The research from South Dakota State University investigated the effects of vaccination type in conjunction with first-induced, timed-artificial insemination (A.I.) protocols. The findings showed synchronized naïve beef heifers vaccinated with a five-way MLV were more likely to have abnormal estrous cycles. This same group of heifers also had significantly lower pregnancy rates than those vaccinated with a well-adjuvanted, inactivated vaccine (INV).

“The vaccination of naïve heifers with an MLV containing infectious bovine rhinotracheitis (IBR) at the start of a fixed-time A.I. protocol had a negative effect on pregnancy success,” says Chris Chase, DVM, MS, PhD, professor at South Dakota State University and coauthor of the study.

“In the end, we had less than 50 percent of animals in the MLV group conceive, so the abnormal cycles were quite high,” he says.

Key study findings
The group of heifers receiving an MLV prebreeding experienced a 48 percent pregnancy rate versus 86 percent in the INV test group. In addition, heifers that experienced an abnormal estrous cycle following MLV vaccination had only a 38 percent conception rate. Those results were attributed to the detrimental effects MLVs can have on fertility for subsequent ovulations.

“The bottom line is that the risks are higher with MLVs,” Chase said. “Their effect on reproductive loss can be very significant in terms of both abortion and early embryonic death where producers might not see what is going on.”

Evaluate the risks
Similar conclusions were drawn from an investigation into the onset of abortions in a University of Wyoming beef herd following administration of MLV vaccines. That study directly implicated MLVs with the 25 percent abortion rate and pregnancy loss, even though the vaccines were administered in full accordance with label directions.

Product safety labels were revised in 2004, declaring some modified-live IBR vaccines safe for use during gestation and prebreeding. However, many producers who have experienced abortions and decreased conception rates following the use of MLVs question their safety. The study published in Theriogenology was sponsored by Novartis Animal Health.

“We’ve known for a long time that MLVs containing IBR virus can cause abortion,” said Doug Scholz, DVM, director of veterinary services, Novartis Animal Health. “But this most recent study underscores the risks of decreased conception rates with MLVs.”

Scholz suggests producers reduce the risks by replacing MLVs with inactivated vaccines that protect against IBR, BVD and other important diseases. These vaccines pose no risk to gestating fetuses and 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.

Click here to access the study “The Effects of Vaccination on Serum Hormone Concentrations and Conception Rates in Synchronized Naïve Beef Heifers.”

All vaccinations of the 59 crossbred heifers in this study were administered at the appropriate time across four test groups. Estrous cycles were synchronized at the time of vaccination and heifers were bred by A.I. All conception rates were then calculated following breeding and pregnancy diagnosis.

—Novartis Animal Health news release