From Tolerance to Transmission: Linking Within-Individual to Community-Level Disease Processes

Variation in individual competence, or the propensity of a host to transmit infection, affects disease dynamics at populationand community-levels. As a result, investigating the elements influencing this individual heterogeneity could have implications for understanding parasite transmission in natural communities. Resistance and tolerance are two important components affecting host competence; resistance involves reducing parasite burden to mitigate infections, whereas tolerance involves maintaining performance during an infection regardless of parasite burden. The performance metrics selected to evaluate tolerance may have a critical impact on the conclusions we draw from such studies, and thus on their ecological relevance. Additionally, challenges associated with estimating tolerance in individuals, particularly in field studies, have resulted in an overrepresentation of laboratory studies of animal tolerance. Here, we experimentally infected house sparrows (Passer domesticus) with West Nile virus (WNV) to investigate two questions: 1) do distinct performance metrics result in similar tolerance estimates among individuals, and 2) is there an efficient alternative to measuring tolerance in field studies? Specifically, we observed four performance traits (body mass, breast muscle score, activity level and flight performance) and compared the associated individual tolerance estimates to WNV infection. We then compared two methods of estimating tolerance in individuals, the scope (using multiple performance-burden measurements per individual) and position (using a single