"There's a lot we don't know": Why public health experts are worried about bird flu

The Centers for Disease Control and Prevention (CDC) issued a health alert last week about a confirmed case of H5N1 in a human.

In late March, a worker on a commercial dairy farm in Texas developed a case of pink eye and later tested positive for the highly pathogenic avian influenza Type A H5N1, also known as "bird flu" or avian flu.

Though the farm worker had no symptoms other than eye inflammation and wasn't hospitalized, they were isolated and received an antiviral treatment, according to the CDC. The public health agency suspects the patient got infected from a sick dairy cow.

While bird flu is nothing new, this is the first time officials have confirmed that the virus has jumped from a cow to a human, and the line of transmission suggests it's easily transmitted between cows. The last time a human tested positive for H5N1 was in April 2022 in Colorado, when an individual got infected from poultry.

The fact that it jumped from cattle to human is unsettling, experts say, and a cause for alarm. But not necessarily because H5N1 will turn into a pandemic this year.

"There's a lot we don't know," virologist Dr. Angela Rasmussen told Salon. "We don't know essentially what kind of risk the cows themselves present."

It seems "clear" that cows can present a risk to each other, Rasmussen elaborated, but what kind of a risk this poses to cow health isn't completely understood. And neither is what kind of a risk this poses to human health.

In birds, HPAI is highly contagious and deadly. The big concern is that the more it jumps from animal to animal, or animal to human, the more likely it is to mutate to better infect humans. Of course, RNA viruses like influenza and SARS-CoV-2, which causes COVID-19, are always mutating. Every replication in a hosts' cells presents a chance for a mutation to emerge. While viruses technically aren't alive, it's their nature to mutate and evolve as they infect hosts' cells and replicate. This is how they reproduce. Thus, understanding how H5N1 in mutating is critical in assessing the potential risk to both animal and human health and how close the U.S. could be to a bird flu pandemic.

Fortunately, bird flu doesn't appear to be jumping from human to human, public health officials say. According to the CDC, the current risk these viruses pose to the public remains low. In fact, the CDC sequenced the genome and compared it to other sequences from cattle, wild birds and poultry. It found no major changes, except one: The human sample had a mutation called PB2 E67K, which is known to connect virus adaptation to mammalian hosts. It has been seen before in people infected with H5N1.

"It is the enzyme that basically copies the virus when it's replicating and that has been associated with mammalian adaptation, but it's only one out of a collection of these mutations that are associated with mammalian adaptation," Rasmussen said. "So that kind of suggests that when the virus was transmitted from the cow to the person it picked up this mutation."

However, Rasmussen emphasized that this "doesn't mean that the virus is better adapted to transmitting between mammals," which is good news. "It just means — as we would expect — it is in a second mammalian host species," she said. "And so it's picking up adaptations that we've already seen that are associated with mammalian adaptation."

Dr. Rajendram Rajnarayanan of the New York Institute of Technology campus in Jonesboro, Ark., also believes the risk for human-to-human transmission is low. However, he's concerned about the virus transmitting to other mammals on a farm, such as pigs.

Pigs, Rajnarayanan said, can get infected with multiple viruses at a time. This could make it easier for the virus to mutate into a new one that could more easily jump from mammal to mammal, like say, from one human to another.

"It needs a mixing pool, like pigs," he said, "to efficiently infect the human airways and then invade different parts of the body and then cause more serious damage."

In the U.S., there haven't been any recent known cases of human-to-human transmission. While they have occurred in other countries, these cases didn't spread beyond close contacts and were contained before becoming an epidemic. Certainly, the consensus among scientists is that the latest case of bird flu isn't a panic-inducing threat to human health right now. It could be in the future, especially if it eventually infects a host like pigs.

"The likelihood of bird flu becoming the next pandemic in the next year is probably pretty low," Dr. Linda Yancey, infectious disease specialist at Memorial Hermann Health System in Houston, told Salon. "The likelihood of bird flu becoming a pandemic in the next century is probably pretty good."

However, this doesn't mean that individuals shouldn't change their behavior. In light of news of the infection, people should refrain from drinking unpasteurized milk.

"It's super important to make sure you're not drinking unpasteurized milk," Yancey said. "Our milk supply is extremely safe. We pasteurize it, but occasionally you'll get people who, for whatever reason, drink unpasteurized milk."

Indeed, one concern top of mind for experts is the potential impact on the milk supply chain. Rasmussen is also worried about the potential economic impact.

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"We don't even really appreciate the full extent of how many cows might be affected," Rasmussen said. "It looks like this is associated with dairy cattle, rather than beef cattle, but there is some crosstalk between those two industries."

This is normally a time when Rasmussen would tell the public to get their vaccines. While there are avian flu vaccines under development, none have been approved for use in humans yet. Notably, according to the CDC's health alert, in the strain identified in the Texas case, there is no indication that bird flu has developed antiviral drug resistance, meaning that even without vaccines certain drugs could possibly work against it.

Still, the CDC is asking healthcare workers across the country to look out for signs of bird flu and consider it as a possibility when patients have been exposed to sick or dead birds, livestock or other animals a week before an onset of symptoms. Humans are also advised to stay away from sick or dead animals.

"This has actually been a crisis in the animal population globally for the last three years," Rasmussen said. "If people want to translate their concern into action, I would strongly suggest advocating for taking more steps and putting more of an investment into pandemic preparation."