US imposes dairy cow tests as concerns grow over bird flu

Dairy cows being moved between US states must be tested for the bird flu under a federal order issued yesterday aimed at containing the spread of the virus across US cattle farms.

The clampdown comes after testing of pasteurised milk from grocery store shelves found genetic traces of the H5N1 virus. Health officials said these fragments of virus did not indicate the milk contained live virus and they had “seen nothing that would change our assessment” that the supply was safe.

Donald Prater, the acting director of the US Food and Drug Administration (FDA) Center for Food Safety and Applied Nutrition, said: “We take the situation very seriously. Based on information currently available, we believe our commercial milk is safe.”

The federal order requires every lactating cow to have a negative result before being moved to a new state.

Michael Watson, an administrator with the US Department of Agriculture’s animal and plant health inspection service, said laboratories had the capacity for “tens of thousands of tests a day”.

Until now, testing was done voluntarily and only in cows with symptoms, but FDA officials suggested some producers had not been fully cooperative. Positive tests will now trigger additional requirements for officials to investigate, milk from affected cows will be dumped and cows that test positive will be kept in situ for 30 days before being retested.

Officials also gave more detailed information about investigations into the potential for milk to contain infectious virus. Genetic traces of H5N1 were found in commercially bought milk in PCR tests. But the tests detect live and dead virus fragments.

Pasteurisation is designed to kill off the most robust bacteria and viruses so would be expected to do the same for bird flu, but further tests are needed to confirm this is the case. The FDA said it had been trying to grow virus from milk found to contain evidence of H5N1, which is the gold standard test to see if there is viable virus in a product.

“Pasteurisation is very likely to inactivate viruses like H5N1,” said Prater, adding that it was a “new and evolving” situation.

The outbreak, which has spread to herds across eight states, has prompted concern because it is the first time bird flu has spread between cows, and transmission between mammals raises the risk of the virus adapting in a way that could make it spread more easily in humans. So far, one human case has been reported in a Texas farmworker, who has recovered, and 44 people have been monitored after possible exposure.

It is not clear how the virus is spreading between cows, but milk is believed to be the primary vector rather than respiratory infection.

“We believe that the primary transfer is through milk … some kind of mechanical transmission,” said Watson. This could be via milking equipment, people moving between facilities or rodents, he added. “At this stage, we’re just speculating.”