Host nutrition and density jointly drive susceptibility and tolerance to parasite infection

Alison Wunderlich, Tadeu Siqueira

Variation in host condition and population context is a major source of heterogeneity in infection risk and a key determinant of host-parasite dynamics. Host nutrition and density are drivers of this variation, yet their combined effects on host susceptibility and tolerance to parasite infection remain poorly understood. Here, we experimentally tested these interactive effects on susceptibility to parasite infections using a fish-ectoparasite system. We conducted a mesocosm experiment manipulating diet quality (high vs. low-protein diets) and host density (low vs. high). Exposure to parasites was standardized by placing infected donor hosts in separated compartments, allowing transmission to non-infected target hosts through a mesh barrier. Parasite abundance per target host, mean infection intensity per tank and host body condition (scaled mass index, SMI) were modeled as a function of diet and density. A high-protein diet increased parasite load across both density treatments, whereas a low-protein diet produced only weak differences between low- and high-density conditions. The combination of high-protein diet and high-density resulted in parasite loads several times greater than those observed under low-protein diet and low-density conditions, representing a sevenfold increase (from 17 to 120 parasites per host, on average). High-protein diets also doubled host body condition relative to low-protein diets. Together, our results show that improved nutritional quality can increase host susceptibility while also supporting higher host condition under infection, consistent with increased tolerance. More broadly, the interaction between host diet and density is critical for understanding host-parasite dynamics and provides new insights into the mechanisms linking host condition, resistance and tolerance to infection.