The H5N1 bird flu situation in the US seems more fraught than ever this week as the virus continues to spread swiftly in dairy cattle and birds while sporadically jumping to humans.
On Monday, officials in Louisiana announced that the person who had developed the country’s first severe H5N1 infection had died of the infection, marking the country’s first H5N1 death. Meanwhile, with no signs of H5N1 slowing, seasonal flu is skyrocketing, raising anxiety that the different flu viruses could mingle, swap genetic elements, and generate a yet more dangerous virus strain.
But, despite the seemingly fever-pitch of viral activity and fears, a representative for the World Health Organization today noted that risk to the general population remains low—as long as one critical factor remains absent: person-to-person spread.
“We are concerned, of course, but we look at the risk to the general population and, as I said, it still remains low,” WHO spokesperson Margaret Harris told reporters at a Geneva press briefing Tuesday in response to questions related to the US death. In terms of updating risk assessments, you have to look at how the virus behaved in that patient and if it jumped from one person to another person, which it didn’t, Harris explained. “At the moment, we’re not seeing behavior that’s changing our risk assessment,” she added.
In a statement on the death late Monday, the US Centers for Disease Control and Prevention emphasized that no human-to-human transmission has been identified in the US. To date, there have been 66 documented human cases of H5N1 infections since the start of 2024. Of those, 40 were linked to exposure to infected dairy cows, 23 were linked to infected poultry, two had no clear source, and one case—the fatal case in Louisiana—was linked to exposure to infected backyard and wild birds.
The Louisiana resident infected with H5N1 bird flu is hospitalized in critical condition and suffering from severe respiratory symptoms, the Louisiana health department revealed Wednesday.
The health department had reported the presumptive positive case on Friday and noted the person was hospitalized, as Ars reported. But a spokesperson had, at the time, declined to provide Ars with the patient’s condition or further details, citing patient confidentiality and an ongoing public health investigation.
This morning, the Centers for Disease Control and Prevention announced that it had confirmed the state’s H5N1 testing and determined that the case “marks the first instance of severe illness linked to the virus in the United States.”
In a follow-up, the health department spokesperson Emma Herrock was able to release more information about the case. In addition to being in critical condition with severe respiratory symptoms, the person is reported to be over the age of 65 and has underlying health conditions.
Further, the CDC collected partial genetic data of the H5N1 strain infecting the patient, finding it to be of D1.1. genotype, which has been detected in wild birds and some poultry in the US. Notably, it is the same genotype seen in a Canadian teenager who was also hospitalized in critical condition from the virus last month. The D1.1. genotype is not the same as the one circulating in US dairy cows, which is the B3.13 genotype.
A person in Louisiana is hospitalized with H5N1 bird flu after having contact with sick and dying birds suspected of carrying the virus, state health officials announced Friday.
It is the first human H5N1 case detected in Louisiana. For now, the case is considered a “presumptive” positive until testing is confirmed by the Centers for Disease Control and Prevention. Health officials say that the risk to the public is low but caution people to stay away from any sick or dead birds.
Although the person has been hospitalized, their condition was not immediately reported. It’s also unclear what kind of birds the person had contact with—wild, backyard, or commercial birds. Ars has reached out to Louisiana’s health department and will update this piece with any additional information.
The case is just the latest amid H5N1’s global and domestic rampage. The virus has been ravaging birds of all sorts in the US since early 2022 and spilling over to a surprisingly wide range of mammals. In March this year, officials detected an unprecedented leap to dairy cows, which has since caused a nationwide outbreak. The virus is currently sweeping through California, the country’s largest dairy producer.
To date, at least 845 herds across 16 states have contracted the virus since March, including 630 in California, which detected its first dairy infections in late August.
Human cases
At least 60 people in the US have been infected amid the viral spread this year. But the new case in Louisiana stands out. To date, nearly all of the human cases have been among poultry and dairy workers—unlike the new case in Louisiana— and almost all have been mild—also unlike the new case. Most of the cases have involved conjunctivitis—pink eye—and/or mild respiratory and flu-like symptoms.
There was a case in a patient in Missouri who was hospitalized. However, that person had underlying health conditions, and it’s unclear if H5N1 was the cause of their hospitalization or merely an incidental finding. It remains unknown how the person contracted the virus. An extensive investigation found no animal or other exposure that could explain the infection.
