Dynamics of resting metabolic rate and innate immune response in malaria-infected Eurasian siskins

Avian malaria, caused by parasites of the genus <i>Plasmodium</i>, is prevalent among wild bird populations worldwide and can have significant impact on avian health and populations. With the rise in global temperatures due to climate change, concerns have arisen about the spread of southern malaria species, that potentially can affect previously unexposed bird populations. We studied juvenile siskins infected with two distinct malaria parasites: <i>Plasmodium relictum</i> (SGS1 lineage) and <i>P. ashfordi</i> (GRW2 lineage). While the former is common in the Northern Palearctic, the latter is primarily found in Central and Southern Africa. We assessed the impact of these infections on siskins' physiological well-being using resting metabolic rate (RMR) and interleukin-6 (IL-6) levels. Changes in RMR reflect the energetic cost of disease, while IL-6 serves as a one of the inflammatory cytokines in the innate immune system's response to infection. Our experimental findings reveal distinct outcomes during the acute phase of SGS1 and GRW2 infections. Infection with SGS1 was marked by reduced RMR and IL-6 levels in siskins. A similar IL-6 pattern was observed in the GRW2 group initially, though it was not sustained. Additionally, GRW2-infected siskins showed distinct RMR dynamics compared to SGS1-infected birds. Our study did not conclusively demonstrate that tropical malaria has more severe effects on siskins. However, similarities with previous studies with SGS1 infected birds and variations in disease progression between the two experimental groups underscore the complexity of host-parasite interactions in avian malaria infections.