A strain of highly pathogenic avian influenza has been silently spreading in US cattle for months, according to preliminary analysis of genomic data. The outbreak is likely to have begun when the virus jumped from an infected bird into a cow, probably around late December or early January. This implies a protracted, undetected spread of the virus — suggesting that more cattle across the United States, and even in neighbouring regions, could have been infected with avian influenza than currently reported.