Host-pathogen interactions under pressure: a review and meta-analysis of stress-mediated effects on disease dynamics

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.