Rise in cases amplifies concerns about consolidation in agriculture.
Holstein dairy cows in a freestall barn. Credit: Getty |
A handful of dairy farms sprawl across the valley floor, ringed by the spikey, copper-colored San Jacinto mountains. This is the very edge of California’s dairy country—and so far, the cows here are safe.
But everyone worries that the potentially lethal bird flu is on the way. “I hope not,” says Clemente Jimenez, as he fixes a hose at Pastime Lakes, a 1,500-head dairy farm. “It’s a lot of trouble.”
Further north and west, in the San Joaquin Valley—the heart of the state’s dairy industry—the H5N1 virus, commonly known as bird flu, has rippled through the massive herds that provide most of the country’s milk. Farmworkers have piled carcasses into black and white heaps. This week the state reported 19 new confirmed cases in cows and more than 240,000 in chickens. Another 50,000 cases were confirmed at a chicken breeding facility in Oklahoma.
Most worrying, though, is the spillover from livestock to humans. So far, 58 people in the United States have tested positive for bird flu. Fifty-six of them worked either on dairy or poultry farms where millions of birds had to be culled.
The Centers for Disease Control and Prevention confirmed that four of the cases in humans had no known connection to livestock, raising fears that the virus eventually could jump from one human to another, though that hasn’t happened yet. On Thursday, a study published in Science by researchers at The Scripps Research Institute said it would take only a single mutation in the H5N1 virus for it to attach itself to human receptor cells.
Large livestock facilities in states across the country, and especially in California, have become the epicenters of these cases, and some researchers say that’s no surprise: Putting thousands, even hundreds of thousands, of animals together in confined, cramped barns or corrals creates a petri dish for viruses to spread, especially between genetically similar and often stressed animals.
More drought and higher temperatures, fueled by climate change, supercharge those conditions.
“Animal production acts like a connectivity for the virus,” said Paula Ribeiro Prist, a conservation scientist with the EcoHealth Alliance, a not-for-profit group that focuses on research into pandemics. “If you have a lot of cattle being produced in more places, you have a higher chance of the virus spreading. When you have heat stress, they’re more vulnerable.”
So far, this bird flu outbreak has affected more than 112 million chickens, turkeys, and other poultry across the US since it was first detected at a turkey-producing facility in Indiana in February 2022. In March of this year, officials confirmed a case of the virus in a Texas dairy cow—the first evidence that the virus had jumped from one livestock species to another. Since then, 720 cows have been affected, most of them in California, where there have been nearly 500 recorded cases.
In the United States, a trend of consolidation in agriculture, particularly dairies, has seen more animals housed together on ever-larger farms as the number of small farms has rapidly shrunk. In 1987, half of the country’s dairy cows were in herds of 80 or more, and half in herds of 80 or fewer. Twenty years later, half the country’s cows were raised in herds of 1,300 or more. Today, 5,000-head dairies are common, especially in the arid West.
California had just over 21,000 dairy farms in 1950, producing 5.6 billion pounds of milk. Today, it has 1,100 producing around 41 billion pounds. Total US milk production has soared from about 116 billion pounds in 1950 to about 226 billion today.
“The pace of consolidation in dairy far exceeds the pace of consolidation seen in most of US agriculture,” a recent USDA report said.
Initially, researchers thought the virus was spreading through cows’ respiration, but recent research suggests it’s being transmitted through milking equipment and milk itself.
“It’s been the same strain in dairy cows… We don’t necessarily have multiple events of spillover,” said Meghan Davis, an associate professor of environmental health and engineering at Johns Hopkins Bloomberg School of Public Health. “Now it’s transmission from one cow to the next, often through milking equipment.”
It’s still unclear what caused that initial jump from wild birds, which are the natural reservoirs of the virus, to commercial poultry flocks and then to cows, but some research suggests that changing migration patterns caused by warmer weather are creating conditions conducive to the spreading of viruses. Some wild birds are migrating earlier than usual, hatching juvenile birds in new or different habitats.
“This is leading to a higher number of young that are naive to the virus,” Prist explained. “This makes the young birds more infectious—they have a higher chance of transmitting the virus because they don’t have antibodies protecting them.
“They’re going to different areas and they’re staying longer,” Prist added, “so they have higher contact with other animals, to the other native populations, that they have never had contact [with] before.”
