The Colorado Department of Agriculture this week stepped up efforts to stop a runaway outbreak of bird flu cases on dairy farms by issuing an order requiring testing for the virus on all commercial cow dairies licensed by the state.
But, separately, new information about the bird flu virus that infected a Colorado farmworker provides reassuring evidence that the virus remains a low risk to human health.
The state has seen at least 51 cases of bird flu on dairy farms since April, meaning nearly half of all commercial dairies in Colorado have been affected. Of those cases, 30 have happened in the past 30 days.
Colorado’s outbreak continues to surge even as others have dwindled nationwide — no other state has seen more than four cases in the past 30 days, and some major dairy-producing states like Wisconsin and California have never reported any cases.
In issuing the order, state veterinarian Dr. Maggie Baldwin said the virus, while not causing deaths of many cattle, has still been a devastating disruption to Colorado’s dairy industry — resulting in quarantines and loss of milk production.
“We have been navigating this challenging, novel outbreak of HPAI in dairy operations for nearly three months in Colorado and have not been able to curb the spread of disease at this point,” Baldwin said in a statement, using a shorthand term for the virus, which is also known as highly pathogenic avian influenza.
The order does not apply to farms that produce raw milk, which are not regulated by the state. Pasteurization kills the virus in milk sold in stores, but raw milk is unpasteurized, meaning there is the potential for it to contain live virus.
Spillover cases
Baldwin noted that, as the dairy outbreaks rage on, they are also generating spillover cases in other animals. Most notably, Colorado has begun seeing infections again in commercial poultry operations.
There have been two major, confirmed outbreaks at egg-laying operations in Weld County, while a third, suspected outbreak is also under investigation. Those outbreaks have resulted in the culling of more than 3.2 million chickens just in July, according to the Department of Agriculture.
Colorado has now seen 33 commercial poultry flocks affected since 2022, with more than 6.3 million domestic birds culled.
Then there’s the human toll. One of those poultry outbreaks led to an unprecedented cluster of cases among workers who were doing the culling. Six workers were confirmed positive for bird flu, though their symptoms were relatively mild and none required hospitalization.
Meanwhile, the Colorado Department of Public Health and Environment announced Thursday that it has identified three new human cases of bird flu — these tied to culling operations at a different poultry farm. The workers, likewise, have mild symptoms and have been offered antiviral drugs for treatment.
Searching for genetic changes
Including the new cases, the state has now identified 11 out of the 14 human cases nationwide since 2022, placing Colorado at the center of the nation’s bird flu epidemic. And the burst of human infections in Colorado has raised questions about whether the flu virus has mutated to make it more capable of infecting people.
But a new CDC analysis eases those concerns. The CDC took a virus sample from one of the Colorado poultry workers and sequenced its genome.
Among the findings:
- The sample’s genetics are closely related to those of viruses found in animals from other recent outbreaks at poultry and dairy facilities. Among human cases, the Colorado worker’s virus is most similar to the virus that infected a dairy worker in Michigan earlier this year.
- The Colorado virus “maintains primarily avian genetic characteristics,” according to the CDC, meaning it isn’t especially adapted to infect people or spread person-to-person. It does, however, have one mutation that makes it better at infecting mammals. That mutation has been found in more than 99% of infections among cattle.
- There were no mutations found that indicate the virus is developing resistance to existing antiviral drugs.
- There are also no changes to the virus suggesting it is more capable of causing harm to humans.
- The virus is closely related to a couple bird flu samples available to vaccine manufacturers, meaning companies could start producing a bird flu vaccine quickly if needed.
So, to recap: Nothing about the Colorado case suggests the bird flu virus has become better able to infect people, hurt people or spread to other people. The CDC said the analysis “supports CDC’s conclusion that the human health risk currently remains low.”
No evidence of silent spread
The CDC also reported some more good news last week: Blood tests of Michigan dairy workers were boring.
Michigan’s public health department conducted what is known as a seroprevalence study of workers with known exposures to infected cows. The goal was to see if workers who showed no symptoms of bird flu actually had antibodies against the virus. If they did, it would suggest that they had been silently infected and that human cases might be more common than known.
Instead, the results from every nonsymptomatic worker tested came back clean — no antibodies against bird flu.
“This is an important finding,” the CDC wrote in a weekly update, “because it suggests that asymptomatic infections in people are not occurring.”
That means the risk remains primarily to farmworkers who have direct contact with infected animals. And that puts the focus on efforts to provide information to farms and to ensure workers have access to — and are able to wear — protective equipment.
“Ongoing cooperation is key to supporting workers’ health and safety, protecting animal health and welfare, and minimizing the spread of the virus,” Kate Greenberg, Colorado’s commissioner of agriculture, said in a statement.