With concern rising about bird flu, there’s a basic question: How does it make the jump from birds to mammals? And how worrisome is that for humans?
Avian influenza, whose scientific name is A(H5N1), can be very deadly in birds, while in mammals it can cause illness with varying degrees, from mild to severe and even deadly. The A in its designation says it’s Type A flu.
Right now, public health experts at the Centers for Disease Control and Prevention are still calling the risk of bird flu to humans “low.” But the agency notes that it continues to monitor bird flu carefully.
“The wide geographic spread of A(H5N1) bird flu viruses in wild birds, poultry and some other mammals, including in cows, is creating additional opportunities for people to be exposed to these viruses. Therefore, there could be an increase in sporadic human infections resulting from bird, cattle and other animal exposures, even if the risk of these viruses spreading to people has not increased. Sporadic human infections in the current context would not significantly change CDC’s risk assessment,” the agency reported.
CDC added that “identification of multiple simultaneous instances of A(H5N1) bird flu viruses spreading from birds, cattle or other animals to people or certain genetic changes in circulating viruses could also change CDC’s risk assessment because they could indicate the virus is adapting to spread more easily from animals to people.”
Person-to-person spread would also be very alarming, the agency added, noting that to become a pandemic, “sustained person-to-person spread is needed.”
Avian influenza in the United States has been detected in big cats, mountain lions, bobcats, brown bears, black bears and polar bears, bottlenose dolphins, grey seals, harbor seals, red foxes, coyotes, fishers, American martens, North American river otters, raccoons, skunks, Virginia possums and Abert’s squirrels. There have been outbreaks in cattle and in mink. But most of the wild mammal infections are considered “dead end,” meaning a mammal ate an infected bird, got infected and died without contributing to spread of the virus.
In Utah, a mountain lion tested positive for bird flu in February 2023 and another in March 2023, both in Salt Lake County, according to the Animal and Plant Health Inspection Service in the U.S. Department of Agriculture. A red fox tested positive in May 2022 in Salt Lake County, as well. In nearby states, a couple of skunks have tested positive, as well as a red fox in Idaho. Avian influenza has been found a few times in Colorado wild mammals, including in skunks, mountain lions, bobcats, red fox and at least one black bear. The USDA map doesn’t say how many animal infections were detected.
A(H5N1), the form of avian flu virus now circulating, has killed “tens of thousands of marine mammals and infiltrated American livestock for the first time,” The New York Times reported. What isn’t clear is how great the risk to humans the bird flu will prove to be.
The story could change as — or if — the virus does.
In the U.S., more than 90 million birds have been killed to try to thwart the virus’s spread.
Deadly toll on birds, marine mammals
The Times article tracks the spread, noting bird flu first reached North America in 2021, detected on a farm in St. John’s Newfoundland. The birds that didn’t die from the virus were culled. The following January, bird flu was found in wild birds in the Carolinas. By summer, hundreds of seals were dying along the Maine Coast, possibly infected from eating sick and dead birds. By fall 2022, it was spreading throughout the continent on its way south to Antarctica.
Notes the Times, “The blow to sea mammals, and to dairy and poultry industries, is worrying enough. But a bigger concern, experts said, is what these developments portend: The virus is adapting to mammals, edging close to spreading to people.”
Experts are in no way predicting a pandemic. But they are paying close attention.
Worldwide, 800 people — including two in the United States — have contracted bird flu over the last nearly 30 years. Most often, the infections have been mild.
How the illness jumped species isn’t entirely clear, though the working theory is the sea mammals contracted the virus through close contact with infected birds’ droppings or because they ate some infected birds. But at some point, it’s likely the virus mutated enough to spread more directly.
“Genetic analysis supports the theory that marine mammals acquired their infections from one another, not birds. Samples of virus isolated from sea lions in Peru and Chile and from the elephant seals in Argentina all share about 15 mutations not seen in the birds; the same mutations were also present in a Chilean man who was infected last year,” the Times reported.
What about humans?
The first detection in mammals in the United States occurred in May 2022, when sporadic infections were found among wildlife in several states.
In November and December 2022, bears in Alaska, Nebraska and Montana were found to be carrying the H5N1 virus.
This year, the young goats were found to have the virus for the first time. They lived on a farm where a poultry flock had tested positive. Then the virus was found in dairy cows in Kansas and Texas. As of this week, cattle in eight states have experienced outbreaks. And the CDC says a number of barn cats are also infected.
The toll on other mammals has thus far been worse. A mink farm in Spain had a major outbreak in October 2022, with some suspected mink-to-mink infection that didn’t involve infected wild waterfowl. There have been other outbreaks of Influenza A in mink, which are susceptible.
Experts believe that previous exposure to different strains of flu will provide some protection if the virus mutates in a way that lets it infect a lot of humans.
The Centers for Disease Control and Prevention has a “highlights” timeline of how the virus has evolved since 2020. That year, it said, gene swapping between poultry and wild bird viruses “led to the emergence” of the bird flu strain known as A(H5N1). There are other strains. That year, five human cases in China were identified among people who had very close contact with birds.
Cases in humans have been found sporadically, though not often. In April 2022, the first infection was reported in the U.S., “though this detection may have been the result of contamination of the nasal passages with the virus rather than an actual infection,” the CDC said. Over the next few months, cases in humans were reported in China, mostly mild, though one individual died. Another human case was recently reported in Texas, the symptoms limited to conjunctivitis, which is red, itchy eyes.
That was worrisome because it is the “first reported cow-to-human spread of H5N1 bird flu,” per CDC.
A cautionary note
People can help stop the spread by taking some precautions. And they can protect themselves at the same time. Per the CDC, “Avoid contact with surfaces that appear to be contaminated with animal feces, raw milk, litter or materials contaminated by birds or other animals with suspected or confirmed avian influenza virus infection.” CDC has information about precautions to take with wild birds, poultry and other animals.
Again, CDC notes that spread from one infected person to someone close to them is “very rare.” Such infection has never been marked by widespread infections. “However, because of the possibility that bird flu viruses could change and gain the ability to spread easily between people, monitoring for human infection and person-to-person spread is extremely important for public health.”