Quantifying iris pigmentation irregularities following an avian influenza outbreak: implications for population-level...

Emerging infectious diseases can have catastrophic impacts on wildlife populations, yet identifying individuals that survived exposure, especially when external symptoms are absent, remains challenging. Since 2021, a virulent strain of highly pathogenic avian influenza virus (HPAIV; H5N1 clade 2.3.4.4b) has caused unprecedented mortality in wild birds. Northern gannets (Morus bassanus) suffered dramatic mortality and reproductive failure at North America's largest colony in 2022. Following the outbreak, Lane et al. reported that a subset of gannets at a UK colony displayed darkened irises associated with HPAIV seropositivity. Here, we build on this work by quantifying iris pigmentation irregularity and evaluating whether greater irregularity predicts prior exposure, measured by anti-nucleoprotein (NP) and anti-hemagglutinin (H5) antibodies with standardized field-based photography. Iris pigmentation was strongly associated (ρ = -0.72) with NP antibody reactivity (S/N ratios), and the probability of NP seropositivity increased with irregularity (50% at 40% irregularity, 65% at 50%, and >90% above ~77%). We observed weaker positive associations with H5 antibody reactivity (percent inhibition, PI%; ρ = 0.30), consistent with differences in antibody persistence and detection dynamics. Among NP-seropositive individuals, NP antibody reactivity did not vary with iris irregularity, whereas H5 antibody reactivity increased with pigmentation extent. Pre-outbreak data defined a narrow range of iris pigmentation irregularity (≤8%), supporting a threshold separating baseline from elevated values. Our findings support earlier evidence that iris pigmentation irregularities reflect prior HPAIV exposure and demonstrate that quantifying their extent provides greater resolution for population-level monitoring. Iris scoring offers a rapid, minimally invasive approach for estimating prior exposure in post-outbreak contexts, particularly where serological sampling is limited. Rather than serving as a diagnostic proxy for individual exposure history, this metric provides an epidemiological indicator when interpreted alongside serological and ecological data. Future research should determine whether iris pigmentation remains stable, fades, progresses or changes, following subsequent HPAIV exposures.