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It’s very hard to say because until just this week, any testing for H5N1 in cows was voluntary, so the data are very limited. We know for sure that 36 herds in nine states have tested positive, but the real numbers may be much higher. One way we can get a better sense for this is to test milk. The FDA tested nearly 300 grocery store samples from 38 states and found fragments of the virus in one out of every five. Grocery milk combines milk from multiple individual cows, so it doesn’t necessarily translate directly to 20% of all cows being infected, but it does indicate that there is much wider spread than we initially realized. Between those numbers, data from genetic sequencing, and the accounts of cattle farmers on the ground in Texas, where the first confirmed cases were found in cows, this may have started spreading between cows back in December 2023. As of right now, there’s no evidence of the virus in beef cattle, but again, there’s very limited testing at this stage.
Sources: STAT, FDA, Dr. A. Rasmussen, Anderson et. al. BioRxiv
Despite the growing concerns about the possibility of a jump to humans, only about 25 people have been tested for H5N1 bird flu in the U.S. as of Wednesday. There are a lot of factors at play in this one. First, the dairy and cattle industries make up hundreds of billions of dollars of revenue. They won’t just cull sick herds like the poultry industry does, in part because cows recover in a few weeks and are more valuable individually than chickens, for example. Both the USDA and CDC have said that farmers have often refused to cooperate with their investigations, likely because none want to bear the risk of going public. There’s an added layer of complication because the human workers who are most at risk for a spillover event are often undocumented, a population who understandably tend to distrust government officials. On top of all that, there’s some jurisdictional jockeying; the CDC said recently that they haven't been officially invited in as a partner by any of the nine states with positive dairy herds, though they’re in regular contact. It’s a sticky situation and one that is starting to really worry public health officials who want to see more testing of animals and humans working on farms.
Almost 900 people have been infected with H5N1 bird flu virus since 2003 when it was first identified in Southeast Asia. In all 900 of these prior cases, the sick person had contact with sick or dead birds. But the farm worker in Texas who tested positive for H5N1 is different because he only had contact with sick cows. A new study looked at the genetic material of the viral sample and found that it had mammalian mutations, meaning it may be the first case we know of that went from a bird to another mammal (a cow) to a human. Unfortunately, the person in Texas did not consent to give any blood samples, and the owner of the farm appears to have refused to allow health officials to investigate further, so the information we can glean is limited. The good news is that the genetics of the viral sample taken from the infected human are still a good match for our human H5N1 vaccines.