A fourth farm worker has been infected with highly pathogenic avian influenza bird flu (H5N1) in the growing outbreak linked to dairy cows, health officials reported Wednesday.
The worker had direct contact with infected dairy cows on a northeast Colorado farm, state and federal health officials said. The man developed conjunctivitis, received antiviral treatment with oseltamivir (Tamiflu), and has recovered.
Three previous cases of human infection linked to cows have been reported in dairy workers in Texas and Michigan since March. Two of those workers also developed conjunctivitis, while one had mild respiratory symptoms. In 2022, the first U.S. case of bird flu was detected in a Colorado farm worker exposed to infected poultry.
"Based on the information available at this time, this infection does not change CDC's current H5N1 bird flu human health risk assessment for the U.S. general public, which the agency considers to be low," the .
Surveillance systems tracking flu in the U.S. have shown no unusual activity, officials said. However, people with prolonged contact with infected birds or other animals, including livestock, or to their environments, are at higher risk of infection.
Health officials across the U.S. have been increasing efforts to find infected farm workers. In Michigan, the Department of Health launched a seroprevalence study to determine whether asymptomatic infections have occurred among people working with infected cattle, with CDC providing technical assistance.
The Colorado man was being monitored when he developed symptoms because of his work with dairy cows, according to the CDC. Tests at the state level were inconclusive, but samples sent to CDC tested positive. Full results of genetic analysis of the sample are pending.
Critics have said the agency is repeating some of the from the start of the COVID-19 pandemic, including a lack of H5N1-specific tests to help identity cases.
As of Wednesday, more than 135 dairy herds in a dozen states had reported infections with the H5N1 virus that originated in poultry, according to the Agriculture Department.