Pathogen load predicts host functional disruption : a meta-analysis of an amphibian fungal panzootic

The progression of infectious disease depends on the intensity of and sensitivity to pathogen infection. Understanding commonalities in trait sensitivity to pathogen infection across studies through meta-analytic approaches could provide insight to the pathogenesis of infectious diseases. The globally devastating amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), offers a good case system due to the widely available dataset on disruption to functional traits across species. Here, I systematically conducted a phylogenetically controlled meta-analysis to test how infection intensity affects different functional traits (e.g. behaviour, physiology, morphology, reproduction) and the survival in amphibians infected with Bd. There was a consistent effect of Bd infection on energy metabolism, while traits related to body condition, osmoregulation, and behaviour generally decreased with Bd infection. Skin integrity, hormone levels, and osmoregulation were most sensitive to Bd infection (minimum Bd load ln 2.5 zoospore equivalent), while higher minimum Bd loads were required to influence reproduction (ln 10.6 zoospore equivalent). Mortality differed between life stages, where juvenile mortality was dependent on infection intensity and exposure duration, while adult mortality was dependent on infection intensity only. Importantly, there were strong biases for studies on immune response, body condition and survival, while locomotor capacity, energy metabolism and cardiovascular traits were lacking. The influence of pathogen load on functional disruption can help inform pathogen thresholds before the onset of irreversible damage and mortality. Meta-analytic approaches can provide quantitative assessment across studies to reveal commonalities, differences and biases of panzootic diseases, especially for understanding the ecological relevance of disease impact. Read the free Plain Language Summary for this article on the Journal blog.