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Unintended Consequences:
Could MLV Vaccines Be Harming Reproduction?

DES MOINES, Iowa (Sept. 8, 2016) — Perhaps the two most prominent infectious agents implicated in reduced reproductive performance of beef breeding herds are infectious bovine respiratory virus (IBRV) and bovine viral diarrhea virus (BVDV). Both have been linked with reproductive failures, including ovarian and estrous cycle dysfunction, fetal infection and pregnancy loss. Two types of vaccines for immunizing cattle against IBRV and BVDV are widely available, but some controversy exists as to whether vaccines containing modified life virus (MLV) pose a risk to reproduction, as compared to products containing inactivated or “killed” virus when administered to breeding females.

South Dakota State University Extension veterinarian Russ Daley shared findings from research related to the effects that prebreeding vaccination with MLV vaccines may have on reproduction.

As a presenter during the Applied Reproductive Strategies in Beef Cattle symposium, hosted Sept. 7-8 in Des Moines, Iowa, veterinarian Russ Daley was charged with addressing the alleged unintended consequences of using MLV vaccines. The South Dakota State University Extension veterinarian shared findings from research related to the effects that prebreeding vaccination with MLV vaccines may have on reproduction.

Daly described studies involving naïve (not previously exposed or vaccinated) heifers that, when vaccinated with MLV product, exhibited negative effects such as ovarian damage, abnormal estrous cycles and lower pregnancy rates. Results also suggested that because of effects on the estrous cycle, hindered reproductive performance can extend beyond the first cycle.

“I think the take-home message is clear. Don’t give naïve heifers MLV at breeding time,” warned Daly, adding that no problems should result from using MLV vaccine, administered 30 days prior to breeding, on heifers that were previously well-vaccinated.

Daly also shared results from studies applying different prebreeding vaccination intervals which indicate that in well-vaccinated females, MLV vaccines may be used nearer to breeding time than directed by product labels, without significant negative effect. Thus, vaccinations could be given at the time that an estrus-synchronization protocol is initiated, without fear of negative impact to reproduction.

However, Daly advised producers that an increasing amount of emerging evidence suggests that MLV vaccines, even when given at labeled prebreeding intervals, may negatively affect reproductive parameters compared to cattle vaccinated with killed virus vaccines. The documented differences in reproductive performance between MLV-vaccinated cattle and those vaccinated with killed vaccine are not very large — some are statistically insignificant — but differences exist.

“It appears there may be something subtle going on,” said Daly, who advised producers to consult their veterinarian to develop vaccination programs incorporating the best type of vaccine for their individual operations.

Daly spoke during Thursday’s ARSBC session focusing on health and well-being. Visit the Newsroom at, which features comprehensive coverage of the symposium, to view his PowerPoint, read the proceedings or listen to the presentation. Compiled by the Angus Media editorial team, the site is made possible through sponsorship by the Beef Reproduction Task Force.

Editor's Note: This article was written under contract or by staff of Angus Media. To request reprint permission and guidelines, contact Shauna Rose Hermel, editor, at 816-383-5270.