No bird flu virus was found after cooking ground beef to medium to well done, the U.S. Department of Agriculture said in a briefing on Thursday after conducting a study as it addresses concerns over an outbreak of the disease in dairy cattle.

The findings, in which scientists injected high levels of an H5N1 bird flu virus surrogate into ground beef, indicate that properly cooking hamburgers to a temperature of about 145 to 160 degrees Fahrenheit (63 to 71 degrees Celsius) keeps them safe for consumers.

In the cooking experiment, which was intended to further assure consumers of the safety of U.S. ground beef, hamburgers heated to 120 degrees F – or rare – showed the virus surrogate was present at reduced levels, the USDA said.

The agriculture department (USDA) previously said 30 samples of ground beef from retail outlets tested negative for H5N1 virus and that the U.S. meat supply is safe.

The USDA also announced that it was awarding more than $22.2 million to bolster the nation’s ability to respond and control animal disease outbreaks such as H5N1 bird flu, including enhancing laboratory capacity.

The U.S. has confirmed bird flu in dairy cattle in nine states since late March. Older dairy cattle are often processed into ground beef.

Scientists have said they believe the outbreak is more widespread based on U.S. Food and Drug Administration findings of H5N1 particles in about 20% of retail milk samples. The FDA has said tests of retail dairy samples were negative for viable H5N1 bird flu virus but cautioned against consuming raw unpasteurized milk.

The ground beef experiment results come just ahead of the start of peak grilling season with the U.S. Memorial Day weekend later this month.

Wastewater surveillance

Nirav Shah, principal deputy director of the U.S. Centers for Disease Control and Prevention (CDC), told reporters the agency’s newly posted wastewater detection dashboard on influenza A, of which H5N1 bird flu is a subtype, has identified high levels of virus in Illinois and Florida, which have not yet reported positive cases in dairy cattle.

Shah said the wastewater data in concert with the milk findings “suggest that there may be herds that are harboring H5N1 but have not yet tested positive.”

Wastewater tests cannot determine the source of the virus, which could be from humans, infected milk or farm runoff. Scientists are closely watching for changes in the virus that could make it spread more easily among humans.

The CDC has not detected any increased rates of H5N1 in people, Shah said, adding that the agency is working with local officials to understand why these wastewater sites show higher-than-expected levels of influenza A.

“At the dairy farm level, thus far we’ve not seen corresponding reports, but those investigations are still underway,” Shah said.

The CDC has urged states to provide protective gear to all slaughterhouse workers, and anyone involved in the production of milk, including in states without positive herds.

He said CDC so far has not had any individual farm workers take up the agency’s offer last week of $75 compensation to participate in studies of the outbreak.

“We’re not giving up hope. We are in constant conversation with a number of states,” he said.

One dairy farm worker in Texas tested positive for the virus in the current outbreak and reported conjunctivitis. The CDC has said the risk to the general public is low, but the risk to people with exposure to infected birds or animals, including cows, is greater.

—Tom Polansek and Julie Steenhuysen, Reuters