Traces of bird flu virus have been found in milk. What's going on? Is dairy safe?

Fragments of the virus that causes bird flu have been found in samples of pasteurized milk, the Food and Drug Administration said Tuesday, but officials aren't concerned about danger to humans from drinking store-bought milk at this time.

Officials told reporters on Wednesday that the milk tested positive for presence of the virus, but the results do not indicate that the virus is currently infectious, as the pasteurization process works to inactivate viruses.

The initial positive samples came from a limited number of tests done by researchers on milk at grocery stores. Federal agencies are now doing their own, more robust tests. The FDA said it is conducting further analysis on the positive samples, and is working with the U.S. Department of Agriculture, the Centers for Disease Control and Prevention and state partners to investigate.

"To date, we have seen nothing that would change our assessment that the commercial milk supply is safe," Donald Prater, the acting director of the Center for Food Safety and Applied Nutrition at the FDA, told reporters.

On Wednesday, officials announced a new federal order that will require all dairy cattle to be tested for bird flu before being transported across state lines. Any cows that test positive will have to wait 30 days and retest before being moved, said administrator for the USDA's Animal and Plant Health Inspection Service Michael Watson.

The avian influenza A(H5N1) virus has recently infected herds of dairy cows in several states, including Texas, Kansas and Michigan. Herds in New Mexico and Idaho are presumed infected, officials said.

Both the FDA and the USDA said the current commercial milk supply is safe because of the pasteurization process and the "diversion or destruction of milk from sick cows."

The agency pledged to take the situation seriously and release results from multiple studies in the coming days or weeks.

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What is bird flu?

Avian influenza, or bird flu, is a disease that's caused by an avian influenza Type A virus infection, according to the CDC.

The disease primarily spreads through contact with infected birds, which can infect their respiratory tract and intestines, according to the CDC. Infected birds can shed viruses in their saliva, nasal secretions and feces. Bird flu is very contagious among birds, and some of the viruses can sicken or kill certain domesticated species, including chickens, ducks and turkeys.

Is anyone at risk from drinking milk?

Pasteurized milk — which makes up 99% of milk produced on dairy farms and sold in stores — is treated to make sure any viruses in the milk are dead. Experts say drinking this kind of milk is safe, even if fragments of bird flu have been found in some samples.

However, raw milk does not receive this treatment.

Is raw milk safe to drink?

The FDA says people should not drink raw milk. The agency also says the dairy industry shouldn't manufacture or sell raw milk or raw milk products, including cheese made with raw milk, from any cows showing symptoms of illness or exposed to other cows that have bird flu.

How did virus fragments get into milk?

While the pasteurization process is designed to make milk safe by inactivating viruses, it doesn't destroy the virus fragments.

"It's important to understand that fragments, such as genetic material, of those pathogens can still be found in pasteurized milk," Prater said. Still, "pasteurization is very likely to effectively inactivate heat-sensitive viruses, like H5N1 in milk from cows and other species."

Officials said Wednesday that current studies on how effectively pasteurization kills off bird flu virus are underway, but the process has proven effective in similarly acting viruses for decades, and thermal inactivation has been successful at inactivating bird flu in eggs, which occurs at a lower temperature than in milk.

The FDA and its partners are testing samples of milk from store shelves and conducting further analysis on any that show they have bird flu virus to ensure that the virus fragments are inactive, Prater said. Those results will be available in the coming days, he said.

What is pasteurization?

Named after the scientist Louis Pasteur, pasteurization is a treatment process that applies heat to liquids for short amounts of time, according to the Virginia Department of Health. The process destroys pathogenic microorganisms in some foods and beverages that can cause spoilage and prolongs storage time.

Pasteurization was adopted by the FDA as a public health measure to kill dangerous bacteria and viruses, and largely eliminates the risk of getting sick from milk. In 1987, the FDA prohibited the interstate sale of raw, or unpasteurized milk.

Can humans get bird flu?

There have been a few cases of humans being infected with various strains of bird flu, usually through close contact with sick or dead animals. But experts say the risk to humans is currently low because the disease has not mutated to spread person-to-person.

There have been two human cases in the U.S. tied to this strain of bird flu. Among them: On April 1, Texas health officials said a person became infected with bird flu after close contact with infected cows. The CDC said the only symptom reported by the patient was pink eye, or conjunctivitis.

Public health officials told reporters that 23 people have been tested for bird flu during the current outbreak among cattle and 44 people are being actively monitored after suspected exposure.

Officials say risk to public health remains low. The CDC says most cases in humans have occurred after a person had unprotected exposure to sick or dead infected animals. Nirav Shah, principal deputy director of the CDC, said "we have not observed changes to the virus' genetic makeup that would suggest an enhanced the ability to spread to humans or among humans."

"This is definitely an evolving situation," Dr. Diego Diel, director of Cornell University's virology laboratory at the Animal Health Diagnostic Center, which worked to sequence the virus, previously told USA TODAY. "Right now, I think the viruses that have been detected in cattle and in other animals in these farms don't contain any mutations that would indicate increased transmissibility or pathogenesis to humans."

Contributing: Eduardo Cuevas, USA TODAY.

This article originally appeared on USA TODAY: Bird flu virus found in pasteurized milk, FDA says its safe to drink