Evidence is mounting that modified-live IBR vaccines are causing abortions and unsettled cows.
In 2004 the FDA approved the labeling of MLV IBR vaccines for use in pregnant cows after the cow had been vaccinated with the same MLV prior to breeding. This two-part labeling instruction is very specific and was intended to protect the cow from abortion. However, researchers are now finding a first MLV vaccination given to open cows does not prevent the second MLV vaccine from causing abortions.
Kerry Barling, a veterinarian from Iola, Texas, says he was seeing and hearing of increased IBR-related abortions, so he began talking up the need for a study of the issue. As a result he obtained and analyzed the numbers from university veterinary diagnostic labs in Kansas, South Dakota, Minnesota and Nebraska.
Barling found the four labs reported a total of 235 IBR abortions from 2000 to 2009. Of those, 71% happened from 2005 to 2009, after the labeling change. To be included in this study, each case had to test positive for IBR in one of four methods.
Over the entire 10-year study period, 86% of IBR-related abortions in vaccinated herds came from herds with a history of recent MLV IBR vaccination. After the labeling change, from 2005 to 2009, 62% of IBR-related abortions in vaccinated herds came from cows vaccinated with MLV IBR during gestation. Another 25% were from cows vaccinated with MLV IBR, but with unknown timing.
This study proves the epidemiologic association that modified live IBR vaccines given to pregnant cows increases the risk of abortions.
Barling says the veterinary community has known since 1968 that IBR modified live vaccines cause abortions by destroying the corpus luteum so this was certainly a risky option in the first place.
"You're giving an abortifacient agent to a pregnant cow," Barling says. "The bovine species is the one of the few if any animal species that has a label for MLV use during pregnancy. MLV's are harmful to the fetus. The developing bovine fetus is no exception."
IBR is well known as a respiratory disease in stocker and feedlot cattle but this role as an abortion-causing agent is less well understood, says Clinton Jones, a biomedical professor at the University of Nebraska-Lincoln.
In fact, Jones says, both the virulent strains of IBR and the modified live vaccine strains of IBR go into a form of "latency" -- more or less a hibernation -- after the animals' bodies overcome them. The animals will carry these latent organisms the rest of their lives and the IBR can be awakened from latency and become active again by a stressful event such as pregnancy or a immune suppression.
In addition, a rechallenge by MLV vaccination immediately prior to pregnancy or during pregnancy may lead to virus spread into reproductive tissue such as the corpus luteum. At that time the "infected" animals begin to shed live virus that can infect other animals and cause illness or lowered performance.
Further, Jones says, every exposure to modified live IBR vaccines may increase the number of neurons in an animal's body which are latently infected. In turn, that increases the chances of reactivation.
Jones says modified live IBR vaccines should not be given to pregnant cows or newborn calves.
The danger is well illustrated in a herd of 55 Angus heifers at the University of Wyoming, says Donal O'Toole, university veterinarian. These heifers were given MLV IBR vaccines prebreeding and at 7 months gestation. This was a closely watched herd in excellent health and condition. They had a total 25% pregnancy loss and 10% of the loss was never noticed until they simply were not pregnant at due date. There was never any sign of a fetus from these animals, despite close observation.
This is common, O'Toole says, since the corpus luteum is damaged by the IBR virus. Many of these pregnancy losses happen very early.
Doug Scholz, director of veterinary services for Novartis Animal Health said two dairies in Colorado provide another case study.
One of these dairies was posting a 23% pregnancy loss using MLV IBR. When the managers of that dairy simply switched to a killed IBR vaccine their abortion rate dropped to 8%, Scholz says.
The management of the other dairy was posting a 29% pregnancy loss and took a more radical course of action. It decided to vaccinate all animals two times with a killed vaccine, even though the animals in lactation would drop some production from the reaction. That herd afterward dropped from 29% abortion rate to 10%. It also improved conception rates by 15%.
Jones says if modified live vaccines were engineered so they would not reactivate from latency they might be safer.
In the end, these veterinarians and researchers say, there is now no reason to use a modified live vaccine in breeding cattle. Killed vaccines give excellent response and protection and are much safer and less costly. Modified live vaccines should be reserved for use in terminal cattle, such as in feedlot situations.
Data from Colorado State University's veterinary diagnostic lab showed a tremendous increased in IBR-caused abortions in 2007 through 2009.