It's a hard knock life for some: Heterogeneity in infection life history of salmonids influences parasite disease outcomes

1. Heterogeneity in immunity occurs across numerous disease systems with individuals from the same population having diverse disease outcomes. Proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae, is a persistent parasitic disease negatively impacting both wild and farmed salmonids. Little is known of how PKD is spread or maintained within wild susceptible populations. 2. We investigated an aspect of fish disease that has been largely overlooked, that is, the role of the host phenotypic heterogeneity in disease outcome. We examined how host susceptibility to T. bryosalmonae infection, and the disease PKD, varied across different infection life‐history stages and how it differs between naïve, re‐infected and persistently infected hosts. 3. We investigated the response to parasite exposure in host phenotypes with (a) different ages and (b) heterogeneous infection life histories. Among (a) the age phenotypes were young‐of‐the‐year (YOY) fish and juvenile 1+ fish (fish older than one) and, for (b) juvenile 1+ infection survivors were either re‐exposed or not re‐ exposed to the parasite and response phenotypes were assigned post‐hoc dependant on infection status. In fish not re‐exposed this included fish that cleared infection (CI) or had a persistent infection (PI). In fish re‐exposed these included fish that were re‐infected (RI), or re‐exposed and uninfected (RCI). We assessed both parasite‐centric (infection prevalence, parasite burden, malacospore transmission) and host‐centric parameters (growth rates, disease severity, infection tolerance and the immune response). 4. In (a), YOY fish, parasite success and disease severity were greater and differences in the immune response occurred, demonstrating an ontogenetic decline of susceptibility in older fish. In (b), in PI and RI fish, parasite success and disease severity were comparable. However, expression of several adaptive immunity markers was greater in RI fish, indicating concomitant immunity, as re‐exposure did not intensify infection. 5. We demonstrate the relevance of heterogeneity in infection life history on disease outcome and describe several distinctive features of immune ontogeny and protective immunity in this model not previously reported. The relevance of such themes on a population level requires greater research in many aquatic disease systems to generate clearer framework for understanding the spread and maintenance of aquatic pathogens.