As of 2024, the avian influenza H5N1 has infected a total of 36 people across the country.
The Centers for Disease Control and Prevention (CDC) reported on Tuesday that 16 farmworkers in California have tested positive for H5N1 bird flu. Genetic analyses of nine of these cases linked them to the clade 2.3.4.4b strains, which are closely related to the virus affecting domestic cattle.
Most infections in California have occurred among dairy farmworkers, and a total of 15 cases in other states—including nine in Colorado and six in Washington—have been traced back to poultry farms.
Interestingly, the source of infection for one individual in Missouri remains unknown.
California is facing a significant outbreak, especially after the virus was detected in cattle earlier this year. The CDC reports that 186 livestock herds in the state have been impacted, with an alarming 142 herds affected in just the past month.
Deaths among cattle are rising so quickly that in some cases, farmers have resorted to discarding carcasses by the roadside while waiting for removal services.
While all reported cases are believed to stem from animal-to-human transmission, the increasing number of human infections raises concerns about the potential for human-to-human spread.
If such transmission were to occur, particularly with flu season approaching, the consequences could be severe. Rossman expressed concern, noting the uncertainty regarding how the virus would behave if it developed the ability to spread among people.
The CDC indicated that current data indicate “sporadic instances of animal-to-human spread,” with affected farmworkers in California and Washington primarily experiencing mild symptoms, such as conjunctivitis.
However, Rossman cautions that as the virus mutates, its characteristics may evolve, potentially leading to efficient person-to-person transmission similar to seasonal flu, albeit with a higher mortality rate.