The Food and Drug Administration has begun testing aged raw cow's milk cheese for highly pathogenic avian influenza (H5N1), or bird flu.

The FDA is testing in response to a bird flu outbreak in multiple states, which is affecting people, poultry, and dairy cows across the U.S. Field staff will collect hundreds of samples from warehouses and distribution hubs across the country.

Raw milk cheese is made with unpasteurized milk. In the U.S., cheese that can be made from raw milk needs to be aged for at least 60 days in order to mitigate the risk from any present pathogens.

The first serious human case in the U.S. occurred in an elderly patient in Louisiana, who was hospitalized earlier this month.

The Centers for Disease Control and Prevention believes that it appears that the bird flu virus mutated in the victim. Mutations may make it easier for the virus to infect the upper airways of humans.

• CDC: Bird Flu Virus Likely Mutated in La. Patient
• CDC: Bird Flu Shows Mutations in First Severe Human Case in US