Recent detections have also been reported in Colorado, Kansas and Texas

YAKIMA, Wash. — An expanding H5N1 avian influenza outbreak in Washington dairy cattle reached 40 confirmed herds in two counties by October 30. Three people with occupational exposure to infected cattle have tested positive, including one with underlying health conditions who died November 1.

Washington’s first dairy outbreak involves the B3.13 genotype, identified through sequencing of samples from two affected dairies. This is the strain responsible for sustained cow-to-cow transmission in U.S. dairy herds since H5N1 emerged in cattle in March 2024.

The state’s first two infected herds were confirmed October 8 in Yakima County. By October 30, the Washington State Department of Agriculture (WSDA) reported 39 affected herds there and one in neighboring Benton County. Eighteen were confirmed on October 23 alone.

The concentration raises questions about whether the virus spread rapidly between farms, circulated undetected before testing, or both. It does not necessarily mean the virus has become more contagious.

Three human cases, one death

The Yakima Health District confirmed the first human infection October 15. Two additional cases followed, including the resident who developed severe illness, with underlying health conditions, was hospitalized and died November 1. The other two patients had mild illness and were not hospitalized.

Health officials report no evidence of human-to-human transmission. CDC continues to assess the risk to the general public as low, while people working with infected animals or contaminated materials face greater risk.

B3.13 versus D1.1

The Washington outbreak involves B3.13, which has demonstrated sustained cow-to-cow transmission since 2024. D1.1 was detected in Nevada and Arizona dairy herds in early 2025, with genomic evidence indicating separate introductions from birds rather than the extensive multistate spread seen with B3.13.

D1.1 has also been associated with severe human infections, including the fatal Louisiana case linked to bird exposure in January 2025.

Officials state that there is no evidence so far that Washington’s B3.13 virus has evolved to spread faster or cause more severe disease.

Washington officials said the first two infected dairies had previously tested negative under USDA’s National Milk Testing Strategy. They were identified after cattle developed clinical signs. On the other hand, the Benton County herd was identified through routine surveillance before cows became sick, demonstrating how bulk-milk testing can detect infection ahead of symptoms.

Washington State Veterinarian Dr. Amber Itle said the Benton County detection provided an opportunity to implement controls before further transmission by identifying infected cattle, contaminated milk, equipment, vehicles, clothing, footwear and movement between farms as potential transmission pathways. Wild birds can introduce H5N1, but are not considered the primary source of ongoing B3.13 transmission among dairy cattle.

Dairy biosecurity and worker protection

Washington officials urge producers to strengthen biosecurity, limit cattle movement, avoid sharing equipment and workers between premises, isolate suspect animals and report clinical signs promptly.

Milk from sick or suspect cows must be excluded from the bulk tank. Pasteurization inactivates H5N1, and commercial pasteurized dairy products remain safe.

The human infections also highlight the need for appropriate eye, respiratory and hand protection when working with potentially infected cattle, milk or contaminated equipment.

The Washington outbreak comes with renewed H5N1 dairy detections elsewhere. USDA confirmed B3.13 in Indiana dairy cattle November 2, the state’s first detection, involving two operationally connected herds in Newton County.

Recent detections have also been reported in Colorado, Kansas and Texas. Since March 2024, H5N1 has been detected in dairy cattle in 21 states, although only five have current infections within the last 30 days as of Nov. 4, 2026.

Pennsylvania remains free of H5N1 in dairy cattle, according to the Pennsylvania Department of Agriculture (PDA), which continues mandatory bulk-milk surveillance begun in November 2024.

Pennsylvania achieved Stage 4, HPAI-free status under USDA’s National Milk Testing Strategy in February 2025 after more than 22,000 samples were tested, representing nearly all Pennsylvania dairy farms.

The mandatory testing order was extended in May 2026 through July 12, 2027. Pennsylvania’s Animal Diagnostic Laboratory System shifted from biweekly to monthly surveillance reporting in July 2025.

PDA also offers a voluntary, no-cost Lactating Dairy Cow Health Monitoring Program that can exempt enrolled herds from individual pre-movement testing under applicable requirements.

Dairy vaccine research advances

Meanwhile, H5N1 vaccine research is advancing, although USDA has not authorized a vaccine for routine use in dairy cattle.

A University of Pennsylvania–USDA study published October 28 in Nature Biotechnology reported promising results from an experimental mRNA vaccine tested in eight lactating Holsteins, four of them vaccinated. Following deliberate H5N1 exposure, vaccinated cows had approximately 1000-fold lower peak viral RNA levels in milk, less mastitis and mammary tissue damage, and smaller declines in milk production than unvaccinated cows.

The cattle came from a Pennsylvania dairy herd monitored for H5N1, with viral challenge testing conducted at USDA’s National Animal Disease Center in Iowa. Researchers said larger studies are needed to determine whether vaccination can reduce transmission between cattle.