Erysipelas in layers, caused by the bacterium Erysipelothrix rhusiopathiae (ER), is a very serious infectious disease that causes suffering, high mortality and egg production losses. Despite being considered as one of the biggest health problems in Swedish egg production knowledge on erysipelas in layers has been very limited, making the disease very difficult to combat and prevent. We have however generated new important knowledge on chicken immune responses to ER infection and the genetic variation between ER strains infecting layers. We have e.g. identified the uptake and killing of bacteria by immune cells (phagocytosis) as a central mechanism in the chicken defence against ER. In the proposed project we will continue to uncover the interactions between bacteria and host with focus on how protective immunity is achieved and maintained in chickens. The project comprises 1) studies of immune parameters and factors affecting these during the production period in commercial laying hen flocks, 2) identification of ER and related bacteria in the production environment, 3) monitoring of erysipelas outbreaks for ER-type and chicken immune responses and 4) in vitro studies of immune cells to identify how genetic variation between ER-strains affect phagocytosis. Results from this project will provide novel insight e.g. in to factors influencing the outcome of vaccination against erysipelas in layers and will lead to more effective means to prevent this disease.