How worrying is bird flu’s jump to dairy cows?

Since 2022, a highly contagious strain of bird flu has spread across the US at an unprecedented rate, resulting in the deaths of more than 90 million birds in commercial and backyard poultry flocks, plus thousands more in the wild. Then, in late March, the virus made a jump to another species that surprised many scientists: cows. Dairy cattle in eight states have since tested positive for the virus, the US Department of Agriculture’s tracker showed on Tuesday. So far, there have been no reported cases in Georgia. But there have been other developments that are worrying influenza experts. On Tuesday, the Food and Drug Administration said it had found inactive fragments of the virus in commercially available pasteurized milk, but said consumers who drink it are not at risk of infection. “To date, we have seen nothing that would change our assessment that the commercial milk supply is safe,” the FDA said in a statement.

An employee at one of the Texas dairy farms with positive cases in its cows also contracted the virus, the Atlanta-based Centers for Disease Control and Prevention confirmed April 1. The worker’s case is only the second-ever human case reported in the U.S. The man, who worked in close contact with cows and likely caught the virus from cattle, developed only a mild eye infection and has recovered. The CDC says the threat to the general population remains low and the USDA says there are no signs the virus has changed in ways that could allow it to spread more efficiently to and among people.

Still, experts say any transmission from mammals to humans is an unwelcome opportunity for the virus to adapt in ways that could heighten the risk to humans. The coming weeks could be key to determining whether the virus’ emergence in cows is the start of a growing public health threat or not. As the situation evolves, here’s what you need to know: What is bird flu? Highly pathogenic avian influenza (HPAI) — also known as bird flu — refers to the disease caused by a group of viruses that have spread among wild birds around the world for decades. In birds, the virus spreads in fecal droppings and through respiratory secretions, causing lethargy, tremors, seizures and death.

The virus’ high transmissibility, combined with the fact that most commercial poultry are kept in close quarters, means cases in poultry facilities typically require culling of entire flocks of tens of thousands of birds to stop the spread. A growing list of land and marine mammals have also contracted bird flu, including seals, sea lions, mink, otters, foxes, cats and now, cows. Tens of thousands of mammals across at least 26 countries have died.

There have also been sporadic infections in humans, too, but typically only in people who have had close, unprotected contact with infected animals, primarily poultry. Since 1997, there have been 890 reported human infections globally with the bird flu subtype prevalent among wild birds currently, known as H5N1. Of those, roughly 50% of cases were fatal, but reported symptoms have varied widely, from none at all to severe disease. Overall, the CDC says the spectrum of illness caused by the strains circulating right now is unknown.

How did cows catch it? Bird flu was likely originally transmitted to cows by wild migrating birds, according to a USDA memo published April 16. The agency adds there is now evidence of cow-to-cow spread in some recent cases. The virus may have also spread from dairy cattle to nearby poultry and barn cats on some farms, too, but the exact pathways are not yet known. In the latest cases, the sickened cows have survived but other animals, including cats and poultry, have died. Compared to birds, cows are experiencing mild symptoms: low energy, fever, dehydration and reduced appetite, plus thick, discolored milk and a decrease in production, the USDA says. The raw milk of infected cows has been found to contain high concentrations of H5N1 and USDA has warned dairy workers to avoid any exposure to unpasteurized milk. To tamp down the spread, the agency has recommended “particular attention” to disinfecting milking equipment, as well as pens and other surfaces.