Large-scale circulation of High Pathogenicity Avian Influenza (HPAI) viruses of the H5Nx lineage (clade 2.3.4.4b) has driven substantial mortality in wild birds, with particularly heavy impacts on long-lived seabird species. In the North Atlantic in 2022, northern gannet colonies experienced severe losses, with more than 40% mortality among breeding adults observed at affected sites. The demographic and immunological consequences for survivors in these long-lived species remain uncertain and are critical to understanding population resilience.
To assess immunity after the outbreak, researchers collected samples from the Rouzic gannet colony in France during 2023 and 2024. The sampling targeted survivors of the 2022 outbreak to determine whether individuals retained detectable anti-influenza antibodies and functional neutralisation activity in the years following epizootic events. The report summarizes serological findings and links them to observable phenotypic markers and modelling of population-level effects.
Serological assays conducted on samples from 2023 and 2024 revealed that surviving northern gannets maintained persistent individual anti-AI antibody levels across years. In addition to binding antibodies, many immune individuals showed measurable seroneutralisation titres, indicating the presence of antibodies able to neutralise virus in vitro. These findings demonstrate that at the individual level, gannets can retain a detectable and apparently long-lived humoral response to HPAI infection following the 2022 outbreak.
A notable field observation was that a majority of seropositive individuals exhibited dark irises. The presence of darker iris coloration correlated with serological evidence of prior HPAI exposure in this population, making iris darkening a potentially useful visible marker of prior infection in field assessments. The study reports that most immune individuals showed this phenotype, although the abstract does not provide exact sensitivity or specificity estimates for using iris colour as a surrogate for seropositivity.
The authors used a modelling approach to explore how long-lasting immunity affects population persistence under scenarios of repeated outbreaks. Model results emphasize that in long-lived species such as gannets, the proportion of individuals retaining immunity between years is a key determinant of future population declines: higher rates of lasting immunity substantially limit decreases in population size when outbreaks recur. Thus, durable individual immunity can buffer populations against the demographic impacts of recurring epizootics.
Despite these protective effects at the individual level, the study highlights that population-level immunity remained limited. Only a subset of individuals retained immunity between years, which means that populations can still be vulnerable to subsequent waves of HPAI unless a larger fraction of birds acquire and maintain protective immunity.
The combination of field serology, observation of a visible marker (iris darkening), and modelling provides pragmatic tools for post-outbreak monitoring of long-lived seabirds. Evidence of long-lasting antibody responses supports the idea that persistent humoral immunity can contribute to population resilience. However, limited overall seroprevalence or incomplete retention of immunity between years indicates that populations may not be broadly protected against future HPAI events.
For conservation and management, these findings suggest prioritizing longitudinal monitoring of serostatus in survivors, documenting phenotypic markers like iris colour, and incorporating immunity duration into population viability assessments. Such integrated approaches can better inform interventions and risk assessments for colonies at risk of repeat HPAI incursions.
The abstract summarizes key outcomes but does not report several methodological and quantitative details. Sample sizes, precise seroprevalence values, assay platforms and thresholds, exact seroneutralisation titre distributions, statistical analyses, and the parameter values used in the models are not provided in the abstract and would need to be obtained from the full preprint for a complete appraisal. Likewise, specific timelines linking individual infection histories to antibody persistence were not detailed in the abstract.
Overall, the study documents durable individual antibody responses and an observable phenotypic correlate in gannets after a major HPAI outbreak, while showing that limited population-level immunity still leaves colonies at risk if outbreaks recur.