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In the 14 months since H5N1 was first detected in Texas cattle, more than 1,060 herds have tested positive across 18 states. Given that testing of dairy cows (or their milk) is largely voluntary - and only lactating dairy cows are usually tested - how many herds have actually been infected is a subject of some debate.
A couple of weeks ago, in Nature: A Mathematical Model of H5N1 Influenza Transmission in US Dairy Cattle, we looked at a study which suggests that the number of herds - and states - with the H5N1 virus is likely significantly underreported.
Among their findings, they state:
A clear result from Fig. 2 and Table 1 is that some states are particularly likely to be home to infected herds, but have yet to identify and report infections. Most notable are Arizona, Wisconsin, Indiana, and Florida. Arizona has the largest mean herd size in the country (Supplementary Material Section 1), and extensive trade connections with Texas and California (Supplementary Material Section 2.4)—states particularly burdened with infection. Wisconsin, while farther from the epidemic epicenter, has the largest number of dairy herds in the country—6216.
Wisconsin to Begin Monthly Milk Sampling as Part of the National Milk Testing Strategy
FOR IMMEDIATE RELEASE: May 19, 2025
Contact: Molly Mueller, Public Information Officer, (608) 910-1929, molly.mueller@wisconsin.gov
MADISON, Wis. – The Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) announced the implementation of the mandatory National Milk Testing Strategy (NMTS), required by the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS). This comprehensive program aims to bolster highly pathogenic avian influenza (HPAI/H5N1) surveillance of the nation's dairy cattle herds, ensuring swift action to identify and address H5N1 affected herds.
Monthly Sampling Set to Begin
Under the NMTS, Wisconsin will soon initiate mandatory monthly milk sampling. With help from industry partners, it is expected that one milk sample will be obtained per dairy farm each month and tested at the Wisconsin Veterinary Diagnostic Laboratory (WVDL). These samples will primarily be gathered through existing milk quality labs that work with Licensed Bulk Milk Weighers and Samplers, ensuring minimal disruption to dairy operations while obtaining farm-level results.
HPAI/H5N1 Detection and Next Steps
Initial Testing: If H5N1 is detected, DATCP's Division of Animal Health will be notified by the WVDL and the Division of Animal Health will notify the farm.
Follow-Up: DATCP's Division of Animal Health will work with affected farms regarding quarantine requirements. For more information, refer to the What To Do if H5N1 is Confirmed on Your Dairy Farm resource document on DATCP's website.
Milk Standards: Farms may continue shipping milk if it meets “normal" milk standards under ATCP 65.20 (milk that is not bloody, stringy, off-colored, or otherwise abnormal). Additionally, the Food and Drug Administration and USDA continue to emphasize that the commercial milk supply is safe because of the pasteurization process.
Any H5N1 detections will also be confirmed by the National Veterinary Services Laboratories (NVSL).
Ongoing Updates and Resources
DATCP will provide updates on the NMTS collection process as the program develops. Stakeholders are encouraged to visit datcp.wi.gov for additional information, resources, and FAQs and subscribe to receive updates at https://service.govdelivery.com/accounts/WIDATCP/subscriber/new.
The implementation of the NMTS highlights Wisconsin's ongoing commitment to working with federal and state partners to safeguard the dairy industry and ensure public health.
For questions or more information on the testing strategy, contact NMTSfaq@wisconsin.gov or (608) 224-4717.
The full extent of Colorado's H5N1 epizootic only became apparent after more aggressive mandatory testing was instituted. It will be interesting to see what Wisconsin's testing will turn up in the weeks and months ahead.
