One Decade of Active Avian Influenza Wild Bird Surveillance in Belgium Showed a Higher Viroprevalence in Hunter-Harvested Than in Live-Ringed Birds

Active monitoring of avian influenza (AI) viruses in wild birds was initiated in Belgium in 2005 in response to the first highly pathogenic avian influenza (HPAI) H5N1 outbreaks occurring in Europe. In Belgium, active wild bird surveillance that targeted live-ringed and hunter-harvested wild birds was developed and maintained from 2005 onward. After one decade, this program assimilated, analyzed, and reported on over 35,000 swabs. The 2009-2014 datasets were used for the current analysis because detailed information was available for this period. The overall prevalence of avian influenza (AI) in samples from live-ringed birds during this period was 0.48% whereas it was 6.12% in hunter-harvested samples. While the ringing sampling targeted a large number of bird species and was realized over the years, the hunting sampling was mainly concentrated on mallard (Anas platyrhynchos) during the hunting season, from mid-August to late January. Even when using just AI prevalence for live-ringed A. platyrhynchos during the hunting season, the value remained significantly lower (2.10%) compared to that detected for hunter-harvested mallards. One explanation for this significant difference in viroprevalence in hunter-harvested mallards was the game restocking practice, which released captive-bred birds in the wild before the hunting period. Indeed, the released game restocking birds, having an AI-naive immune status, could act as local amplifiers of AI viruses already circulating in the wild, and this could affect AI epidemiology. Also, the release into the wild of noncontrolled restocking birds might lead to the introduction of new strains in the natural environment, leading to increased AI presence in the environment. Consequently, the release of naive or infected restocking birds may affect AI dynamics.