That, researchers believe, could have initiated the spillover from wild birds to poultry, where it has become especially virulent. In wild birds, the virus tends to be a low pathogenic strain that occurs naturally, causing only minor symptoms in some birds.
“But when we introduce the virus to poultry operations where birds live in unsanitary and highly confined conditions, the virus is … able to spread through them like wildfire,” said Ben Rankin, a legal expert with the Center for Biological Diversity, an advocacy group. “There are so many more opportunities for the virus to mutate, to adapt to new kinds of hosts, and eventually, the virus spills back into the wild and this creates this cycle, or this loop, of intensification and increasing pathogenicity.”
Rankin pointed to an analysis that looked at 39 different viral outbreaks in birds from 1959 to 2015, where a low pathogenic avian influenza became a highly pathogenic one. Out of those, 37 were associated with commercial poultry operations. “So it’s a very clear relationship between the increasing pathogenicity of this virus and its relationship with industrial animal raising,” Rankin said.
Some researchers worry that large farms with multiple species are providing the optimal conditions for more species-to-species transfer. In North Carolina, the second-largest hog-producing state after Iowa, some farmers have started raising both chickens and hogs under contracts that require huge numbers of animals.
“So you’ve got co-location at a pretty substantial scale of herd size, on a single property,” said Chris Heaney, an associate professor of environmental health, engineering, epidemiology, and international health at the Bloomberg School of Public Health. “Another concern is seeing it jump into swine. That host, in particular, is uniquely well suited for those influenza viruses to re-assort and acquire properties that are very beneficial for taking up residence in humans.”
In late October, the USDA reported the first case of bird flu in a pig that lived on a small poultry and hog farm in Oregon.
Farmworker advocates say the number of cases in humans is likely underreported, largely because the immigrant and non-English speaking workforce on farms could be reluctant to seek help or may not be informed about taking precautions.
“What we’re dealing with is the lack of information from the top to the workers,” said Ana Schultz, a director with Project Protect Food Systems Workers.
In northern Colorado, home to dozens of large dairies, Schultz started to ask dairy workers in May if they were getting protective gear and whether anyone was falling ill. Many workers told her they were feeling flu-ish but didn’t go to the doctor for fear of losing a day of work or getting fired.
“I feel like there’s a lot more avian flu incidents, but no one knows about it because they don’t go to the doctor and they don’t get tested,” Schultz said. “In all the months that we’ve been doing outreach and taking protective gear and flyers, we haven’t had one single person tell us they’ve been to the doctor.”
Georgina Gustin covers agriculture for Inside Climate News and has reported on the intersections of farming, food systems, and the environment for much of her journalism career. Her work has won numerous awards, including the John B. Oakes Award for Distinguished Environmental Journalism, and she was twice named the Glenn Cunningham Agricultural Journalist of the Year, once with ICN colleagues. She has worked as a reporter for The Day in New London, Conn., the St. Louis Post-Dispatch and CQ Roll Call, and her stories have appeared in The New York Times, Washington Post, and National Geographic’s The Plate, among others. She is a graduate of the Columbia University Graduate School of Journalism and the University of Colorado at Boulder.
So, the ultimate goal of the USDA is to eliminate cattle as a reservoir. When the Agency announced it was planning for this program, it noted that there were two candidate vaccines in trials. Until those are validated, it plans to use the standard playbook for handling emerging infections: contact tracing and isolation. And it has the ability to compel cattle and their owners to be more cooperative than the human population turned out to be.
The five-step plan
The USDA refers to isolation and contact tracing as Stage 3 of a five-stage plan for controlling H5N1 in cattle, with the two earlier stages being the mandatory sampling and testing, meant to be handled on a state-by-state basis. Following the successful containment of the virus in a state, the USDA will move on to batch sampling to ensure each state remains virus-free. This is essential, given that we don’t have a clear picture of how many times the virus has jumped from its normal reservoir in birds into the cattle population.
That makes it possible that reaching Stage 5, which the USDA terms “Demonstrating Freedom from H5 in US Dairy Cattle,” will turn out to be impossible. Dairy cattle are likely to have daily contact with birds, and it may be that the virus will be regularly re-introduced into the population, leaving containment as the only option until the vaccines are ready.
