Viral fragments of bird flu have been identified in samples of milk taken from grocery store shelves in the United States, a finding that does not necessarily suggest a threat to human health but indicates the avian flu virus is more widespread among dairy herds than previously thought, according to two public health officials and a public health expert who was briefed on the issue.

The Food and Drug Administration said Tuesday that it had been testing milk samples throughout the dairy production process and confirmed the detection of viral particles “in some of the samples,” but it declined to provide details.

The presence of genetic fragments of the virus in milk is not unexpected. Pasteurization typically works to inactivate pathogens, said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health. It generally does not remove genetic material, Nuzzo said, but typically renders pathogens unable to cause harm to people.