Stressors constrain host defenses
Hosts invest resources to defend themselves from pathogens by resisting infections or tolerating disease (Råberg et al. 2007, 2009; Svensson & Råberg 2010). Resistance mechanisms control parasite growth and reproduction, reducing infection intensity, while tolerance reduces or compensates for infection-induced pathology without reducing pathogen burden (Boots 2008; Medzhitov et al. 2012). Although the two strategies have different disease implications, both have high energetic requirements (Ayres & Schneider 2009; Cumnock et al. 2018). Consequently, trade-offs exist between immune response and other energetically costly physiological processes, such as reproduction and growth (Lochmiller & Deerenberg 2000), in both vertebrates (Gustafssonet al. 1994) and invertebrates (Schwenke et al. 2016). Induction of an immune response, even without a pathogen, shows similar results (Demas 2004). Furthermore, there is recent evidence that trade-offs between reproduction and immune function exist at the transcriptomic level and may be conserved across animals (Rodrigueset al. 2021).
Given these trade-offs, it follows that under stressful conditions, hosts might not be able to defend themselves optimally from pathogens (Sheldon & Verhulst 1996; Gervasi et al. 2015). For instance, malnutrition can impair immune function by reducing T-cell-mediated immune response (Alonso-Alvarez & Tella 2001), toxicants can immunocompromise a host (Caren 1981) or upregulate host immunity (Pölkkiet al. 2012), and extreme temperature variations can impair immunity leading to species declines (Rohr & Raffel 2010). Consequently, stressors can alter epidemiology through mechanisms that alter host susceptibility to infections. For example, Owen et al . (2021) showed that food-deprived robins (Turdus migratorius ) developed higher West Nile Virus titers and were infectious longer than robins fed normally. Similarly, amphibians exposed to pesticides have experienced eosinophil recusation (a resistance mechanism) and associated increases in trematode infections and subsequent limb malformations (Kiesecker 2002). Conversely, the tolerance of Galapagos mockingbirds (Mimus parvulus ) to infection has been impaired by climatically-induced food stress, exhibiting lower fledging success in dry years (when resources were scarce) compared to wet years, due to their inability to compensate for the cost of parasitic fly nest infestations (McNew et al. 2019).