Is bird flu the next pandemic? Doctor explains what to know.

The government is now testing samples of ground beef sold in retail stores in the nine states where outbreaks of highly virulent bird flu have occurred in dairy cows, while offering assurances that U.S. meat is safe, the USDA said on Monday.

The effort comes after samples of pasteurized milk from around the country tested positive for inactive remnants of the virus known as H5N1, with those samples taken after the the virus was confirmed in dairy herds in nine states: Colorado, Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota and Texas. 

The agency also plans to sample infected beef muscles from culled dairy cows to study whether cooking ground beef reduces the H5N1 virus. 

The agency reiterated recommendations that consumers properly handle raw meats and cook them to a safe internal temperature to kill bacteria and viruses.  

The USDA on Monday started mandating that lactating dairy cows test negative for bird flu before being transported across state lines. 

Widespread in wild birds, H5N1 has also infected poultry and dairy farms, along with barn cats. Cows infected with the virus, which is usually deadly for poultry, typically recover within 10 days. 

A U.S. dairy worker recently became the second known human case of bird flu in this country, according to the Centers for Disease Control and Prevention, which is working with other federal and state agencies to track the spread. 

The outbreaks had one nation, Colombia, moving to restrict imports of U.S. beef, drawing fire from the U.S. Meat Export Federation. "Colombia's attempt to suspend beef imports from specific U.S. states is unworkable and misguides," the trade group said. 

Kate Gibson

Kate Gibson is a reporter for CBS MoneyWatch in New York.