Prior infection induces long-lasting partial immunity to reduce transmission within flocks in an avian host-pathogen system

Population-level differences in the ability to maintain immunity can lead to contrasting survival outcomes between groups and determine how future outbreaks will spread. Pathology and infectiousness during subsequent infections can be influenced by immunity and an increase in resistance or tolerance of individuals. However, determining the longevity and effectiveness of immunity in wildlife population is challenging due to lack of infrastructure for long term monitoring of populations and individuals and host-pathogen specific tests. Further, predicting wildlife disease dynamics requires an understanding of individual-level heterogeneity in pathology and behavior as well as a knowledge of how populations with differing partial immunity will transmit disease, which is currently limited. Using an avian host-pathogen system, we ran two experiments to determine if previously exposed birds had immunity to MG after three years and if Mycoplasma gallisepticum (MG) transmission differed between first and second infection index birds. Birds retained partial immunity to MG for three years after their original infection and showed signs of being resistant to infection. The transmission experiment revealed that although first and second-infection index birds experienced pathology and pathogen growth, only first infection birds transmitted MG. Three years could represent lifetime protection for individuals who survive their first infection and a reduction in risk for other members of their flock. Our research has important implications for understanding MG epidemics in wild populations of birds as well as disease in wildlife more broadly.