Additive metabolic costs of thermoregulation and pathogen infection

Abstract

Thermoregulation and pathogen resistance are two energetically demanding processes that co-occur during seasonal epidemics for many endothermic vertebrates. The ability of hosts to cope with these processes simultaneously may influence population-level disease dynamics. 2.In North American house finches (Carpodacus mexicanus), outbreaks of the bacterium Mycoplasma gallisepticum occur during fall and winter, when ambient temperatures across the host's range are often below thermoneutrality. Here, we examined how ambient temperature influences host energetics and susceptibility to this naturally occurring seasonal pathogen by experimentally infecting wild-caught house finches with M. gallisepticum at either thermoneutral or subthermoneutral temperatures in the laboratory. We quantified the metabolic costs of infection, measures of body condition, two components of the acute-phase response, disease expression and pathogen loads under both temperature regimes. 3.The metabolic costs of simultaneous infection with M. gallisepticum and thermoregulation were additive and significant (combined costs of 4·71kJ per night; within the range of the daily energy requirements of passerine moult). Contrary to our predictions, house finches at subthermoneutral temperatures had lower disease expression and higher circulating levels of the cytokine interleukin-6 in response to experimental infection with M. gallisepticum than finches at thermoneutral. However, pathogen loads did not differ between the two temperature treatments. Finches from both treatments expressed fever in response to infection, but the magnitude of fever did not vary with ambient temperature. 4.Despite the significant energy costs of infection and thermoregulation, house finches from both temperature treatments maintained body mass and pectoral muscle condition, suggesting that birds housed at subthermoneutral consumed more food to maintain energy balance. In the field, competition for finite resources would be expected to exacerbate the effects found here and force infected birds to spend more time at feeders where M. gallisepticum is transmitted. 5.Overall, our results indicate that moderate cold stress alters house finch immunity, energetics and disease pathology, but does not alter infectiousness as measured by pathogen load. The effects of ambient temperature on host response and energy demands could directly or indirectly contribute to seasonal and geographical variation in disease dynamics in free-living house finches. More broadly, our results suggest that even subtle changes in abiotic factors such as temperature can alter host disease expression, with broad implications for disease dynamics. © 2012 The Authors. Functional Ecology © 2012 British Ecological Society.

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Functional Ecology

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