H5N1 HPAI detected in Wisconsin herd

MADISON, Wis. — Nearly two years after it first appeared in a Texas dairy herd, the Wisconsin Department of Agriculture, Trade and Consumer Protection has confirmed that H5N1 highly pathogenic avian influenza has been detected in a Dodge County dairy herd.

Confirmation of the disease presence in Wisconsin was announced by DATCP Dec. 14. According to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, it is the first confirmed case this month and one of only two cases confirmed in the past 30 days; the other being in California, Nov. 26. The Wisconsin case brings the total number of confirmed cases to 1,084 in 19 states, since the virus was first identified March 25, 2024.

“There are three things we know,” said Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory in Madison. “We know that pasteurization is 100% effective in killing the virus; that’s the most important thing. The second is that the National Milk Testing Strategy and the system that Wisconsin built are working and are critical to continue to monitor. The third is that we have a huge infrastructure to be able to understand more about what’s going on and how we can manage it.”

Poulsen said the Wisconsin case was detected through Wisconsin’s compliance with the NMTS, with no sign of clinical symptoms noted on-farm.

“We don’t have any evidence at this time of any other herds,” Poulsen said. “We know this because we’re doing surveillance now. We did drop back a bit on surveillance while we had gold status, so now we’re continuing to look. I can’t guarantee it won’t change, but it appears we haven’t seen any spread.”

While full sequencing was not completed as of Dec. 16, Poulsen said the case in Dodge County does not appear to be related to the movement of lactating dairy cattle.

“This is a closed herd and it’s very well-managed,” Poulsen said. “We haven’t seen any spread yet and I think that’s really important. Sequencing will tell us what type of strain this is.”

Poulsen said the predominant strain of H5N1 that spread herd-to-herd in many western dairies with the movement of lactating dairy cattle was B3.13. Endemic, or long-term infection with the B3.13 strain remains a problem due to lack of approved vaccines.

“We don’t really see that here,” Poulsen said. “In Wisconsin, in this region, we don’t move lactating cows like that — we’re talking 50,000-70,000 animals a week, nationally. We don’t have that dairy model in the state.”

Poulsen said the absence of sick cows presenting clinical symptoms of the disease leads him to believe this case is a different strain, similar to one that presented about a year ago in the Southwest U.S.

“I think it’s possible this could be a different introduction, like what we saw in Arizona and Nevada in January and February 2025,” Poulsen said. “We know that the Wisconsin DNR has released that from migratory wildfowl; there is D1.1 present in all major Wisconsin waterways, and this has been the case for many months.”

Like Wisconsin’s case, the Arizona and Nevada cases Poulsen referred to were discovered via those states’ implementation of the NMTS, rather than through reports of sick cows.

“The data really pointed at these introductions as being a high-plains desert issue, where you have a higher chance of birds comingling in cow water sources, cows that are on dry lot dairies, with birds that would be migrating through, stopping and eating cow feed or at the bunker,” Poulson said. “We really don’t have that in Wisconsin. We have pigeons and blackbirds, but those are not considered significant mechanisms for disease transfer.”

Migratory bird issues that plague western dairies did not appear to be a concern in the Upper Midwest for a variety of reasons, Poulsen said.

“Our thoughts were that we were low risk because of our topography,” Poulsen said. “Our cows are in barns, our feed is covered in bunker silos, and the birds have plenty of other waterways with abundant food sources.”

Poulsen characterized the situation as a potential good news-bad news scenario.

“The good news is that if it’s a different strain, without sick cows or reports of sick people, maybe it will burn  out like we’ve seen previously in Nevada and Arizona,” Poulsen said. “The challenge is that due to the wildlife reservoir, we are continuing to see more virus in the environment. We need to know if the virus is chnaging and we need to adapt our management practices.”

Increased virus load in the environment changes the scope of dealing with the issue, to protect both human and animal health, Poulsen said.

“That’s a global discussion,” Poulsen said. “It could involve the vaccination of poultry and it would have an impact on trade.”

Poulsen said for the time being, until more concrete information is available about the strain and any potential spread, the state of Wisconsin will continue to stay the course of increased bulk surveillance testing, testing of individual lactating animals going to shows and exhibitions, as well as testing to comply with the federal order for out-of-state movement of lactating animals. Poulsen said the state is asking on-farm veterinarians to be vigilant.

On-farm, Poulsen continues to urge each dairy farmer to take action in protecting their farm by implementing increased biosecurity protocols, such as cleaning boots, shoes and clothing, and controlling traffic on and off the farm.

“The Amazon truck does not need to drop off by the parlor,” Poulsen said. “It’s more challenging when you’re talking about the hoof trimmer, who really needs to be in the barn. Those are the things you need to talk with your veterinarian about, to determine how to best mitigate the risk.”

Resources for implementing biosecurity are available through the websites of USDA, DATCP, the National Milk Producers Federation, the Dairy Business Association and the Wisconsin Cheese Makers Association, Poulsen said.

Every day that passes and every new case that presents since the initial identification of the virus in dairy cattle provides the opportunity for greater understanding, Poulsen said.

“There are a lot of questions out there, but there are a lot of people working on it,” Poulsen said. “We don’t want people to panic — we’re getting information very quickly to understand how to best manage it. We’ve been a leader in this response for almost two years now, so we have a lot of experience. It’s unfortunate that we found it in Wisconsin, but that’s the whole purpose of surveillance: to prevent intentional negative consequences in the future. The bottom line to remember is, there is very little risk to public health and pasteurization keeps dairy products completely safe to consume.”