When highly pathogenic avian influenza (HPAI) first hit U.S. dairies, it threw the industry into unfamiliar territory. With so many unknowns, the immediate focus was on slowing the spread and caring for the cows that were affected.
Now, with more of the puzzle pieces coming together, researchers are beginning to step back and look at the bigger picture, examining how the virus affects cows not only in the days and weeks after infection, but what it may mean for their health and performance long after.
On a recent episode of “The Dairy Podcast Show”, Jennifer Spencer and Juan Pinedo, Extension dairy specialists with Texas A&M, came together to better understand what this virus is doing to reproduction — and if infected cows will be paying the price for years.
What Does HPAI Mean for Cow Fertility?
Spencer and Pinedo are just starting to study HPAI’s long-term effects, and their work is one of the first to measure how it may impact reproduction in U.S. dairy herds. Early signs point to a real effect on reproductive performance, particularly in younger animals, though the science is still evolving.
“We really want to know if it does impact reproduction,” Spencer says. “We want to let the producers know so they can understand if they might have to cull heavier to make sure that they’re managing this and replacing the cows in a timely manner — for sustainability of the herd and also to help maintain or improve their profitability.”
Like all major health events, HPAI is predicted to have an impact on cow fertility or pregnancy loss. But for Spencer and Pinedo, they are trying to figure out to what degree.
“We know that when cows get sick, they shift their energy toward fighting an infection rather than reproducing,” Spencer explains. “If these cattle that are infected with HPAI are having reduced milk production, feed intake and rumination, then there’s a high probability it’s impacting reproductive efficiency and their performance.”
Pinedo adds this pattern isn’t unique to HPAI, but still worth studying.
“Just like with any other systemic disease, having a detrimental effect in repro performance is something that we will expect,” Pinedo explains.
Dig Into Herd Data
In March 2025, Spencer’s team received rapid-response funding from USDA APHIS to study how HPAI affects reproduction in dairy herds. They designed a retrospective observational study, analyzing on-farm records from January 2021 through each herd’s HPAI outbreak and beyond.
“We’re looking at data from January 2021 until they had an HPAI outbreak,” Spencer says. “How we’re determining that is based not just on what the producer says, but by also analyzing the records and looking for that drastic drop in milk production, because that’s kind of the overall sign when they had an HPAI outbreak.”
To give the team a wide view of how HPAI is affecting herds in different settings, the project spans across three dairy regions with five to 10 dairies per region. These areas include the:
- South Central
- Western U.S.
- Pacific Northwest
“We want to get information from different environments to have a better idea of the geographical differences, and what they deal with heat-stress-wise, or the feed availability,” Spencer says. “This will give us a 30,000' view of what is happening.”
A Hit on Heifers
While the full analysis is still in progress, the team has already taken a close look at one South Central dairy, and the early patterns are raising important questions in heifers. The study found conception rates dropped during the outbreak year but appeared to rebound the following year.
“For heifers, they actually had about a 5% decrease in their conception rates during 2024 from March until December, but that appears to go back up in 2025,” Spencer reports.
Pinedo added specific figures to put the changes into perspective.
“There was a 52% conception rate, and they dropped during the outbreak to 45%. The year after, they came back to 50%,” he notes.
These numbers show that while conception rates began to recover after the outbreak, they didn’t fully return to pre-HPAI levels. Spencer notes heifers needed more services per conception, suggesting the virus may have lingering effects on reproductive efficiency.
“For heifers, we saw an increase in services per conception, but they’ve remained higher. They went from about a 1.5 up to like 2, 2.2 number of times bred for their heifers, and it’s continuing into 2025,” she adds.
Spencer admits that wasn’t what they expected.
“We didn’t think that heifers would be impacted,” she says.
Complicating things further, some of the heifers in question were born to cows infected late in gestation.
“Some of those heifers were actually from cows that were in their third trimester of their pregnancy, so that may be a contributing factor,” Spencer adds. “There’s so many moving parts in it.”
How Are Cows Being Impacted?
For cows, the pattern is more complex. Unlike heifers, which showed a relatively clear dip and rebound in conception rates, mature cows showed more varied responses to HPAI. Some herds experienced noticeable declines during the outbreak, while others were less affected.
“We did see conception rates appeared to decrease in all of the lactations,” Spencer notes. “But for the first and third and greater lactations, they seem to be going back up, whereas the second lactation seems to be kind of having a harder time rebounding.”
Those second-lactation cows are noteworthy because many were first-lactation animals during the outbreak itself.
“It seems as if perhaps a first-lactation animal, which you might think would be more resilient to recover, maybe, is having more longer-term effects on at least reproduction, as opposed to older cattle,” Pinedo adds.
These unexpected patterns have the researchers taking a closer look at the number to try and determine why younger animals are taking a bigger reproductive hit.
How Much Should We Read Into Early HPAI Data?
While the data is eye-opening, both researchers are quick to point out that the findings are still early, and there’s a lot they don’t yet know.
“This is research that is at the very early stages, and it’s a retrospective observational study,” Pinedo emphasizes. “You really want to control confounders; it’s nothing that we could jump into conclusions [about] right now.”
He lists the kinds of changes every producer lives with year to year:
“You have bull genetics that will have changed, repro program that will have changed, feed that will have changed, heat abatement technologies, so many confounders that affect repro that have to be controlled,” he says.
Spencer sees the same complexity in the field of research.
“What if they changed a breeder? Or what if they started using precision technologies on their heifers, so their heat detection rate went up? These are things we have to take into consideration,” she says.
For now, the research is still in its early stages, but the data suggest HPAI does have reproductive effects worth paying attention to, especially in younger animals. As the team continues to analyze records and track herd performance, these early insights can help producers keep a closer eye on animals who were impacted by the virus and make more informed decisions for the long-term health and fertility of their herds.
