A milk sample from a Weld County dairy tested positive for highly pathogenic avian influenza, also called bird flu, during routine testing Friday, October 23, 2026. The finding changes Colorado's status under the National Milk Testing Strategy to "Affected." Dairy cattle moving to other states must now meet new federal testing rules.

The sample came from a bulk tank, which holds milk from the whole herd. The first result came back non-negative for the H5N1 virus, clade 2.3.4.4b. The National Veterinary Services Laboratories has since confirmed the positive result.

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The strain appears to be B3.13. This strain has kept spreading among dairies across the country. It has not been found in wild birds. That means the virus likely spread from one dairy to another. Whole genome sequencing may help show where it came from.

The herd has not shown major signs of illness. The dairy has noted some recent cases of mastitis, an udder infection, and a slight drop in milk production.

The dairy is under a state quarantine. The quarantine limits movement of lactating dairy cattle, which are cows producing milk. The dairy has also added biosecurity measures and weekly bulk tank testing. Workers now use personal protective equipment to lower their risk of infection. Officials hope quick detection and these steps will also protect other Colorado dairies. The quarantine will stay in place until the dairy has two negative bulk tank tests at least a week apart.

Under the new status, lactating dairy cattle must test negative for Influenza A virus before moving to another state. The test must be done at an approved National Animal Health Laboratory Network lab, using a test the network has approved. For groups of 30 or fewer animals, every animal being moved must be tested. For groups of more than 30, only 30 animals total must be tested. Samples must be collected and tested no more than seven days before the move.

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Nonlactating dairy cattle do not need testing to move to other states at this time because of their lower risk. These include heifers, dry cows and bull calves.

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service will pay for all premovement testing done at network labs. That means the testing costs nothing for the producer or the person sending the sample. Costs for collecting or shipping samples are not covered at this time. Full details are in the agency's guide, "Requirements and Recommendations for Highly Pathogenic Avian Influenza (HPAI) H5N1 Virus in Livestock For State Animal Health Officials, Accredited Veterinarians and Producers."

Signs of illness reported in affected dairies include:

Lower milk production. Some cows have a sudden, sharp drop, and badly affected cows may give thick, concentrated milk that looks like colostrum. Other signs include eating less, sticky or loose manure, low energy, dehydration, a low-grade fever and breathing problems, including a clear runny nose.

High amounts of virus shed by an infected dairy can contaminate the area around it. This puts other dairies at risk. It also threatens nearby commercial poultry farms and backyard flocks. H5N1 is highly deadly to birds. Stronger biosecurity helps contain the virus and protect poultry producers from catastrophic losses. It is also peak migration season, and wild birds could be another source of the virus. All poultry facilities are urged to increase biosecurity.

Dairy biosecurity guidance is available through the Secure Milk Supply Program and the FARM Program. Producers should consider pasteurizing all milk products fed to calves. Limiting contact with wild birds and their droppings may lower the risk of exposure. More information on wild bird biosecurity is on the Colorado Department of Agriculture's HPAI website. Guidance for poultry operations and backyard flocks is available through the U.S. Department of Agriculture's Defend the Flock program.

This detection is a reminder that the virus is still spreading and that staying alert is essential. Veterinarians and producers should report any suspected cases right away by calling (303) 869-9130, by using the Reportable Disease Case Report Form or by contacting their local Veterinary Medical Officer.