Avian Influenza A(H5) Outbreak | Center for Outbreak Response Innovation

Current US Outbreak

As of December 10, 2024:

Critical Epidemiological Updates

California public health officials have identified a suspected case of H5N1 in a child in Marin County. The source of the infection is currently unknown, and officials are awaiting confirmatory testing by the CDC. If confirmed positive, this would be the third case in the US with no known source and the fourth case with no known source in North America. Marin County is located about 30 miles from Alameda. If the case is confirmed positive, hypotheses of potential sources should be shared with the public to inform research and risk communication and serology studies may be necessary to identify if there is community transmission.
A recent publication in Science identifies a single mutation in the currently circulating H5N1 strain that would change the virus from being best adapted for birds to best adapted for humans. This mutation is believed to be a prerequisite for human-to-human transmission and, if observed in nature, could signal the beginning of a human pandemic.
Arizona health officials have identified two human cases of H5 among people working with infected poultry in Pinal County. CDC confirmatory testing is ongoing.

Routine Surveillance Updates

The Centers for Disease Control and Prevention (CDC) is reporting 58 confirmed human cases of H5 in the United States Outbreak as of December 10, 2024. Two of these cases have not had a known source of infection, and reported no contact with animals or raw milk, while 21 have had contact with poultry and 35 have had contact with dairy cows. In addition to those cases reported by the CDC, Arizona is reporting two suspected cases in poultry workers and Marin County is reporting one suspected case in a child. If all three cases are confirmed, this would bring the national total for the outbreak to 61.
CDC reports that for the duration of the outbreak, 330 tests for poultry workers and 147 tests for dairy workers have been sent to CDC for H5N1 testing, resulting in a positivity rate since March 2024 of 6.36% (21/330) and 23.81% (35/147), respectively.
The United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) has reported 249 new infected cattle herds across two states (NV, CA) in the last 30 days, bringing the total for the outbreak to 742 cattle herds in 16 states. This represents a slight decrease compared to the previous 30 days.