Replacement cows had been quarantined. They tested negative for bovine viral diarrhea virus (BVDV). They joined the herd without incident.
Months later, pregnancy rates fell.
For a research herd with well-established vaccination protocols, extensive record keeping and routine testing, the results didn’t make sense.
At first glance, the obvious suspects were vaccination, biosecurity or a missed persistently infected (PI) animal. The investigation eventually pointed somewhere much less obvious: The replacement cows weren’t the problem. The PI calves they were carrying were.
The case, described by George Perry, professor at Texas A&M University, to Dr. Jim Rhodes, veterinarian with IDEXX Laboratories, during a recent webinar, also highlights why preventing virus exposure remains equally important for protecting reproductive performance.
“We did everything you were supposed to,” Perry says. “The cows were quarantined for over two weeks. They were all tested. They were all negative. Everything was healthy. Everything was good. They went out with our cows. The part you don’t think about then is the calf that’s in them becomes a new introduction when they’re born.”
The replacement cows were not the source of the problem. Two PI calves born after the cows entered the herd became the source of virus exposure during the breeding season, contributing to an unexpected decline in pregnancy rates and ultimately changing the herd’s BVD surveillance protocol.
How PI Calves Entered a Well-Vaccinated Herd
The story began when the research center purchased six pregnant cows for an unrelated research project.
The cattle completed quarantine, and every cow tested negative for BVDV before joining the herd. Then calving season arrived.
Two calves died shortly after birth. Because they were born in December, outside the herd’s normal calving season. Perry says the losses didn’t immediately raise concern. Another calf later died following calving complications. Three calves survived and appeared healthy. Nothing suggested BVD.
The real surprise came months later during pregnancy checks.
The research center had consistently achieved breeding season pregnancy rates of roughly 70% to 80% in previous years. Following the introduction of those cows, final pregnancy rates fell below 60%.
“When we’re below 60%, that kind of kills the breeding group,” Perry says.
The unexpected decline prompted Perry and his team to begin looking for an explanation. Fortunately, the research center had something many commercial operations do not: extensive historical samples collected through ongoing research projects.
“We had blood samples on all the calves before weaning,” Perry explains. “We started going back and testing everything.”
The results revealed what routine testing of the replacement cows had missed. Two of the apparently healthy surviving calves were persistently infected.
The cows themselves had tested negative, but the PI calves they carried became a source of virus exposure after they were born.
Can BVDV Reduce Pregnancy Rates in Vaccinated Herds?
Perry says the experience reinforced findings from earlier reproductive research.
In a separate study involving nine vaccinated cow herds, four herds had at least one animal with evidence of BVDV infection during the breeding season. These exposed herds recorded breeding season pregnancy rates of 68% compared with 88% in noninfected herds.
As Perry explains: “If they’re vaccinated, it’s the same as me and you getting the flu vaccine. If you get exposed to a good enough dose, you still get the flu. It’s just milder. These animals fight it off. It’s a transient infection, but it kills conception rates.”
The findings don’t suggest vaccination failed. Instead, they reinforce that transient BVD exposure can still affect reproductive performance when sufficient virus exposure occurs.
The Texas research herd reflected those findings. In this case, two PI calves were enough to alter pregnancy rates across the herd.
Five Questions to Ask When BVD May Be Affecting Pregnancy Rates
Perry’s experience highlights several questions worth revisiting when pregnancy rates decline unexpectedly.
- Were pregnant replacement females introduced before conception rates declined?
- When are calves from purchased females tested for PI status?
- Could fence-line contact, commingling or another biosecurity breach have introduced recent BVD exposure?
- Is there reason to suspect an undetected PI animal?
- Does the timing of reproductive losses coincide with recent virus exposure?
When Should Calves Be Tested for BVD?
The investigation also changed how the research herd manages replacement cattle. Rather than waiting until weaning, every calf born on the operation is now tested shortly after birth using an ear-notch sample.
Purchased pregnant cows receive additional attention as well. They may enter the herd after testing negative themselves, but are separated again before calving so newborn calves can be tested before joining the rest of the herd.
“We’ve shifted so everything’s done then,” Perry says. “We pull the ear notch at birth. If they come up positive at birth, we can remove that calf before the breeding season gets hit. We’re eliminating that exposure as early as we can.”
He adds: “If you test at weaning, you’re already into the next breeding season.”
Why Vaccination Alone Isn’t Enough for BVD Control
The changes implemented by Perry’s team mirror guidance from the American Association of Bovine Practitioners, which recommends combining vaccination with testing for PI animals and biosecurity rather than relying on vaccination alone. The guidelines also note that a PI animal continually exposes susceptible cattle to the virus, making identification and removal of PI animals a critical component of effective BVD control.
As Rhodes summarizes: “We’ve got to do testing. We have to have biosecurity, and that biosecurity also includes vaccination.”
The experience ultimately changed how the research herd viewed BVD surveillance. Testing replacement cows alone wasn’t enough. The calves they carried represented a second introduction into the herd, one that routine protocols hadn’t fully accounted for.
As Rhodes reflects after discussing the case: “Experience is a brutal teacher, but you really learn.”
