No abstract

Glucocorticoid stress hormones are important for energy mobilization as well as regulation of the immune system, and thus these hormones are particularly likely to both influence and respond to pathogen infection in vertebrates. In this study, we examined how the glucocorticoid stress response in house finches (Haemorhous mexicanus) interacts with experimental infection of the naturally-occurring bacterial pathogen, Mycoplasma gallisepticum (MG). We also investigated whether infection-induced concentrations of corticosterone (CORT), the primary glucocorticoid in birds, were associated with the expression of sickness behavior, the lethargy typically observed in vertebrates early in infection. We found that experimental infection with MG resulted in significantly higher CORT levels on day 5 post-infection, but this effect appeared to be limited to female house finches only. Regardless of sex, infected individuals with greater disease severity had the highest CORT concentrations on day 5 post-infection. House finches exposed to MG exhibited behavioral changes, with infected birds having significantly lower activity levels than sham-inoculated individuals. However, CORT concentrations and the extent of sickness behaviors exhibited among infected birds were not associated. Finally, pre-infection CORT concentrations were associated with reduced inflammation and pathogen load in inoculated males, but not females. Our results suggest that the house finch glucocorticoid stress response may both influence and respond to MG infection in sex-specific ways, but because we had a relatively low sample size of males, future work should confirm these patterns. Finally, manipulative experiments should be performed to test whether the glucocorticoid stress response acts as a brake on the inflammatory response associated with MG infection in house finches.

While avoidance of sick conspecifics is common among animals, little is known about how detecting diseased conspecifics influences an organism's physiological state, despite its implications for disease transmission dynamics. The avian pathogen Mycoplasma gallisepticum (MG) causes obvious visual signs of infection in domestic canaries ( Serinus canaria domestica ), including lethargy and conjunctivitis, making this system a useful tool for investigating how the perception of cues from sick individuals shapes immunity in healthy individuals. We tested whether disease-related social information can stimulate immune responses in canaries housed in visual contact with either healthy or MG-infected conspecifics. We found higher complement activity and higher heterophil counts in healthy birds viewing MG-infected individuals around 6–12 days post-inoculation, which corresponded with the greatest degree of disease pathology in infected stimulus birds. However, we did not detect the effects of disease-related social cues on the expression of two proinflammatory cytokines in the blood. These data indicate that social cues of infection can alter immune responses in healthy individuals and suggest that public information about the disease can shape how individuals respond to infection.