Bird flu spread in dairy cows for months before it was detected. How did officials miss it?

Get the Full StoryBefore this outbreak, bird flu has never been found in U.S. dairy cows.

The outbreak of the bird flu virus in dairy cows across the United States was first officially reported toward the end of March. But scientists say the virus was actually spreading undetected as early as November, according to a preliminary analysis of the virus s genome.

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