Mortality is a central parameter in the layer sector with implications for animal health and welfare, production and sustainability. Little is known about the pathology leading to mortality in commercial laying hens in general and particularly in non-cage housing systems. Transition to non-cage housing systems are currently happening in many parts of the world. Baseline knowledge of causes of mortality in non-cage housing systems are relevant to ease this transition. The objective of this study was to investigate the causes of mortality and risk periods for disease management in commercial laying hens housed in organic and barn/aviary production in Denmark. We followed seven commercial layer flocks (three organic, four barn/aviary) on six farms from placement at 17-18 weeks of age to slaughter. The first ten dead hens (culled or found dead) every other week were sampled throughout the production period. Two experienced poultry pathologists performed lege artis necropsies of all carcasses. Bacteriological culturing was performed on
indication. Production data was collected from the industry-owned database ‘E-kontrollen’, including weekly mortality numbers, feed and water consumption, and egg production (number, egg weight, egg size). A total of 1,801 laying hens were examined. Based on the observed pathology and bacteriology at least one possible cause of mortality was determined for 1,758 hens, of which 226 hens could have died from two to four independent causes. The cause of mortality could not be established for 43 hens, either due to no identifiable lesions (13 hens) or due to autolysis (30 hens). We collected 1,562 bacteriological samples from 755 hens, of which 609 bacterial isolates were obtained (pure or mixed culture). Bacterial strains were identified by Maldi-Tof. We identified the following causes of mortality: salpingitis-peritonitis (24.8%), cannibalism (18.3%), egg bound (13.3%), pecking (11.7%), uraemia/nephropathy (8.8%), chronic salpingitis (6.3%), septicaemia (5.6%), internal haemorrhage (5.3%), gastro-intestinal disorders (5.0%), trauma (3.4%), neoplasia (3.2%), cardio-vascular disorders (1.5%), arthritis (1.1%), polyserositis (0.8%), bumble foot (0.4%), anaemia (0.4%), miscellaneous non-infectious (0.4%), miscellaneous infectious (0.3%). We observed differences in prevalence of causes of mortality between housing systems and between flocks. The largest differences between causes of mortality in laying hens housed in organic systems compared to barn/aviary systems were observed for pecking (3.9% versus 17.3%), cannibalism (12.7% versus 22.2%) and gastro-intestinal disorders (10.2% versus 1.3%). Salpingitis-peritonitis and egg bound were amongst the five most prevalent causes of mortality in all seven flocks. The range of the five most prevalent causes of mortality in the seven flocks were: salpingitis-peritonitis (9.1-35.9%), cannibalism (6.9-27.1%), egg bound (9.2-18.8%), pecking (2.0-52.5%), and uraemia/nephropathy (3.3-12.1%). Patterns in occurrence of causes of mortality over time suggested risk periods for disease management in terms of hen age. The following bacterial strains were isolated in pure or mixed culture: Eschericia coli (347, 57.0%), Staphylococcus spp. (82, 13.5%), Erysipelothrix rhusiopathiae (47, 7.7%), Gallibacterium anatis (59, 9.7%), Enterococcus spp. (44, 7.2%), Streptococcus spp. (9, 1.5%), miscellaneous (19, 3.1%). The results contribute with baseline knowledge of causes of mortality in laying hens housed in organic and barn/aviary systems including a detailed description of pathological findings. Understanding
the complexity underlying the mortality risk can guide intervention strategies and preventative measures to improve health and welfare of laying hens in non-cage housing systems.