The Canadian Food Inspection Agency (CFIA) has toughened import requirements on U.S. breeding cattle over the H5N1 bird flu virus, the agency said in a statement on Friday.

U.S. sellers of raw milk appear undeterred by federal health warnings for consumers to avoid drinking unpasteurized milk in light of a bird flu outbreak that has affected dairy herds in nine states and sickened at least one dairy farm worker.

Cattle prices have tumbled as the US government steps up measures to contain the spread of a bird flu outbreak among the country’s cow herd. The US Department of Agriculture at the start of this week banned infected cattle from crossing state borders and said it would test ground beef for bird flu particles,

A wave of bird flu has devastated poultry populations and spread to dairy cows in the US and led to a number of other animals' deaths, raising fears of widespread human transmission.

A USDA analysis shows the spread in cattle started from a single spillover event from birds in the Texas panhandle that may have happened in early December.

Federal testing and surveillance of the outbreak has so far lagged since initial detection in dairy cows more than a month ago.