Testing will initially focus primarily on states where cattle-to-human transmission is known to have occurred or the virus is known to be present: California, Colorado, Michigan, Mississippi, Oregon, and Pennsylvania. If you wish to track the progress of the USDA’s efforts, it will be posting weekly updates.
The milk-related risk of H5N1 is only from raw milk; pasteurized milk does not contain live virus and is safe to drink. Pasteurization, which heats milk to a specific temperature for a specified amount of time, kills a variety of bacteria and viruses, including bird flu. Influenza viruses, generally, are considered susceptible to heat treatments because they have an outer layer called an envelope, which can be destabilized by heat. Studies that have specifically looked at the effectiveness of heat-killing treatments against H5N1 have repeatedly found that pasteurization effectively inactivates the virus.
The advent of pasteurization is considered a public health triumph. Its adoption of a safe milk supply contributed to a dramatic reduction in infant deaths in the early 20th century. Before that, milkborne infections—including human and bovine tuberculosis, brucellosis, salmonellosis, streptococcal infections, diphtheria, and “summer diarrhea”—were common killers of infants.
As such, public health officials have long advised people against consuming raw milk, which has no evidence-based health benefits. Raw milk consumption, meanwhile, is linked to higher rates of outbreaks from pathogens including Salmonella, Listeria monocytogenes, toxin-producing E. coli, Brucella, Campylobacter, and many other bacteria.
Risky drinking
Since H5N1 was found spreading among dairy cows in March, health experts have warned about the additional risk of consuming raw milk. Still, consumption of raw milk has continued, and surprisingly increased, as supporters of the dangerous practice have accused health officials of “fearmongering.”
When the retail sampling of Raw Farm’s milk came back positive, the California Department of Food and Agriculture (CDFA) conducted testing at the company’s locations, which were negative for the virus. The CDFA will now begin testing Raw Farm’s milk for bird flu twice a week.
The recalled milk has lot code 20241109 and a “best by” date of November 27, 2024, printed on the packaging.
“Drinking or accidentally inhaling raw milk containing bird flu virus may lead to illness,” California’s public health department said. “In addition, touching your eyes, nose, or mouth with unwashed hands after touching raw milk with bird flu virus may also lead to infection.”
Some US dairy workers who contracted the virus from infected cows reported having had milk splash in their eyes and face. A common symptom of H5N1 infections in humans during the dairy outbreak has been conjunctivitis, aka eye inflammation.
So, in all, Missouri’s case count in the H5N1 outbreak will stay at one for now, and there remains no evidence of human-to-human transmission. Though both the household contact and the index case had evidence of an exposure, their identical blood test results and simultaneous symptom development suggest that they were exposed at the same time by a single source—what that source was, we may never know.
California and Washington
While the virus seems to have hit a dead end in Missouri, it’s still running rampant in California. Since state officials announced the first dairy herd infections at the end of August, the state has now tallied 137 infected herds and at least 13 infected dairy farm workers. California, the country’s largest dairy producer, now has the most herd infections and human cases in the outbreak, which was first confirmed in March.
In the briefing Thursday, officials announced another front in the bird flu fight. A chicken farm in Washington state with about 800,000 birds became infected with a different strain of H5 bird flu than the one circulating among dairy farms. This strain likely came from wild birds. While the chickens on the infected farms were being culled, the virus spread to farmworkers. So far, two workers have been confirmed to be infected, and five others are presumed to be positive.
As of publication time, at least 31 humans have been confirmed infected with H5 bird flu this year.
With the spread of bird flu in dairies and the fall bird migration underway, the virus will continue to have opportunities to jump to mammals and gain access to people. Officials have also expressed anxiety as seasonal flu ramps up, given influenza’s penchant for swapping genetic fragments to generate new viral combinations. The reassortment and exposure to humans increases the risk of the virus adapting to spread from human to human and spark an outbreak.
California’s infections bring the country’s total number of affected herds to 255 in 14 states, according to the USDA.
In a new release Thursday, California health officials worked to ease alarm about the human case, emphasizing that the risk to the general public remains low.
“Ongoing health checks of individuals who interact with potentially infected animals helped us quickly detect and respond to this possible human case. Fortunately, as we’ve seen in other states with human infections, the individual has experienced mild symptoms,” Tomás Aragón, director of California’s Department of Public Health, said. “We want to emphasize that the risk to the general public is low, and people who interact with potentially infected animals should take prevention measures.”
