Host genetics, lung T-cell immunity, and laying activity determine the disease outcome in avian influenza virus-infected chickens.

Avian influenza viruses pose a threat to both wild and domestic bird populations worldwide. In particular, chickens kept outdoors have contact with wild birds and thus an increased risk of infection with influenza viruses. Increasing use of high-performance chicken breeds in growing poultry production reduces genetic diversity whereas local breeds may offer valuable traits such as disease resilience important for sustainable agriculture. This study investigates the immunological performance of three local chicken breeds (Altsteirer, Ramelsloher, and Bielefelder) in Germany to assess their potential resilience to infectious diseases such as avian influenza. Viral resilience was assessed by inoculating 6-week-old chickens with a highly pathogenic avian influenza virus H7N1 and the moderately pathogenic recombinant avian influenza virus TG05-HAR65. Differences in mortality, immune response, transmission, virus shedding, and viral load in certain organs were observed. The Ramelsloher chickens had the lowest clinical score and the highest survival rate. Chicken-to-chicken transmission was only observed in the Altsteirer breed. Lung infiltration by T cells was detected in the Bielefelder and Ramelsloher breeds. In addition, 35-week-old laying hens at peak laying performance were infected with TG05-HAR65, to assess the impact of laying activity on viral resilience and immunocompetence. Compared with juvenile chickens, clinical signs, virus shedding, and seroconversion were detected at later time points. At the end of the experiment, most hens showed egg yolk peritonitis, likely caused by the influenza virus infection. This study indicates differences in the immunocompetence and viral resilience of local chicken breeds and provides breeding and biosafety management recommendations for organic farming.