At least one person in Texas has been diagnosed with bird flu after coming into contact with dairy cows that are believed to be infected, state officials said on Monday.
The announcement adds a worrying dimension to an outbreak that has affected millions of birds and sea mammals around the world and, more recently, cattle in the United States.
So far, there are no signs that the virus has evolved in ways that would help it spread more easily among people, federal officials said.
The patient had worked directly with sick dairy cows, said Lara M. Anton, a spokeswoman for the Texas Department of State Health Services. “We tested about a dozen people with symptoms who worked in dairies, and only one person tested positive” for the virus, he said in an email.
The patient’s main symptom is conjunctivitis; the individual was treated with an antiviral drug and is recovering, according to the Centers for Disease Control and Prevention.
Announced by the Department of Agriculture the first cases in dairy in Texas and Kansas last week, and after a few days, to an additional herd in Michigan. Preliminary testing suggests that cattle in New Mexico and Idaho may also be infected.
The virus was identified as the same version of H5N1, a subtype of influenza, that spreads among birds in North America.
The CDC is working with state health departments to track other people who may have come into contact with infected birds and animals, the agency said Monday. It also urged people to avoid exposure to sick or dead birds and animals, and to raw milk, feces or other potentially contaminated materials.
This is only the second case of H5N1 bird flu in humans in the United States; The first in 2022. The risk to the general public remains low, experts said. But testing and analysis continues, and many questions remain unanswered.
“This is a rapidly evolving situation,” the USDA said in its announcement last week.
Here’s what to know:
What is bird flu?
Bird flu, or avian influenza, is a group of influenza viruses that are primarily adapted to birds. The particular virus in these new cases, called H5N1, was first identified in 1996 in geese in China, and in people in Hong Kong in 1997.
In 2020, a new, highly pathogenic form of H5N1 emerged in Europe and spread rapidly around the world. In the United States, it was affected more than 82 million farmed birdsthe worst bird flu outbreak in US history.
Since the virus was first identified, sporadically Cases have been found in people in other countries. But most resulted from prolonged, direct contact with birds.
H5N1 does not seem to have adapted to spread efficiently among humans, experts say.
How did cows get bird flu?
Cattle are not thought to be a high-risk species.
“The fact that they’re susceptible — the virus can replicate, can make them sick — that’s something I can’t predict,” said Richard Webby, an influenza virologist in St. Jude Children’s Research Hospital.
But this year, reports of sick cows began to emerge in Texas and New Mexico. Dead birds were also found on some of these farms, and laboratory testing confirmed that some cattle were infected with bird flu.
There are various ways in which the virus may have entered the cattle. The most likely route, some experts said, is that infected wild birds, which shed the virus in their feces, saliva and other secretions, contaminated the cattle’s food or water.
But other free-ranging animals known to be susceptible to the virus, such as cats and raccoons, may have also brought the virus to dairy farms.
How are the cows affected?
Although the virus is usually fatal in birds, it appears to cause relatively mild disease in cattle.
“It didn’t kill the animals, and they seemed to recover,” said Dr. Joe Armstrong, a veterinarian and cattle production expert at the University of Minnesota Extension. Last week, the USDA said that no plans to “depopulate,” or kill, affected herds, which is the standard procedure when poultry flocks become infected with the virus.
The disease mainly affects older cows, which develop symptoms that include loss of appetite, low-grade fever and a significant decrease in milk production. The milk produced by cows is often “thick and pale,” according to Texas officials. The virus has also been found in unpasteurized milk samples collected from sick cows.
It is not yet clear whether the bird flu virus is the sole cause of all the symptoms and illnesses that have been reported, experts warn.
How widespread is the problem?
Vague. Last Friday, the USDA’s National Veterinary Services Laboratory confirmed bird flu infections in two flocks in Texas, two flocks in Kansas and one flock in Michigan.
Initial testing suggested that additional herds in Texas, New Mexico and Idaho may also have the virus, but those findings have not been confirmed by the national laboratory. So far, the virus has only been found in dairy cows and not in beef cattle.
But because cattle are not regularly tested for bird flu, and the disease is relatively mild, there may be other infected herds that go undetected, experts said.
And the movement of cattle between states can transport the virus to new locations. The affected dairy in Michigan recently imported cows from an infected herd in Texas. When the cattle are brought in, the animals do not show any symptoms. The Idaho farm also recently imported cattle from an affected state, Idaho officials said.
How does it spread?
That is a key, and still unanswered, question. It is possible that infected cattle acquire the virus independently, especially if a shared source of food or water is contaminated.
However, a more worrisome possibility is that the virus spreads from cow to cow. On Friday, the USDA noted that “transmission between cattle cannot be ruled out.”
Some scientists say they would be surprised if there isn’t any level of cow-to-cow transmission. “How else can it move so fast?” said Dr. Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch.
If the virus is easily spread between cows, that could lead to larger, longer outbreaks. It would also give the virus more opportunities to adapt to its new mammalian hosts, increasing the risk that it would develop mutations that make it more dangerous to humans.
How do officials know if bird flu is adapted to spread between people?
Analysis of the genetic sequence of the virus from infected birds, cattle and humans may reveal whether H5N1 has acquired mutations that help it spread to humans.
Scientists are closely monitoring infections in birds and marine mammals and, now, cattle. So far, the virus doesn’t seem capable of spreading efficiently between people.
In 2012, scientists showed that H5N1 can spread through the air between ferrets – a popular model for studying the transmission of respiratory viruses to humans – after developing five mutations.
A bird flu sample isolated from a Chilean man last year had two mutations that indicate adaptation to infectious mammals. But those mutations have previously been seen without the virus ever evolving to spread between people, experts said.
Are dairy products safe to consume?
Federal officials stress that commercially processed milk remains safe to drink. Dairies are required to keep milk from diseased animals into the human food supply, and milk sold across state lines must be pasteurized, a process in which the milk is heated to kill potential pathogens. Pasteurization “has consistently proven to inactivate bacteria and viruses, such as influenza, in milk,” the Food and Drug Administration said in a new online milk safety guide.
Dr. Gail Hansen, a veterinary public health expert and independent consultant, agreed that the risk of contamination from pasteurized milk is likely to be “very low.” He added, “I don’t want people to stop drinking milk because of this.”
But the possibility cannot be completely ruled out, he said, expressing some concern that federal officials are “overconfident in the face of so many unknowns.” If cows shed the virus in their milk before they show signs of illness, that milk has the potential to enter the commercial milk supply, he said. And different pathogens may require different temperatures and duration of pasteurization; the specific conditions required to inactivate this particular virus remain unclear, said Dr. Hansen.
The risk of contracting the virus by consuming unpasteurized, or raw, dairy products remains unknown, the FDA said. Raw milk is known to pose a variety of potential disease risks beyond avian influenza.
