Genetic parameters for welfare, infestation level, and inflammatory response traits in cage-free laying hens experimentally infested with northern fowl mites (Ornithonyssus sylviarum).

Northern fowl mite (NFM) infestation in laying hens remains an important economic and welfare challenge within North American cage-free egg production systems. Controlled challenge trials provide a framework for generating standardized phenotypes of host response. Therefore, our objectives were to quantify phenotypic variability and estimate (co)variance components and genetic parameters for indicators of hen welfare, mite burden, and skin response traits following an experimental NFM challenge. A total of 1030 laying hens were infested at 24 weeks of age (WOA) and evaluated at 26, 30, and 40-41 WOA. Mite burden was assessed using an ordinal mite count (MC) score ranging from 0 to 7 (> 10,000 mites). Skin inflammatory response traits included inflammation area (IA; cm²), skin inflammation score (SI), and skin lesion score (SL). Additional welfare traits were recorded using standardized scoring systems. Hens were genotyped with a 54 K SNP panel. Variance components were estimated using animal models fitted with single-trait genomic BLUP (GBLUP), and genetic correlations between traits were obtained using two-trait GBLUP implemented under a Bayesian framework. Heritability estimates for MC, SI, IA, and SL ranged from 0.04 ± 0.03 at 40 WOA to 0.10 ± 0.04 at 26 WOA; 0.04 ± 0.02 at 40 WOA to 0.07 ± 0.04 at 30 WOA; 0.08 ± 0.07 at 40 WOA to 0.11 ± 0.05 at 30 WOA; and 0.04 ± 0.03 at 40 WOA to 0.05 ± 0.03 at 26 WOA, respectively. Genetic correlations among traits varied in both magnitude and direction, indicating complex and age-dependent relationships between mite burden and host response. These results demonstrate that traits associated with the hen response to NFM infestation are measurable, heritable, and can be improved through genomic selection. These approaches have the potential to improve hen welfare and productivity while reducing the reliance on chemical interventions, thereby supporting the long-term sustainability goals of the poultry sector.