The outbreak of H5N1 bird flu among U.S. dairy cows has now spread to three herds in California, the nation's largest milk-producing state with about 1.7 million dairy cows, federal and state health officials have confirmed.
Fourteen states and 197 herds have now been affected by the unprecedented outbreak in dairy cows, which was first confirmed by federal health officials on March 25.
In a statement, California Department of Food and Agriculture Secretary Karen Ross said the spread of the virus to California was not unexpected. “We have been preparing for this possibility since earlier this year when [Highly pathogenic avian influenza or HPAI] detections have been confirmed on dairy farms in other states,” Ross said. “Our extensive experience with HPAI in poultry has given us ample preparation and expertise to address this incident, with worker health and public health as our top priorities.”
Virus on the run
The California herds are thought to have been infected by cattle being moved, despite a federal order requiring testing of cattle before they are moved between states. So far, health officials believe all dairy farm infections in the affected states stem from a single spillover event from wild birds to dairy cows in Texas. The virus is thought to have spread from cow to cow, as well as via contaminated milking equipment, dirty hands and boots.
Wild bird populations around the world have been devastated by H5N1 in recent years, with its spread in the U.S. first documented in 2022. But unlike previous outbreaks of avian influenza in wild bird populations, the current strain of H5N1 spreading—clade 2.3.4.4b—has proven unusually adept at jumping to different mammals. The outbreak in the U.S. dairy industry, for example, was the first time the virus has been documented to cause an outbreak in cows.
Since the U.S. dairy cow outbreak began, federal officials have said they are well-equipped to stop the spread. However, the number of affected herds has continued to rise, with 17 herds reported infected in the past 30 days, according to the U.S. Department of Agriculture. The state with the most affected herds — 64 — is Colorado, the only state to mandate bulk milk testing for H5N1. Since that July 22 mandate, the state has identified 11 infected herds through bulk testing. Overall, testing for H5N1 has been limited, and experts believe official counts of infected herds may be significantly undercounted.
Risk of reassortment
For the time being, the risk to the general population is still considered low and the virus does not pose a risk via pasteurized milk and dairy products or well-cooked meat. The influenza virus is easily inactivated by heat treatments. The risk is also relatively low for infected cows, most of which recover completely within a few weeks.
However, farm workers exposed to infected animals are at risk of contracting the virus. So far, four dairy farmers and 10 poultry farmers have contracted the bird flu virus responsible for the dairy outbreak. The infections have been mild, with some only having sore eyes and others having classic flu symptoms. Officials have seen no evidence of human-to-human spread in any of these cases. However, experts fear that continued adaptation to mammals and exposure to humans will give the virus ample opportunity to switch to a more dangerous, contagious virus, potentially causing the next pandemic.
The concern was on full display at a July 30 press conference, when the Centers for Disease Control and Prevention announced a $5 million effort to vaccinate farmworkers against seasonal flu. The fear is that farmworkers could become human mixing bowls for H5N1 and seasonal flu this year. Influenza viruses are known to undergo reassortment, a process in which different strains of influenza viruses can swap parts of their genomes with each other when they co-infect a host. This can create genetically distinct strains, perhaps with new abilities. By vaccinating farmworkers against seasonal flu, health officials hope to prevent the mixing of bird flu and the human-adapted seasonal flu.
