H5N1 infects dairy cows, study shows

Nature Communications published a study on May 24 showing that H5N1 avian influenza can infect dairy cattle at a low infectious dose and has a strong affinity for the bovine mammary gland. The paper adds experimental evidence to a dairy-cattle outbreak that first drew attention in the United States in 2024, when H5N1 spread through herds and spilled back into poultry, wild birds and some other mammals. The researchers said the findings did not by themselves show efficient human transmission, but they did identify a livestock setting in which the virus can persist. The work arrives as animal-health authorities and producers in several countries continue to tighten controls around recurring avian-flu outbreaks. ### What did the new study actually show in cows? The Nature Communications paper said H5N1 infected dairy cattle with a low infectious dose and replicated especially well in mammary tissue. The authors described a “strong tropism” for the bovine mammary gland, a finding that helps explain why milk and udder infection became central features of the U.S. dairy outbreak. The researchers also reported transmission barriers. (nature.com) Their results indicated that while the virus can establish infection in cattle, spread is not unlimited across routes and conditions, which the paper described as identifiable constraints on transmission. ### Why are scientists focused on the mammary gland? The bovine mammary gland matters because it offers a clear site where the virus can replicate in dairy animals. The study said that tissue preference challenges standard assumptions about influenza A host range and tissue specificity, which have historically centered on respiratory infection. (nature.com) U.S. public-health agencies have already been tracking the dairy dimension closely. (nature.com) The U.S. Centers for Disease Control and Prevention says H5 bird flu is causing outbreaks in poultry and U.S. dairy cows, with sporadic human cases in dairy and poultry workers, while maintaining that the current public-health risk to the general public is low. ### Does this mean H5N1 is about to spread widely in people? The CDC says the current public-health risk remains low. (nature.com) The new paper does not say the virus has gained broad human transmissibility; instead, it shows that cattle can be infected efficiently enough to sustain concern about animal reservoirs and worker exposure. The distinction matters because cattle infection and human adaptation are not the same event. A separate Nature Communications study published in January reported polymerase mutations that helped H5N1 adapt early to cattle and improved replication in bovine cells and tissues, as well as cells of other mammals including humans, but that paper did not say sustained human-to-human spread had occurred. (cdc.gov) ### What are authorities and animal facilities doing now? (cdc.gov) Alberta zoos and wildlife facilities said they were increasing protections for captive birds after outbreaks were detected on commercial poultry operations east of Edmonton and near Ribstone Creek on May 11, and northeast of Drumheller on May 16. CBC reported that facilities were moving some birds, limiting exposure and adding other protective measures. (nature.com) The Wilder Institute/Calgary Zoo said on May 21 that highly pathogenic avian influenza had been confirmed within 200 kilometres of the zoo. The zoo said it closed one aviary and added foot baths at two bird exhibits as a precaution. ### What does the trade and food-control picture look like? Britain’s bird-flu guidance says meat from poultry and farmed game birds originating in disease-control zones is subject to rules on marking, separation and movement for human consumption. (cbc.ca) The guidance underscores that outbreaks continue to carry routine regulatory consequences for farms, processors and distributors. (calgaryzoo.com) South Africa’s poultry cull trade recovered in the final quarter of last year after earlier bird-flu disruptions, African Farming reported on May 24. The report said the live-bird trade remains important to township markets and that health regulations are being used to reduce the risk of spreading avian influenza. ### What comes next for this story? May 24’s study gives veterinary and public-health agencies a new experimental basis for watching dairy herds, milk-associated transmission routes and exposed workers. (govwire.co.uk) The CDC said it is continuing surveillance of people with animal exposures, while animal facilities in Alberta and poultry operators in Britain and South Africa are maintaining or updating controls tied to current outbreaks and trade rules. (cdc.gov) (africanfarming.com)