SACRAMENTO, California — Health officials across the U.S. are working to prevent a potentially dangerous combination virus as avian flu rips through one of the nation's largest milk-producing regions during the height of flu season.
Public health experts have long warned that avian flu poses a significant pandemic threat to humans, and the number of infections among dairy workers in California continues to grow. The timing of the outbreak will soon collide with the seasonal flu, complicating efforts to track bird flu and raising the risk that the two viruses could mix, potentially creating a virulent combo that could spread beyond dairy workers to the rest of the population.
Despite what California officials say is a proactive approach, public health experts outside the state say too little is being done to track and respond to avian flu, which has spread to 105 dairy farms since the virus was first found here in August. The stakes are high: Approximately half of documented human H5N1 avian flu cases in the past two decades were deadly, according to the World Health Organization.
“It will mutate to become increasingly optimal in humans as soon as it gains any foot in the door for human-to-human transmission,” said Michael Mina, chief science officer at digital health company eMed. “How far that goes and how fast it means the virus starts to transmit … is almost entirely unknown right now.”
Other states have had a handful of avian flu cases in humans this year, including one in Texas and 10 in Colorado. But as the largest dairy-producing state in the country, with over 1.7 million cows, California’s response could serve as a test case for how to deal with large numbers of infected cows or people.
Since the California Department of Public Health disclosed the state’s first presumptive infection on Oct. 3, 11 human infections have been confirmed, according to the state. All of the individuals — who work at nine different farms — have had direct contact with infected dairy cattle.
“It’s important to note that to date, there has been no evidence of human-to-human transmission of bird flu in the U.S.,” a CDPH spokesperson said in an email.
But between March 31 and Oct. 14, only 25 individuals in the state have been tested for avian flu, according to CDPH. Workers who are symptomatic are first screened for flu before additional bird-flu-specific testing is performed. California is home to over 17,500 dairy workers, most of whom are in the Central Valley.
The CDC set aside more than 100,000 doses of seasonal flu vaccine for the 12 states with outbreaks. Five thousand of those doses are meant for California’s dairy workers, and counties with many herds can now order them, but they weren't available until Oct. 14.
Getting regular flu vaccines into dairy workers’ arms is meant to help detect avian flu better. With fewer people exhibiting regular flu symptoms, health workers should be better able to find avian flu cases. The effort is also intended to cut down on the chance that avian flu goes through a process known as reassortment that could result in a virus that can transmit human to human.
“That alone, without contemplating high pathogenicity, equates to huge numbers of increased hospitalizations and deaths nationally and globally,” said Mina of eMed.
It’s something the world has seen before. The swine flu pandemic of 2009 was a “quadruple-reassortant virus” made up of swine, avian and human flu genes. Over 60 million people got sick between April 2009-2010, and almost 12,500 people died .
State officials say proactive preparation has enabled quick detection of human avian flu infections in farmworkers with exposure to infected dairy cows. Those efforts — concentrated in California’s Central Valley — are leveraging what public health officials learned from an avian flu tabletop simulation earlier this summer and Covid lessons learned even earlier than that.
“Once we first heard about what's happening in Texas and dairy herds in other states, and knowing we're a large dairy state, we activated sort of an instant management team early,” California State Epidemiologist Dr. Erica Pan said in an interview, referring to the first infections in dairy herds that officials traced back to the Texas panhandle.
Anyone testing positive is being offered antivirals, and so are their close contacts, who are also told to monitor for symptoms for 10 days. If any develop symptoms, CDPH recommends they be tested for bird flu. So far, cases have all been mild, with most experiencing conjunctivitis, also known as pink eye.
The state has repeatedly warned that it expects more cases to be detected in individuals who work with infected dairy cattle.
“[The cases] represent the tip of an iceberg and a massive risk and game of chicken we are playing with this virus,” said Mina, who advocated for broad testing at the start of the Covid-19 pandemic. “The only problem is that we are likely on the losing end. We still have done exceedingly little around surveillance testing and diagnostic testing so we don't know how frequently it is spreading to humans or among humans.”
Jennifer Nuzzo, a professor of epidemiology and director of the Pandemic Center at Brown University School of Public Health, also argued that too little information is being provided about human avian flu infections.
“I've been very frustrated that we haven't had crucial details like the date of symptom onset and the date they were actually tested,” Nuzzo said. “These are outbreak standard data to report, but they're not being reported, and the absence of these data make it really hard to understand what's going on.”
In California, the response has been divided into human and animal health, with agriculture departments taking the lead on testing milk to find infected herds, then flagging those farms to public health officials who look for infected people, according to interviews with four public health departments in the Central Valley.
“California is doing bulk testing, so I think they're identifying a lot of herds through aggressive disease surveillance, and I think identifying infected herds is allowing them to better identify human cases,” said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security who was a founding associate director of the CDC’s Center for Forecasting and Outbreak Analytics.
In the meantime, until the extra doses of seasonal flu vaccines arrive, dairy-producing counties are pushing vaccines in their regularly scheduled seasonal flu outreach, setting up walk-up clinics at the Kern County swap meet or contracting with mobile health clinics in Tulare to visit farms.
