What to know about the bird flu outbreak in the US after virus fragments found in milk samples

Public health officials are continuing to monitor as an outbreak of avian flu, also known as bird flu, continues to spread across the country.

The strain, known as H5N1, has sickened several mammals this year, according to the Centers for Disease Control and Prevention (CDC), the U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA).

Several dairy cows have been infected, resulting in milk samples showing inactive remnants of the virus, and one human case has been confirmed.

Health officials say the food supply is safe and the risk to the general public is currently low.

Here's the latest to know on the outbreak.

What is bird flu?

Avian influenza, or bird flu, is an infectious viral disease that primarily spreads among birds and is caused by infection with Influenza A viruses.

These viruses typically spread among wild aquatic birds but can infect domestic poultry and other bird and animal species, according to the CDC.

Although bird flu viruses normally don't infect humans, there have been rare cases of infection. To confirm infection, laboratory testing is required.

Signs and symptoms of infection in humans often include sore throat, cough, fever, runny or stuffy nose, headache, muscle or body aches, fatigue and shortness of breath. Less common symptoms include nausea, vomiting, diarrhea and seizures.

Infections can range from no symptoms or mild illness, such as flu-like symptoms, to more severe illness, such as pneumonia that could require hospitalizations, the CDC says.

How did the outbreak begin?

In early March, the USDA announced a bird flu strain that had sickened millions of birds across the U.S was identified in several mammals this year.

At the time, three states had reported cases of bird flu in mammals in 2024, including striped skunks found in Washington state, a mountain lion in Montana and a raccoon in Kentucky.

A few weeks later, federal and state public health officials said they were investigating an illness among primarily older dairy cows in Kansas, New Mexico and Texas and causing symptoms including decreased lactation and low appetite.

The USDA said in a statement at the time that "there is no concern about the safety of the commercial milk supply or that this circumstance poses a risk to consumer health."

First human case of bird flu

Earlier this month, the CDC said a human case of bird flu was identified in Texas and linked to cattle. The infected individual worked directly with sick cattle and reported eye redness as their only symptom.

This is the second human case of H5N1 ever reported in the U.S. but the first linked to cattle.

However, there have been no reports and no evidence to indicate there is person-to-person transmission, a CDC Director Dr. Mandy Cohen told ABC News at the time.

The CDC said it considers the health risk assessment to the general public to be low.

Inactive fragments found in milk samples

Earlier this week, reports emerged of bird flu fragments found in samples of pasteurized milk. However, the fragments are inactive remnants of the virus and cannot cause infection as the commercial milk supply undergoes pasteurization.

Federal agencies maintain the U.S. commercial milk supply remains safe because milk is pasteurized and dairy farmers are required to dispose of any milk from sick cows, so it does not enter the supply.

"To date, we have seen nothing that would change our assessment that the commercial milk supply is safe," the FDA said in an update.

The FDA said in its update that fragments of the virus are likely inactivated by the pasteurization process.

"The discovery of bird flu virus fragments in commercial milk is significant, not because it poses a direct threat to public health, but because it indicates a broader exposure among dairy cattle than we previously understood," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor. "This calls for an expanded surveillance of both the virus's presence and its potential impact on food safety."

He added, "It's crucial to continue rigorous testing to determine if any live virus can survive the process. Understanding the dynamics of this virus in dairy products will help us refine our risk assessments and ensure public health safety."

The FDA said it is collaborating closely with the CDC's food safety group surveillance team to monitor emergency department data and flu testing data for any unusual trends in flu-like illness, flu or conjunctivitis. There is currently no data showing any unusual trends or activity.

ABC News Sony Salzman contributed to this report.