Modulating disease phenotype in a songbird: A role for inflammation in disease tolerance?

Individual animals vary greatly in their responses to infection, either killing off the invading pathogen (resistance) or minimizing the per‐pathogen costs of infection on host fitness (tolerance). Though we understand little about the physiological drivers of tolerance in wild animals, phenotypically, it manifests as milder clinical signs of disease. Here, we use a well‐described disease system, finch mycoplasmosis, to evaluate the role of inflammation in disease tolerance. House finches (Haemorhous mexicanus) infected with the bacterial pathogen Mycoplasma gallisepticum (MG) develop conjunctival pathology that satisfies the cardinal signs of inflammation. We report on a captive trial performed in 2016 and replicated in 2018 that tested whether chemotherapeutics, specifically nonsteroidal anti‐inflammatory drugs (NSAIDs), can reduce lesion severity, thus pushing individuals toward more tolerant phenotypes. Though birds treated with NSAIDs in the first trial developed milder pathology per unit pathogen load, we found no effect of treatment in the second trial, perhaps due to natural variation in baseline tolerance within the source population across years. Second‐trial control birds developed markedly milder pathology than first‐year controls, suggesting that the effect of trial swamped the effect of treatment in this study. Moving forward, using birds from a population in which the disease is absent or only recently emerged—and so tolerance has not yet been selected for—may better elucidate the role of pro‐inflammatory mediators in disease tolerance. Research Highlights: An individual's disease phenotype reflects how its immune system responds to tissue injury caused by an infectious pathogen. Limiting the pro‐inflammatory response may be one‐way animals express milder clinical signs to disease consistent with disease tolerance. Exogenous treatment with nonsteroidal anti‐inflammatory drugs may similarly push individuals toward tolerant phenotypes, however, population‐level swings in baseline tolerance can mask treatment effects. [ABSTRACT FROM AUTHOR]