Cows in a dairy farm

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Dairy cows in multiple US states have fallen ill with bird flu, according to the US Department of Agriculture (USDA). The virus has killed millions of birds worldwide, but this is the first time it has been detected in cattle.

How many cows are affected by bird flu?

As of 25 March, milk samples collected from two dairy farms in Kansas and one in Texas have tested positive for a subtype of avian influenza called H5N1, according to The New York Times. This strain is highly lethal in birds. A cattle throat swab taken from a dairy farm in Texas also tested positive. No cows have died from the virus so far.

US agencies began testing cattle for bird flu on 22 March after farms in Texas, Kansas and New Mexico reported some dairy cows had fallen ill, and that dead wild birds were on their properties.

On the affected farms, about 10 per cent of milking cows appear to be sick, most of whom are older. It is unclear if bird flu is responsible for all of the animals’ illnesses. US agencies are moving quickly to conduct additional testing.

How did the cows catch the virus?

The USDA says it appears the cattle caught the virus from infected wild birds. However, it is unclear exactly how the virus transmitted between the species.

Most mammals who fall ill with bird flu are carnivores, such as foxes or seals, meaning they most likely contracted the virus from eating dead and infected birds. As cows don’t eat birds, it is harder to explain the source of transmission, says Richard Webby at St. Jude Children’s Research Hospital in Tennessee. It could be that faeces or saliva from wild birds contaminated the cattle’s water or food.

“The biggest question I can’t quite get my head around is how to explain infections across such a geographic spread of states,” says Webby.

The worst-case scenario is the virus spread among the cows, but this is perhaps unlikely, he says. That is because we haven’t found evidence that bird flu can transmit between mammals.

Do the sick cows increase the risk of bird flu spreading to people?

The risk of contracting bird flu remains low for most people. Initial testing of samples from infected cattle haven’t found genetic changes that would suggest the virus has become more transmissible to humans.

Yet any time a mammal becomes ill with bird flu, it provides the virus an opportunity to gain the necessary mutations to transmit between mammals, says Webby. “But we still need some answers to put this into perspective: most of all, how many cows show evidence of being infected with the virus?” If few cows are ill, this will most likely be another dead end for the virus, much like foxes, bears and other previously infected animals were.

Is milk safe to drink?

Yes, milk is still safe to drink. The USDA already requires that dairy farms only send milk from healthy cattle in for processing. Milk from infected cows has also been destroyed so it won’t enter the food supply.

Even if some contaminated milk entered the supply chain, pasteurisation kills bacteria and viruses, including influenza.


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