In recent weeks, a highly pathogenic strain of the bird flu virus has killed tens of thousands of birds, and, alarmingly, has begun to spread among mammals. Yesterday, the CDC reported that a dairy worker in Texas was infected, reviving longstanding fears that the disease could mutate in ways that allow it to spread widely among humans.
Bird flu is an influenza virus, also known as H5N1, that primarily infects birds. It was first identified in the mid-1990s but rose to prominence around 2003, with a large outbreak in poultry in China, said Dr. Louise Ivers, executive director of the Massachusetts General Hospital Center for Global Health and professor of medicine at Harvard Medical School.
The virus does not usually infect humans. Between 2022 and 2023, the CDC closely followed more than 6,000 people who had been exposed to the virus in the United States, and only one became infected. Between January 2003 and February 2024, there were 887 laboratory-confirmed cases in people in 23 countries, according to the World Health Organization. But mortality among those infected in some previous outbreaks has been extremely high: of the 887 people infected around the world, 462 died.
The two people infected in the United States with the current strain have had mild symptoms, but it’s too soon to draw conclusions about how harmful the new strain is to humans, said Dr. Jennifer Nuzzo, professor of epidemiology and director of the Pandemic Center at Brown University School of Public Health.
Were it to spread widely in people, infectious disease experts worry it could prove catastrophic.
Experts say the current threat to public health remains low. To date, there have been no confirmed cases of human-to-human transmission.
But influenza viruses are known for their ability to mutate, or change, in ways that could help them spread more easily among humans. Flu viruses are also difficult to contain, because they are contagious at the earliest stages of illness and initial symptoms are usually non-specific – they can be confused with allergies or a cold.
The widespread geographic distribution of the current outbreak, coupled with the fact that the virus is infecting a growing number of mammals, vastly increases the opportunity for it to change. Last year, tens of thousands of seals and sea lions died off the coast of South America. Last month, a sick goat in Minnesota became the first US livestock to contract H5N1, and last week, the USDA announced that cows had been infected for the first time on farms in Texas and Kansas.
“Every time it makes a copy of itself, there’s the potential that a mutation could occur that gives it enhanced abilities to infect —it’s kind of like just rolling the dice each time,” said Nuzzo.
The current outbreak, and the rapidity with which it now seems to be spreading among cows, is a testament to the ability of the virus to shape-shift.
It seems to be infecting all sorts of animals that we didn’t previously see it infecting,” Nuzzo said. “I mean, it was called bird flu because it was predominantly affecting birds, but that’s no longer the case. The most recent development of seeing it in cows is quite alarming just because it shows that the virus is doing things that we didn’t expect it to be able to do.”
This highly pathogenic strain of H5N1 has been devastating wild flocks of birds in New England since early 2022. After waxing and waning over the previous two years, it surged in Massachusetts in January. In recent months, scores of sea ducks, loons, and seagulls have littered the beaches along the North Shore, washing up in Nahant Beach, Salisbury, Newbury, Newburyport, Plum Island, and Manchester-by-the-Sea.
In the summer of 2022, H5N1 killed more than 180 gray and harbor seals off the coast of Maine. Local scientists are tracking it closely.
Scientists have been tracking various versions of bird flu for decades – all the while speculating and warning this could be the “next” big pandemic. So far, that has not happened. “This isn’t currently a threat to humans,” Nuzzo says. “I just want to make that clear. I don’t think people are going to get sick from this virus. I want to make sure that we don’t give the virus the ability to make people sick. So we need to act swiftly to prevent this virus from getting entrenched in more species.”
Nuzzo notes that, though alarming, it’s “not entirely surprising,” that a dairy worker was infected since humans are around sick cows. It’s reassuring that the person is experiencing mild symptoms, which are largely confined to eye inflammation.
It remains unclear whether the source of the transmission was the cow itself, or whether the human and the cow contracted it from the same source, possibly a bird, she notes.
The good news is that we know a lot more about this virus than we did about COVID-19 when it emerged. Existing antiviral medications are effective against most forms of the virus. (The dairy worker infected in Texas is reportedly taking Tamiflu).
“The scope of its geographic spread and its shift now to mammals means that there’s just more of it circulating,” says Ivers. “The more of it’s circulating, the more opportunity it has to combine with other types of flu viruses to adapt to its environment and to potentially become a form that will have more ability to transmit from human to human. That’s the reason for concern. But right now we have not seen human-to-human transmission.”
The 20th century saw devastating pandemics in 1918, which killed 50 million people. Less-deadly pandemics followed in 1957 and 1968. “On average, we have three flu pandemics a century, which doesn’t make our odds look very good in terms of escaping another pandemic,” Nuzzo says.
