The report is the latest evidence that the outbreak in dairy herds is spreading undetected in cows, and the spillover into people at highest risk of exposure is going unnoticed.

The patient was experiencing "flu-like symptoms" and had "health conditions that can make people more vulnerable to illness," Wyoming's health department said. She was likely exposed to the H5N1 virus through direct contact to an infected poultry flock at her home.

Suspicions that there may be more bird flu going around undetected were backed by a voluntary serosurvey among veterinary practitioners who work with cattle, CDC investigators said. Out of 150 participating veterinary practitioners across the country,