Authorities in the US have confirmed viral fragments of avian influenza – or bird flu – in one-in-five samples of commercial milk.

The Food and Drug Administration (FDA) said that it had received some initial results from its nationally-representative commercial milk sampling supply.

It had confirmed last week that it was carrying out this sampling after confirming the presence of highly pathogenic avian influenza (HPAI) in some pasteurised milk samples, which came after the virus was confirmed in dairy cattle on some farms.

Initial results from this testing show that around 20% of the 297 samples tested positive for bird flu viral fragments through quantitative PCR (qPCR) tests, with a greater proportion of positive results coming from areas with infected herds.

The FDA said it continuing to analyse the results of the sampling.

The FDA reiterated that a positive qPCR test does not necessarily represent actual virus that may be a risk to consumers.

Additional testing is required to determine whether intact pathogen is still present and if it remains infectious, which would help make a determination if there was any risk of illness associated with consuming the product, the FDA said.

Meanwhile, investigations funded by the National Institutes of Health (NIH) indicate an absence of infectious virus in retail milk.

The FDA said the results do not provide a reason to change its assessment that the commercial milk supply in the US is safe.

The Centers for Disease Control and Prevention (CDC) has shown no increase in human cases of flu and no cases of bird flu specifically, beyond the one known case of an individual who was in contact with an infected dairy herd.

Last week, the US Department of Agriculture (USDA) issued a federal order making it mandatory to test dairy cows before interstate transfer (transfer between US states) to protect the industry from highly-pathogenic bird flu.

Prior to interstate movement, dairy cattle are required to receive a negative test for influenza ‘A’ virus (the pathogen that causes bird flu) from an approved laboratory.

Owners of herds in which dairy cattle test positive for the pathogen will be required to provide epidemiological information, including animal movement tracing.

These steps are required immediately for lactating dairy cattle, while the requirements for other classes of dairy cattle will be based on scientific factors concerning the virus and its evolving risk profile.