Determinants of highly pathogenic avian influenza spread during bird migration

Abstract Migratory waterfowl are central to the global spread of the Goose/Guangdong lineage of H5 highly pathogenic avian influenza (HPAI), yet the parameters and processes shaping their role remain poorly understood, with evidence scattered across studies focused on narrow sets of parameters. Here, we synthesized current knowledge to identify 43 parameters and developed a spatially explicit agent-based model of waterfowl migration to evaluate the influence of 28 of these parameters on nine HPAI outcomes. Our results show that migratory waterfowl contribute most to HPAI spread and risk under conditions of high bird-to-bird transmission rate, long infectious periods and greater aggregation of birds at stop-over sites. When aggregation level is held constant, lower overlap in migratory routes among individuals (‘spatial alignment’) further amplifies waterfowl contributions by extending infection maintenance in waterfowl population and the environment. These results suggest that a few viral parameters shape much of infection dynamics, but also that migration behaviour, via aggregation and spatial alignment, plays a comparably large role within our model. Viral traits strongly shaped infection prevalence in waterfowl and incursion risk to poultry, whereas migratory behaviour more strongly shaped outbreak timing, maintenance and spatial distribution. These findings underscore the importance of integrating waterfowl movement ecology into HPAI risk models and assessments.