The release noted that in the past four months, the health department has distributed more than 340,000 respirators, 1.3 million gloves, 160,000 goggles and face shields, and 168,000 bouffant caps to farm workers. The state has also received 5,000 doses of seasonal flu vaccine earmarked for farm workers and is working to distribute those vaccines to local health departments.
Still, herd infections and human cases continue to tick up. Influenza researchers and other health experts are anxiously following the unusual dairy outbreak—the first time an avian influenza is known to have spilled over to and caused an outbreak in cattle. The more opportunities the virus has to spread and adapt to mammals, the more chances it could begin spreading among humans, potentially sparking an outbreak or even a pandemic.
Enlarge/ A warning sign outside a laboratory testing the H5N1 bird flu virus at The Pirbright Institute in Woking, UK, on Monday, March 13, 2023.
More than a month after a person in Missouri mysteriously fell ill with H5-type bird flu, investigators in the state are still identifying people who became ill after contact with the patient, raising questions about the diligence of the ongoing health investigation.
On September 6, Missouri’s health department reported the state’s first human case of H5-type bird flu, one that appears closely related to the H5N1 bird flu currently causing a nationwide outbreak among dairy cows. But the infected person had no known contact with infected animals—unlike all of the other 13 human cases identified amid the dairy outbreak this year. Those previous cases have all occurred in dairy- or poultry-farm workers. In fact, Missouri has not reported bird flu in its dairy herds nor recent poultry outbreaks.
Given the unexplained source of infection, health investigators in the state have been working to track the virus both backward in time—to try to identify the source—and forward—to identify any potential onward spread. The bird flu patient was initially hospitalized on August 22 but recovered and had been released by the time the state publicly reported the case.
In an update Friday, September 27, the Centers for Disease Control and Prevention relayed that Missouri officials have now identified four more health care workers who experienced mild respiratory illnesses after caring for the person with bird flu. None of the four workers were tested for flu at the time of their illnesses and all have since recovered.
Testing new cases for antibodies to H5N1
The four newly identified cases bring the total number of health care workers who fell ill after contact to six. Missouri investigators had previously identified two other health care workers who developed mild respiratory symptoms. One of those workers was tested for flu around the time of their illness—and tested negative. But the other, like the four newly identified cases, was not tested. That person has since submitted a blood sample to test for bird flu antibodies, which would indicate a previous infection.
In addition, a household contact of the bird flu patient also fell ill at the same time as the patient, suggesting a possible common source of the infection.
The illnesses are concerning, given the fear that H5N1 bird flu could begin spreading from human to human and spark a widespread outbreak or even a pandemic. However, it can’t be overlooked that a plethora of other respiratory viruses are around—and SARS-CoV-2 transmission was relatively high in Missouri at the time—it’s impossible to draw any conclusions at this point about whether the illnesses were bird flu infections.
But, the illnesses do clearly raise concern about the health investigation, which is being conducted by Missouri officials. “The slow trickle of info is the most concerning part,” infectious disease expert Krutika Kuppalli wrote on social media Friday. The CDC can get involved at the request of a state, but such a request has not been made. For now, the CDC is only providing technical assistance from Atlanta.
In its update today, the CDC emphasized that “to date, only one case of influenza A(H5N1) has been detected in Missouri. No contacts of that case have tested positive for influenza A(H5N1).” The agency added that blood testing results for H5 antibodies are pending.
MDHSS reported that the person, who has underlying medical conditions, was hospitalized on August 22 and tested positive for an influenza A virus. Further testing at the state’s public health laboratory indicated that the influenza A virus was an H5-type bird flu. The Centers for Disease Control and Prevention has now confirmed that finding and is carrying out further testing to determine if it is the H5N1 strain currently causing a widespread outbreak among US dairy cows.
It remains unclear if the person’s bird flu infection was the cause of the hospitalization or if the infection was discovered incidentally. The person has since recovered and was discharged from the hospital. In its announcement, MDHSS said no other information about the patient will be released to protect the person’s privacy.
The report marks the 15th human case of an H5-type bird flu infection in the country since 2022. But, the case stands out—and is quickly generating alarm online—because the man reported no contact with animals. All 14 of the previous cases occurred in farmworkers who had contact with either dairy cows or poultry that were known to be infected with H5N1.
