To protect the US livestock industry from the threat posed by highly pathogenic H5N1 avian influenza (bird flu), the United States Department of Agriculture (USDA) has issued a federal order to make it mandatory to test dairy cows before interstate transfer.

Last month, USDA confirmed that bird flu had been detected in sick cattle on two dairy farms.

Unpasteurised, clinical samples of milk from sick cattle collected from two dairy farms in Kansas and one in Texas, as well as an oropharyngeal swab from another dairy in Texas, tested positive for the strain of bird flu.

Then this week, the US Food and Drink Administration (FDA) confirmed that some pasteurised milk samples have “indicated the presence” of bird flu.

According to the FDA, pasteurisation is likely to inactivate the bird flu virus but the process is “not expected to remove the presence of viral particles”.

Bird flu tests for dairy cows

USDA has outlined a number of actions that it is taking with its federal partners to help get ahead of the disease and limit its spread.

USDA’s Animal and Plant Health Inspection Service (APHIS) announced a federal order requiring various measures to be undertaken including mandatory testing for interstate movement of dairy cattle.

Prior to interstate movement, dairy cattle are required to receive a negative test for Influenza A virus at an approved National Animal Health Laboratory Network (NAHLN) laboratory.

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

Dairy cattle moving interstate must adhere to conditions specified by APHIS.

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

Mandatory reporting

USDA has said that laboratories and state veterinarians must report positive Influenza A nucleic acid detection diagnostic results in livestock to APHIS.

Laboratories and state veterinarians must also report positive Influenza A serology diagnostic results in livestock to APHIS.

USDA has identified spread between cows within the same herd, spread from cows to poultry, spread between dairies associated with cattle movements, and cows without clinical signs that have tested positive.

On April 16, APHIS microbiologists identified a shift in an H5N1 sample from a cow in Kansas that could indicate that the virus has an adaptation to mammals.

Centers for Disease Control and Prevention (CDC) conducted further analysis of the specimen sequence, which did not change their overall risk assessment for the general public, because the substitution has been seen previously in other mammalian infections and does not impact viral transmission.

Additionally, APHIS’ National Veterinary Services Laboratories found H5N1 in a lung tissue sample from an asymptomatic cull dairy cow that originated from an affected herd and did not enter the food supply.

The novel movement of H5N1 between wild birds and dairy cows requires further testing and time to develop a critical understanding to support any future courses of action, USDA stated.

In a statement, USDA said: “While we are taking this action today, it is important to remember that thus far, we have not found changes to the virus that would make it more transmissible to humans and between people.

“While cases among humans in direct contact with infected animals are possible, our partners at the US Centers for Disease Control and Prevention (CDC) believe that the current risk to the public remains low.”

USDA also added that affected cows recover after supported care with little to no associated mortality.