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What to know about the risks of the bird flu outbreak

Richard Webby, a virologist at St Jude Children’s Research Hospital, expresses uncertainty regarding the situation, stating, “There are many unknown factors at this time.” Questions arise about the prevalence of the virus in dairy cattle and its potential implications for humans. However, clarity on these matters remains elusive. The emergence of the H5N1 bird flu strain was first observed in North America among wild migratory birds toward the end of 2021, subsequently spreading to poultry farms. Presently, cases are surfacing among dairy cows and a major egg producer, with one individual contracting the virus after close contact with cows.

Webby notes that the current variant of the H5N1 virus is challenging existing understandings about influenza. Unlike previous outbreaks, this one has affected a broader range of wild bird species and has endured for a longer duration. Additionally, the virus has been observed more frequently in mammals, both in natural habitats and on farms, sometimes resulting in widespread infections and fatalities.

Dr. Peter Rabinowitz, director of the UW Center for One Health Research, remarks that the global situation concerning avian influenza is unprecedented and uncharted.

While the recent bird flu outbreak at the largest U.S. chicken egg producer raises concerns about potential impacts on egg prices, federal officials and scientists emphasize that the risk to the public remains low. As of now, the virus has not undergone significant mutations that would make it substantially more dangerous. The one reported human case appears to be consistent with typical transmission patterns, occurring through direct exposure to an infected animal.

However, scientists continue to monitor the outbreak closely. Genetic sequencing of the virus collected from infected cattle has not revealed any major changes that would explain the infections. Similarly, sequencing of the virus from the Texas patient showed minor alterations, including one mutation associated with viral adaptation to mammals, consistent with previous human cases.

However, there is no evidence from previous infections to suggest that this mutation increases the likelihood of the virus spreading among humans.

Angie Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, explains that although this mutation occurred when the virus transmitted from a cow to a person, there is nothing particularly concerning about it.

Rasmussen emphasizes that there is no indication that the virus has adapted to efficiently spread among humans or routinely cause severe disease.

However, she warns that the recent human case and infections in dairy cattle serve as clear reminders: “The less transmission between humans or between cows and humans, the fewer opportunities the virus has to acquire these mutations.”

Human-to-human transmission of bird flu is exceedingly rare, and it is uncommon for people to contract any form of bird flu. During the current outbreak, this strain of H5N1 has only been identified in a small number of humans worldwide over the past few years, with no documented cases of human-to-human transmission.

In the Texas case, the individual’s only symptom was redness in the eye after contact with cattle. This marks the second known case of H5N1 infection in a human in the United States. In 2022, a poultry worker in Colorado developed a mild illness after exposure to sick chickens.

While some recent human infections in other countries, such as Ecuador, Chile, and China, have resulted in severe illness, Rasmussen notes that this virus typically does not infect humans efficiently but can occasionally cause significant disease.

Historically, human infections with avian influenza have often been linked to close contact with birds, particularly in markets or on farms. Rasmussen explains that individuals exposed to bird feces, dead birds, or large numbers of live birds are at higher risk of exposure.

Unlike seasonal influenza viruses that commonly infect humans, H5N1 does not easily target the upper respiratory tract, making human-to-human transmission rare.

However, the virus can bind to receptors in the lower respiratory tract, potentially leading to severe pneumonia in individuals who contract respiratory infections from bird flu. Angie Rasmussen explains that because these receptors are located deep in the lungs, those infected with bird flu can experience severe illness.

Scientists remain vigilant for any signs that the virus may adapt to better target the upper respiratory tract. Additionally, the recent case in Texas raises concerns about “mucosal exposure,” where individuals may have touched their eyes after coming into contact with the virus, though the implications for transmission are unclear.

Regarding transmission among dairy cattle, researchers are investigating whether significant spread occurs between animals. Cases have been reported in herds across several states, with evidence suggesting that infected wild birds may have initially introduced the virus. However, there are indications of mammal-to-mammal spread.

Joe Armstrong, a veterinarian at the University of Minnesota, has been monitoring reports of a mysterious illness affecting dairy cattle in various states since early February. While the exact extent of avian influenza’s involvement in these cases remains uncertain, Armstrong suspects that the virus is spreading among cows, based on reports from farmers.

Armstrong highlights the difficulty in completely segregating livestock and wildlife, especially when wild birds and other wildlife are involved. Despite cows falling ill, Armstrong notes that the illness is not proving fatal. Federal officials reassure the public that the commercial milk supply remains unaffected because dairy products are pasteurized, mitigating any risk.

The concern among scientists revolves around the potential sustained transmission of avian influenza among mammals. The exact mechanism of transmission between mammals and the extent to which infections occur after contact with infected birds remain unclear. However, scientists worry that prolonged mammal-to-mammal transmission provides the virus with more opportunities to adapt and acquire mutations that could enhance its ability to infect mammals.

Notable instances of avian influenza outbreaks in marine mammals in South America and on a mink farm in Spain have raised concerns. In these cases, the virus had developed specific mutations adapted to mammals, which have not yet been observed in cows. Louise Moncla, a virologist at the University of Pennsylvania, highlights the unusual nature of the situation, given that cows are typically infected by a different influenza strain. Unlike pigs, which are known to serve as intermediary hosts for both human and bird viruses, there is no evidence suggesting that cows play a significant role as intermediary hosts for these viruses.

Amid the ongoing outbreak in livestock, there is a potential risk of exposure to other animals or workers themselves. Peter Rabinowitz notes the importance of paying attention to these workers, as they often serve as early indicators of transmission events. In case of human spread, there is the possibility of tapping into existing bird flu vaccines and adapting them accordingly.

Federal health officials emphasize their serious approach to the situation surrounding avian flu outbreaks in the United States.

Dr. Mandy Cohen, director of the Centers for Disease Control and Prevention (CDC), underscores the nation’s extensive preparation for avian flu outbreaks over the past two decades. She compares this to the initial stages of the COVID-19 pandemic, highlighting the lack of tests, treatments, and vaccines for the novel virus.

The United States maintains a limited supply of vaccines developed for early strains of H5N1, which could be utilized in the event of human-to-human transmission. These older vaccines can be enhanced with immune-stimulating ingredients called adjuvants to broaden the immune response, covering mismatched strains more effectively. Additionally, Dr. Wilbur Chen from the University of Maryland’s School of Medicine suggests that mRNA technology could facilitate the production of new vaccines.

Dr. Chen mentions that part of the ongoing preparation may involve manufacturing limited quantities of vaccines in anticipation of potential human cases.

Dr. Ashish Jha, dean of Brown University’s school of public health, anticipates the possibility of additional human cases, primarily among farm workers. However, he advises against mobilizing a larger pandemic response, such as mass vaccine production, at this stage. Jha explains that the likelihood of such a response being necessary is currently very low. He suggests considering broader vaccination efforts only if there is evidence of widespread transmission among non-farm workers.

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