The finding in a person without such an exposure raises the possibility that the H5N1 virus is spreading from person to person, undetected, or is spreading via an undetected animal source.
But, while the case raises concern, some infectious disease experts are cautious not to sound the alarm without more data on the case and potential exposures.
“[U]ntil such data is collected and analyzed, my level of alarm is only mildly heightened,” Caitlin Rivers, a senior scholar at the Johns Hopkins Center for Health Security and founding associate director of the Center for Forecasting and Outbreak Analytics at the CDC, said online.
“I am encouraged that this case was detected through existing surveillance systems, which bodes well for our ability to identify any additional cases in the future,” she added. “Federal, state, and local health officials maintained flu surveillance through the summer months in response to the H5 situation, and that was definitely the right move.”
But Rivers, like many of her colleagues, has long worried about H5N1’s potential to jump to humans and spark a pandemic.
A small study in Texas suggests that human bird flu cases are being missed on dairy farms where the H5N1 virus has taken off in cows, sparking an unprecedented nationwide outbreak.
The finding adds some data to what many experts have suspected amid the outbreak. But the authors of the study, led by researchers at the University of Texas Medical Branch in Galveston, went further, stating bluntly why the US is failing to fully surveil, let alone contain, a virus with pandemic potential.
“Due to fears that research might damage dairy businesses, studies like this one have been few,” the authors write in the topline summary of their study, which was posted online as a pre-print and had not been peer-reviewed.
The study authors, led by Gregory Gray, were invited to two undisclosed dairy farms in Texas that experienced H5N1 outbreaks in their herds starting in early and late March, respectively. The researchers had a previously approved research protocol to study novel respiratory viruses on dairy farms, easing the ability to quickly begin the work.
Rare study
“Farm A” had 7,200 cows and 180 workers. Illnesses began on March 6, and nearly 5 percent of the herd was estimated to be affected during the outbreak. “Farm B” had 8,200 cows and 45 workers. After illnesses began on March 20, an estimated 14 percent of the herd was affected.
The researchers first visited Farm A on April 3 and Farm B on April 4, collecting swabs and samples at each. Based on the previously approved protocol, they were limited to taking nasal swabs and blood samples from no more than 10 workers per farm. On Farm A, 10 workers provided nasal swabs and blood samples. On Farm B, only seven agreed to give nasal swabs, and four gave blood samples.
While swabs from cows, milk, a dead bird, and a sample of fecal slurry showed signs of H5N1, all of the nasal swabs from the 14 humans were negative. However, when researchers looked for H5N1-targeting antibodies in their blood—an indicator that they were previously infected—two of the 14, about 14 percent, were positive.
Both of the workers with previous infections, a man and a woman, were from Farm A. And both reported having flu-like symptoms. The man worked inside cattle corrals, close to the animals, and he reported having a cough at the time the samples were taken. The woman, meanwhile, worked in the cafeteria on the farm and reported recently recovering from an illness that included fever, cough, and sore throat. She noted that other people on the farm had similar respiratory illnesses around when she did.
The finding suggests human cases of H5N1 are going undetected. Moreover, managing to find evidence of two undetected infections in a sample of just 14 workers suggests it may not be hard to find more. The Centers for Disease Control and Prevention estimates that around 200,000 people work with livestock in the US.
Known infections in humans have all been mild so far. But experts are anxious that with each new infection, the wily H5N1 virus is getting new opportunities to adapt further to humans. If the virus evolves to cause more severe disease and spread from human to human, it could spark another pandemic.
Federal officials are also worried about this potential threat. In a press briefing Tuesday, Nirav Shah, the CDC’s principal deputy director, announced a $5 million effort to vaccinate farm workers—but against seasonal flu.
Shah explained that the CDC is concerned that if farm workers are infected with H5N1 and the seasonal flu at the same time, the viruses could exchange genetic segments—a process called reassortment. This could give rise to the pandemic threat experts are worried about. By vaccinating the workers against the seasonal flu, it could potentially prevent the viruses from comingling in one person, Shah suggested.
The US does have a bird flu-specific vaccine available. But in the briefing, Shah said that the use of that vaccine in farm workers is not planned for now, though there’s still active discussion on the possibility. The lack of severe disease and no documented human-to-human transmission from H5N1 infections both argue against deploying a new vaccine, Shah said. “There has to be a strong and compelling case,” he added. Shah also suggested that the agency expects vaccine uptake to be low among farm workers.