“Because of Covid-19 we had relationships established with our dairy industry partners, we did outreach of Covid vaccines with the local dairy cooperatives in Tulare County because we wanted to get farm workers vaccinated for Covid,” said Carrie Monteiro, a spokesperson for the Tulare County Health and Human Services Agency. “So we had those relationships established, which proved to be beneficial for us.”
There’s no statistics yet on how well this outreach is working, as the counties don’t keep occupational data on who is getting shots. Overall, Californians are getting vaccinated against flu at the same rate as last year, said Brian Micek, a spokesperson with CDPH.
Since the beginning of the outbreak in dairy cattle, some farmers have expressed reluctance to allow public health officials onto their farms to conduct testing and surveillance. USDA requires farmers to test lactating dairy cattle before crossing state lines, and offers financial support to farmers who lose milk production from sick animals. But tight margins in the dairy industry and limited federal support for impacted farms mean that farmers may be less inclined to test.
USDA’s voluntary bulk milk testing program has enabled some states and farmers to comply with federal testing regulations and conduct additional surveillance, but just 64 herds have enrolled and just three in California.
“The Department does not believe mandatory bulk milk testing is necessary at this time,” wrote USDA spokesperson Will Clement in a statement. “Since the beginning of this outbreak, USDA has been working closely with states, and if requested, working with states to help them implement testing programs.”
The Food and Drug Administration plans to beef up milk testing later this month , deploying a separate study across participating states to discern how far the virus has spread.
California’s bulk milk testing, required by the state “where there is elevated risk of the disease,” has contributed to the state’s high caseload and the state has plans to continue expanding its testing, according to Steve Lyle, a spokesperson for the California Department of Food and Agriculture. The state is currently testing more than 350 dairies with a combined total of more than 1 million cattle.
“California is the largest dairy state in the nation, and we’re implementing a scale of testing that hasn’t been employed previously in any other state,” wrote Lyle in a statement.
On the human health side, the state has tried to lean into preparation, relying on lessons learned and technology developed during Covid-19.
The state has around 700,000 doses of antivirals in the state stockpile, and sent doses to 18 local health departments ahead of the outbreak. Some of those doses are going to the close contacts of infected workers to take prophylactically so they don’t get sick too.
Local health departments are using CalCONNECT, an IT system developed for Covid-19, that consolidates information for local health departments from other systems, like disease surveillance and vaccine registries. Some counties are deploying wastewater surveillance, another Covid-era technology, to keep track of where the virus is showing up, although it’s complicated by the presence of infected wild birds.
The state also reopened a mass procurement system so counties can quickly request more PPE to distribute to farms. A huge part of California’s response has been getting PPE to dairies ahead of outbreaks, especially gloves, goggles and face shields. Tulare County officials, for instance, say they distributed a million pieces of PPE to farms before their first positive case.
Still, experts say more needs to be done for workers, like financial compensation for missed work.
“We probably need to be thinking about ways of supporting our workforce, more than just recommending PPE,” said Dr. Meghan Davis, associate professor at the Johns Hopkins Bloomberg School of Public Health and a veterinarian. “There likely are other things that we need to be thinking about doing to help prevent their exposure.”
With dairy workers traveling between counties and dairy owners operating in multiple jurisdictions, the Central Valley’s dairy producing counties have been trying for a uniform approach.
In August, the six counties sent out a joint advisory to health care providers, telling them what to look for in their dairy worker patients, and reminding them to immediately report cases to local health departments. It’s meant to provide a kind of backstop, if patients or their employers aren’t reporting cases to the health department their doctors will.
It’s important information to get out, because clinicians may not know what they’re dealing with and it could be putting farm workers at risk, said Amy Liebman, chief program officer for workers, environment and climate with the Migrant Clinicians Network.
“The bottom line is we … think that the symptoms are mild, and as a result, we don't really have the documentation that we need,” she said, noting that workers may not seek care for mild illness. “We're not testing everybody, and we're not testing everybody when they have these symptoms, and so I don't think we really understand the extent of it at all.”
Public health is treading lightly and trying to make it easy for dairy farms to participate and comply. As the state epidemiologist Dr. Erica Pan noted, “There’s a lot of competing priorities as these farms are dealing with their sick cattle.”
Officials are trying to use more social media to get the word out to workers, and texting to check in on people. Gone are the days of “contact tracing,” which carries a lot of Covid-era baggage.
“We’re really framing this as ‘these are health checks for your employees’ and to the workers themselves,” Pan said. “I think we’re kind of getting away from terms like monitoring. These are health checks, this is what we can do to help your workers stay healthy.”
Flu antivirals must be given within days of symptom onset, meaning there is only a small window to administer medicine that could potentially save lives.
“Fortunately, the cases have been mild, but it's a real gamble to assume that the rest of them will be,” Nuzzo said.
CLARIFICATION: After publication, the FDA clarified that the separate study of milk testing in California is not a pilot.
CLARIFICATION: This report has been updated to show the availability of seasonal flu vaccine to California dairy workers.
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