If there is a reduction in small carnivores, or exposure to their feces, that should reduce the possibility of ducks being infected. I don’t think the infection is something that gets reduced or contracted in noticeable form over night. Probably population trends in skunks, foxes, etc… and the amount of water. Widespread flooding where small carnivores are forced to retreat from wetlands or things like distemper or rabies probably play a role. Interesting question anyway. My purely anecdotal results were zero this year. My worst day was a seven bird limit and 3 were infected probably 8 or 10 years ago.