Health officials are making a renewed call for vigilance against bird flu, but some experts are puzzling over why reports of new human cases have stopped.

The first major global review of bird flu in cats reveals an emerging threat of a potential human pandemic. Spring is here, birds are on the move, and a new threat is spreading alongside them: bird flu (H5N1).

Over a 14-month period, 70 people in the US were confirmed to have bird flu. But since early February, the number of new diagnoses stands at zero—leaving experts to question why reports of new human cases have essentially stopped.

The most recent infections confirmed by the Centers for Disease Control and Prevention were in early February.