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).