Bovine leukemia virus (BLV) is present in most U.S. dairy herds, but many producers do not know it. Because infected animals often appear healthy, the virus can circulate quietly for years before its impact becomes visible.
“BLV is often present long before it becomes an issue. So, if you’re not looking for it on the farm, chances are it’s there, but if you’re not looking, you don’t know or you don’t see it,” Tasia Kendrick, associate professor at Michigan State University, says on a recent episode of “The Dairy Health Blackbelt Podcast.”
Kendrick studies BLV epidemiology and control strategies in dairy herds.
That quiet presence can make BLV difficult to recognize. On many farms, the infection only becomes visible after production or health problems begin to accumulate.
“I’ve talked to quite a few producers, and it’s not a problem until it is. And then, all of a sudden, the animals dying are condemned at slaughter. It’s too late to do anything about when we get to that point,” Kendrick says.
BLV at a Glance:
- Present in 80% to 90% of U.S. dairy herds
- Up to 40% to 50% of animals infected within affected herds
- Impacts immune function, longevity and production
- Spreads primarily through blood-to-blood transfer
Subclinical Production and Immune Effects of BLV
Part of the challenge with BLV is that infected animals often appear normal during daily observation. However, research increasingly shows the virus can affect multiple aspects of herd performance.
“Those animals may appear to be normal, but you may be treating them for other ailments through their entire life, and then they leave the herd early, which leads to profit loss for that producer,” Kendrick says.
The economic impact often comes from small performance losses that accumulate across the herd. Reduced milk production, shorter productive lifespans and additional health treatments can all contribute to lower overall profitability.
The virus affects the immune system directly, which can influence both disease resistance and vaccine response.
“The way the virus works is it lays latent in the immune system. It targets B cells, one of your immune cells, and it can lay latent and dormant until it doesn’t,” Kendrick says.
Because BLV infects immune cells, affected animals may be more susceptible to secondary diseases or respond less efficiently to vaccination programs. Over time, those subtle effects can reduce both longevity and productivity.
How BLV Spreads on Farms
Understanding transmission is central to controlling the virus. BLV spreads primarily through the transfer of infected blood between animals.
“The virus itself targets B cells, which are just cells of the immune system that are in the blood system. So it’s a blood-to-blood transfer that moves it from animal to animal,” Kendrick says.
Routine management procedures can inadvertently contribute to transmission if proper precautions are not taken. Shared needles, contaminated equipment or procedures that transfer even very small amounts of blood between animals can spread the virus.
Vertical transmission is also possible.
“The virus is small enough that it can go through the placenta wall, so there is some dam-to-calf transfer as well as colostrum,” Kendrick says. “If raw colostrum or milk is fed, there’s potential for live virus to infect the animal that way.”
These pathways mean infections can occur both in the milking herd and during early life stages.
Practical Management of BLV
Because dairy operations differ widely in their management practices, BLV control strategies often need to be tailored to individual farms.
“It’s not one solution for every farm because every farm is managing their animals differently from colostrum all the way up to the milking herd,” Kendrick says.
Still, several practical steps can help reduce transmission risk.
“It’s really thinking about management strategies of what you can do to reduce blood-to-blood transfer, whether that’s single-use needles, single-use sleeves, fly control, anything that’s going to decrease the chances of blood transfer,” Kendrick says.
Key Strategies to Reduce BLV Transmission
1. Reduce blood transfer.
- Use single-use needles.
- Use new palpation sleeves for each cow.
- Maintain strict hygiene during procedures.
2. Manage colostrum carefully.
- Avoid feeding raw pooled colostrum.
- Freeze or pasteurize colostrum when possible.
3. Control biting flies.
4. Establish herd status through targeted testing.
- Testing every animal may not be necessary to understand the scope of infection.
“If you test 40 specific animals across the lactation in your herd, you get a pretty good picture of the prevalence, so you have a starting point and you don’t have to test every animal in your herd,” Kendrick says.
The Michigan State University BLV website recommends testing the 10 most recently calved cows that are greater than three days in milk from each lactation group (first, second, third and fourth-plus).
Looking More Closely
BLV has circulated in the dairy industry for decades, often without drawing much attention, but growing evidence of its effects on immunity, productivity and longevity is prompting veterinarians and producers to take a closer look.
For many herds, the first step is simply recognizing the virus may already be present.
