- Fecal glucocorticoid metabolites (FGMs) are commonly used as indicators of an animal’s stress response in behavioral and eco-physiological studies. Stress in wild animals represents an immediate measure of the physiological response to changes in the environment, and, potentially, a prospective assessment of the animal’s health and well-being. In wild mammals, stress responses have been linked to changes in nutritional availability and predation, as well as anthropogenic disturbances including direct human contact and habitat loss. The recent global rise in infectious disease emergence in wildlife and humans, along with host stressors often implicated in adverse health consequences, raises the question whether stress has the potential to increase disease susceptibility, facilitate the spread of infections, and linking physiological effects of environmental change with disease dynamics. As such, a non-invasive measure of stress physiology may thus provide an integrative, mechanistically relevant measure of animal health at the interface of environmental drivers and immunological and infection consequences.
Here I use a well-studied model system, African buffalo (Syncerus caffer), to investigate links between physiological stress (measured as FGMs) for studying disease processes and immune-mediated interactions between hosts in natural populations, and parasite/pathogen existence. For chapter 1, an adrenocorticotropic hormone (ACTH) experiment was performed on 12 buffalo to evaluate two commercially available FGM assays (radioimmunoassay and enzyme immunoassay) as a non-invasive tool for assessing stress in buffalo in field studies. For chapter 2 and 3, a longitudinal dataset of 200 buffalo (sampled every six months for four years) were used, to assess FGMs, and infections by viral and bacterial pathogens, gastrointestinal parasites, as well as a range of immune measures and physiological (hematological and biochemical) parameters.
The ACTH-induced plasma cortisol peak was detectable in FGMs at 10 – 20 hours post-injection (chapter 1). Acute viral infections rather than chronic infections contributed to stress in buffalo, and consequently increased the risk of obtaining some chronic bacteria (chapter 2). While accounting for fixed animal effects and current exposures, results support the hypothesis that buffalo with elevated FGMs had elevated pro-inflammatory immune responses (chapter 3: increased IFNγ, Il-12 responses, bacterial competency, the proportion of neutrophils), but weak evidence suppressing responses to chronical subclinical infections, such as strongyles (chapter 3). Together, findings of chapter 1-3, suggest that measuring FGMs as a stress marker can be used as an indicative tool associated with animals’ environment, its physiological responses, some patterns of infectious diseases, and immunological consequences. Therefore, providing a widely used tool for scientists and wildlife managers interested in African buffalo management and conservation.