“Am I going to be the next case of bird-to-mammal spillover? Hill, an ecologist who studies how diseases spread among animals, remembers his thoughts. “The chances are low, but your imagination will take you there.” It’s not out of the question.
Hill knows this better than anyone. She is part of a small army of researchers working to collect crucial data amid an unprecedented outbreak of H5N1, a highly contagious form of avian flu, to find answers to questions that many between us would prefer not to consider. Among them: If bird flu can rage uncontrolled across the world and adapt to infect cats, skunks, cows and seals, if it can get into our milk supply, then why are human infections so rare? And what would it take for that to change?
Answering these questions has become more urgent in recent weeks. Since arriving on the Atlantic coast via migratory birds in late 2021, the virus has killed tens of thousands of wild birds and hundreds of seals. The scale of the pandemic on Massachusetts beaches, Hill said, is far worse than it has ever been since its arrival. In April, scientists discovered it had spread among cows and entered the country’s milk supply. Frighteningly, scientists at the University of Arizona recently said that the spread of the virus to cows likely occurred in December and that the virus has since been spreading quietly and uncontrolled on livestock farms across the country. country.
Public health officials have downplayed the immediate threat to human health, noting that human infections are rare and that viral fragments found in milk were likely inactivated by pasteurization. The Centers for Disease Control and Prevention closely tracked more than 6,000 people who were exposed to the virus between 2022 and 2023; only one was infected.
But some local epidemiologists are sounding the alarm. Dr. Nahid Bhadelia, founding director of the Boston University Center for Emerging Infectious Diseases, is part of a growing group of disease experts who warn that the federal government is moving too slowly. As of last week, only 25 people nationwide had been tested for the virus, federal health officials said at a press briefing — an effort Bhadelia says is far too anemic given the changing superpowers of the virus and the consequences of its adaptation. to human-to-human transmission.
“We have a small window and we are not taking advantage of it by increasing our testing to get a true understanding of the extent of this virus,” Bhadelia said. “We need to understand how it is transmitted, how it evolves, and stop transmission so that we don’t get to a position where we have to think about how we can make a broader response to this in humans. We are not doing enough.
The growing list of mammals known to have contracted the virus, she said, should serve as a warning.
Since 1996, when the H5N1 virus was first identified in waterfowl in China, the virus has spread to the Middle East, Africa, Europe and the United States and has infected more than 48 mammal species, including tigers, leopards, domestic cats and dogs, mink and pigs. , seals, donkeys, horses and cows.
Although human cases are extremely rare, they can be fatal: of 887 confirmed human cases since 1996, more than half of the people – 463 – died. To date, no human-to-human transmission has been observed, a viral characteristic that would be necessary to trigger a global human pandemic. But the apparent infection of a Texas farmworker through contact with a cow last month — the first known case of a mammal infecting a human — brings us even closer.
Infected birds are known to have the virus in their saliva and droppings, and most previous cases of human infection are believed to have occurred when individuals contracted the virus on their hands and then touched their eyes, nose, or mouth. . The virus can also be inhaled when airborne.
The good news is that scientists such as Hill and his collaborators Wendy Puryear and Jonathan Runstadler, virologists at the Cummings School of Veterinary Medicine at Tufts University, have developed and refined powerful tools to foster collaboration and avert calamity .
To better understand the phenomenon of spread, document the evolution of the virus and track its global progression, they have spent the last decade compiling vast libraries of genetic samples from dead and infected animals, uploading them into databases shared. The most widely used database, GISAID, contains more than 188,333 genetic sequences associated with more than 31,095 individual viral samples.
“We monitor these sequences for known specific mutations that are linked to adaptation or a high probability of spillover in mammals,” Hill said.
When new mutations are discovered, Hill said, scientists in labs around the world inject the sequences into mice, ferrets, ducks and other lab animals and document the impact. Are viruses more deadly? Does the mutation shift the impact from gastrointestinal transmission to respiratory transmission? Scientists have not determined how the virus spreads among cows, although some believe milking machines “aerosolize” the virus, allowing it to spread through droplets in the air.
Andy Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health who studies influenza, said he and others are closely monitoring mutations in key areas necessary for their spread in humans.
First, H5N1 would likely need to better bind to receptors on the surface of human cells, which differ from those found in birds, in order to inject its DNA into human cells.
Second, the avian virus, in its current form, has difficulty commandeering the cellular machinery necessary for its reproduction once it enters human cells.
Body temperature is another important barrier. The birds, Pekosz notes, “run hot,” with an average body temperature of 102 to 104 degrees Fahrenheit. The upper respiratory tract of humans is around 91.5 degrees Fahrenheit. This small difference can have a big impact on the virus’s ability to replicate.
“We have evidence from many different mammalian infections that each of these three things can change,” if the correct mutations occur in the viral DNA, Pekosz said. “Luckily, we haven’t seen all three of those things change at once.”
BU’s Bhadelia worries federal officials aren’t doing enough to prevent a mutation from taking hold in humans. One reason the spillover is less on humans than on other mammal species, she suggests, may be that other mammals are much more likely to come into contact with infected birds than are birds. humans. Now that the virus appears endemic on American farms, that likely won’t be the case.
Bhadelia said widespread testing is needed both for farm animals and for agricultural and dairy workers. Others suggested that farmworkers be equipped with goggles, masks and other protective gear.
The USDA, Bhadelia notes, is trying to strike a delicate balance between protecting the agricultural sector and maintaining trust with farm owners to ensure access to continued monitoring and the broader threat to public health. But this caution could come at a cost. Each new infection in a human host represents an opportunity for the virus to potentially acquire new mutations that better optimize its survival in its new home.
“If I was a gambler,” she said. “I suspect there are already other farm workers who may have been infected.”
Globe correspondent Alex Viveros contributed to this report
Adam Piore can be contacted at adam.piore@globe.com.
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