or decrease in response to resource limitation or environmental stress gradients. Our results carry implications for conservation and disease emergence and reveal areas for future work.  

We conducted a systematic literature search and meta-analysis to evaluate the impacts of three broad types of environmental stressors on disease dynamics. First, we confirmed that pathogen exposure in laboratory studies typically negatively affected host fitness. We then proceeded with our main meta-analyses focused on two specific questions: Q1) were stressor fitness effects more severe for infected vs. uninfected hosts?, and Q2) was infectivity more susceptible to environmental stress than host fitness traits? To address these questions with data from primary studies, we used infection intensity and prevalence as proxies for infectivity and survivorship and fecundity as proxies of host fitness.

To identify studies that evaluated the effects of environmental stressors on infectivity and host fitness traits in host-parasite systems, on February 9th of 2021, we conducted a systematic literature search in Web of Science using the search terms: (parasit* OR pathogen* OR disease) AND (environment* OR temperature OR pollution OR resource OR provision* OR toxi* OR contamination) AND (infection OR load OR yield OR resistance) AND ("birth rate" OR "death rate" OR surviv* OR mortality OR reproduct* OR fecundity). We limited our search to journal articles published in English between 2010 and 2020 and scanned titles and, if relevant, abstracts of all 20,684 hits. This initial screening effort was split and carried out by two experienced independent reviewers (AVS and BW). We identified ten additional studies from references of selected studies. One experienced reviewer or two student reviewers further examined articles documenting effects of environmental stressors on infectivity and host fitness.

We classified stressors into three groups: 1) environmental factors, which can vary naturally but are also subject to human-induced perturbation (hereafter “endogenous environment”); 2) presence or quantity of chemical pollutants (hereafter “chemical pollution”), that lead to negative expected outcomes for hosts; and 3) resource availability for hosts (hereafter “resource limitation”).