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Number of paired swab samples in filtered environmental surface water from Farmes Pool (red) and Tamarack Pool (black) field sites at Agassiz National Wildlife Refuge, Minnesota yielding influenza A virus isolates in ovo each month September 2018–April 2019 when held at 4°C in the laboratory. Twenty samples were virus isolation positive from each field site upon initial screening in September 2018. (Online version in colour.)