The highly pathogenic avian influenza H5N1 virus that spilled from wild birds into US dairy cows late last year may have recently seeped from a dairy farm in Colorado to a nearby poultry farm, where it then infected five workers tasked with culling the infected chickens
In a press briefing Tuesday, federal officials reported that four of the avian influenza cases have been confirmed by the Centers for Disease Control and Prevention, while the fifth remains a presumptive positive awaiting CDC confirmation.
All five people have shown mild illnesses, though they experienced variable symptoms. Some of the cases involved conjunctivitis, as was seen in other human cases linked to the H5N1 outbreak in dairy cows. Others in the cluster of five had respiratory and typical flu-like symptoms, including fever, chills, sore throat, runny nose, and cough. None of the five cases required hospitalization.
The virus infecting the five people is closely related to the virus infecting the chickens on the poultry farm, which, in turn, is closely related to virus seen in infected dairy herds and in other human cases that have been linked to the dairy outbreak. The affected poultry farm is in Colorado’s northern county of Weld, which has also reported about two dozen outbreaks of avian influenza in dairy herds.
Dairy to poultry hypothesis
In one fell swoop, Colorado’s poultry farm outbreak has more than doubled the number of human avian influenza cases linked to the dairy cow spillover, bringing the previous tally of four cases to nine. While officials have previously noted instances where it appeared that H5N1 on dairy farms had moved to nearby poultry farms, this appears to be the first time such spread has led to documented human infections.
The link between the poultry farm cases and neighboring dairy farms is still just a hypothesis, however, Nirav Shah, the principal deputy director at the CDC, emphasized to reporters Tuesday. “It is a hypothesis that needs and requires a full investigation. But that is a hypothesis at this point,” he said of the link between the dairy farms and the poultry farm. So far, there is no direct evidence of a specific source of the poultry farm’s infection, and the route of infection is also unclear.
Throughout the outbreak of H5N1 on dairy farms, officials have noted that the primary way the virus appears to spread to new farms is via the movement of cows, people, and machinery between those facilities. There remains no evidence of human-to-human transmission. But milk from infected cows has been found to be brimming with high levels of infectious virus, and milk-contaminated equipment is a prime suspect in the spread.
In the press briefing Tuesday, Eric Deeble, acting senior advisor for H5N1 response with the US Department of Agriculture, noted the poultry are very susceptible to avian influenza and are easily infected. “It does not take much to introduce this into a flock,” Deeble said. The USDA is now working on a “trace-back” investigation on how the Colorado poultry farm was infected.
Searing spread
As for how the farm workers specifically became infected with the virus, health officials pointed to high temperatures that prevented workers from donning protective gear. The poultry farm is a commercial egg layer operation with around 1.8 million birds. Given the presence of bird flu on the premises, all 1.8 million birds need to be culled, aka “depopulated.” This is being carried out using mobile carts with carbon dioxide gas chambers, a common culling method. Workers are tasked with placing the birds in the chambers, which only hold a few dozen birds at a time. In all, the method requires workers to have a high degree of contact with the infected birds, going from bird to bird and batch to batch with the carts.
Amid this grim task, temperatures in the area reached over 100° Fahrenheit, and massive industrial fans were turned on in the facility to try to cool things down. Between the heat and the fans, the approximately 160 people involved in the culling struggled to use personal protective equipment (PPE). The required PPE for the depopulation involves a full Tyvek suit, boots, gloves, goggles, and an N95 respirator.
“The difficulty with wearing all that gear in that kind of heat, you can imagine,” said Julie Gauthier, executive director for field operations at the USDA’s Animal and Plant Health Inspection Service (APHIS). The industrial fans blowing large amounts of air made it yet more difficult for workers to keep goggles and respirators on their faces, she said.
The CDC and the USDA are both involved in further investigations of the poultry farm outbreak. CDC’s Shah noted that the team the agency deployed to Colorado included an industrial hygienist, who can work on strategies to prevent further transmission.
To date, at least 161 herds in 13 states have tested positive for avian influenza since the dairy outbreak was confirmed in March. Since January 2022, when US birds first tested positive for the H5N1 virus, 99 million birds in the US have been affected in 48 states, which involved 1,165 individual outbreaks.
