A University of Wyoming herd of replacement heifers lost 25 percent of its calf crop to abortions and lost pregnancies after vaccinating pregnant heifers with modified-live vaccines (MLV) containing infectious bovine rhinotracheitis (IBR) virus, according to an article recently published in the Journal of the American Veterinary Medical Association (JAVMA).

A case report detailing the onset of abortions in the university-owned and well-managed herd was featured in the Pathology in Practice section of JAVMA, which spotlights diagnostic case studies. The episode marks the latest piece of evidence implicating MLV IBR vaccines in cases of abortion and pregnancy loss, even when those vaccines are administered in full accordance with label directions.

Donal O’Toole, MVB, PhD, professor, Department of Veterinary Science, University of Wyoming, authored the case study for JAVMA. “Diagnosticians now have enough experience with these episodes to say that post-vaccination abortion can occur following on-label use of vaccines with MLV IBR virus,” he said. “It is not just a problem when MLV IBR vaccines are used off-label, as has been known for years.”

Veterinary diagnostic labs in several states have reported increases in abortions in cows and heifers with a history of MLV use since the vaccines were approved for use in pregnant cows in 2004.1,2

In a separate study tracking abortion submissions to university diagnostic laboratories in Kansas, Nebraska, South Dakota and Minnesota between 2000 and 2009, abortion cases more than tripled from 2004 to 2005, the first production year following the label change and widespread use of MLV vaccines in pregnant cattle.

“The connection between IBR MLV vaccines and abortions is not a new discovery,” said Kerry Barling, DVM, of Bull Creek Veterinary Services in Iola, Texas. “Veterinarians became aware of pregnancy losses related to MLVs when the vaccines were first introduced in the 1960s. There’s a substantial body of scientific literature that document the risks of pregnancy loss with modified-live vaccines.”

According to Barling, who compiled the diagnostic lab data, “the increase in abortions since 2005 may be due to incorrect adoption of MLVs on cattle that had not been vaccinated before, but it’s looking more like there’s an inherent safety issue with the IBR fraction of MLVs for use in pregnant animals.”

JAVMA Case Study – University of Wyoming Herd

A group of 55 healthy, pregnant Angus-cross heifers owned by the University of Wyoming were vaccinated according to label directions with a MLV vaccine at 7-8 months of gestation. The vaccines were administered in accordance with label directions, eliminating the possibility abortions were caused by improper administration.  

Pregnancies were confirmed by ultrasound at vaccination and all heifers had received a MLV pre-breeding. Previous MLV vaccinations had occurred in May and October 2009.

According to the report, “Abortions began 32 days post-vaccination. Diagnostic tests were conducted on six of seven aborted fetuses and one heifer that died two days after developing signs of abortion. Six heifers were found to be open at the end of the trial. The loss of 14 pregnancies among 55 heifers equaled a 25.4 percent reproductive loss for the herd.”

Diagnostic tests found numerous presentations of IBR pathology as well as BoHV-1 antigens present in all aborted fetuses. No antigens of BVDV were detected in aborted fetuses and there was no evidence of other infectious ailments.

The potential for MLV IBR vaccines to cause abortion isn’t the only issue that raises questions, said O’Toole. “I also have concerns that MLV IBR virus may infect the ovaries, particularly corpora lutea, and compromise early pregnancy.”

Safety Issues Evolve

Doug Scholz, DVM, director of veterinary services, Novartis Animal Health, said the high incidence of IBR abortion in the United States decreased in the 1980s and 1990s once veterinarians realized the risks of using MLV IBR vaccines in pregnant cattle. As a result, USDA required MLVs to carry a “do not use in pregnant cows or in calves nursing pregnant cows” label restriction.

In 2004 the labels were revised again stating their safety for use during gestation, with certain strict requirements for vaccinating pregnant cows with MLVs. The revised label requires that animals must be treated with the same MLV pre-breeding to build IBR immunity in the dam so that the subsequent vaccination during pregnancy will not harm the fetus.

According to Scholz, many cattle veterinarians and producers who have experienced abortions following MLV use are now questioning their safety.

“We’ve long known that modified-live vaccines (MLVs) containing IBR virus can cause abortion,” he said. “The research published in JAVMA underscores the risks of using an MLV in pregnant cows because the protocol strictly adhered to label guidelines but still experienced a 25 percent pregnancy loss.”

The abortion data make a compelling case for reducing risk of losing pregnancies by replacing MLVs with inactivated vaccines, such as Vira Shield®, that protect against IBR, BVD and other important diseases and pose no risk to gestating fetuses.

Vira Shield will not cause IBR abortions because the virus has been inactivated and cannot revert to virulence. It has been proven safe over many years and can be used at any phase of the estrous or gestation cycle, regardless of the animal’s vaccination history.

“Choosing Vira Shield for breeding or pregnant animals protects investments in reproductive programs, supports sustained pregnancies and eliminates undue risk to calf crops,” Scholz concluded.

To learn more about Vira Shield and cattle health management tips that drive maximum performance, visit www.virashield.com, www.youtube.com/cattletalk or contact your Novartis Animal Health representative.


  1. IBR Abortions on the Increase, Lablines, Colorado State University Vol 14, No 1, Spring/Summer 2009
  2. Isolation of Infectious Bovine Rhinotracheitis Virus from Clinical Cases of Multiple Abortions in Recently Vaccinated Herds; Texas Veterinary Medical Lab Annual Report, 2010