Commercial laying hens have undergone extensive breeding and husbandry changes aimed to increase productivity. Unfortunately, the changes have also been linked to health issues like reproductive disorders, skeletal problems and susceptibility to diseases. While Norwegian laying hens show low mortality and few outbreaks of infectious diseases, the prevalence of non-infectious diseases and general morbidity is not well-documented. An example of a highly prevalent non-infectious condition is a keel bone fracture (KBF). Although common worldwide, few studies have investigated macro- and microscopical characteristics of the fractures in order to better understand the etiology and pathophysiology behind KBF. The keel’s ossification, a process that might be linked to KBF etiology, has never been studied in detail. The overall aim of this thesis was to generate new knowledge about the health challenges in Norwegian commercial aviary housed laying hens. We described causes of carcass condemnation in slaughtered laying hens (n= 459’584 hens) and addressed seasonal variation, by gathering historical meat inspection data from one slaughterhouse in the period 2018-2020. Secondly, 48 farms with indoor aviary housed laying hens were visited to conduct a field necropsy of hens found dead (n= 482 hens) at end-of-lay (65-75 weeks of age), to describe common pathological lesions and tentative cause of death. To investigate KBF characteristics, one flock with indoor aviary housed laying hens was visited repeatedly from 17-57 weeks of age (WOA) with approximately four weeks between visits. At each visit the keels from 30 hens were radiographed and a subset of 5 hens were further euthanized for histological examination of the keel. Radiographical- (n=330 hens) and histological- (n=50 hens) fracture characteristics were assessed and the keels’ maturation was additionally described based on histology. The main findings from this PhD-project are the following: • Common causes of carcass condemnation in laying hens were abscess/cellulitis, peritonitis and dead-on-arrival (DOA). Substantial variation between batches was seen in total carcass condemnations and DOA. The analysis indicated a higher estimated prevalence of total carcass condemnations during winter compared to other seasons. • Common pathological lesions in hens found dead on farm at end-of-lay were KBF, fatty liver, emaciation and salpingitis. Common causes of mortality were salpingitis and fatty liver hemorrhagic syndrome. • Description of radiographical- and histological fracture characteristics in one flock housed in an indoor aviary housing system showed a high frequency of simple, complete- or incomplete fractures without characteristics typical of high impact collisions. This indicates that nontraumatic causes to KBF may be of higher importance than conventional beliefs. • Histological description of the keel bone’s maturation showed endochondral ossification and a substantial variation in the individual degree of ossification between individuals within the same age. The cartilage in the keel can contain vessel-carrying cartilage canals. Medullary bone was present in the keel bone at all examined ages (17-53 WOA). Altogether, this thesis provides new insights into health challenges Norwegian commercial laying hens are facing at end-of-lay. Results on carcass condemnation and in-field necropsy can be used to inform future studies investigating these challenges, ultimately improving the hens’ health and wellbeing. Our results on KBF characteristics suggest that other explanations than high impact collisions should be explored further, and that different KBF can have different causes. Our description of keel maturation is novel and contributes to the understanding of the keel’s structure.

Norges forskningsråd