Disease control: Vaccine causes concern
With increased use of modified live vaccines in pregnant cattle, some researchers have concerns about possible adverse reactions.
UW Veterinary Science Professor Donal O’Toole noticed that the vaccines may be linked to abortions.
“In one instance, heifers in the University of Wyoming were appropriately vaccinated prior to breeding with a modified-live vaccine and then vaccinated again at seven months’ gestation,” notes O’Toole.
Abortions began 30 days after vaccination, resulting in a reproductive loss of approximately 25 percent. The cause was the infectious bovine rhinotracheitis (IBR) virus. At that time there was no test to tell vaccine from wild strains.
“What we found at the Wyoming State Vet Lab when we started looking into episodes like this was an association between abortion and recent use of modified-live IBR vaccines. This was despite the fact that the products were administered following label directions,” O’Toole explains.
Fetuses from the heifers in the UW herd had typical lesions caused by IBR. There was no evidence of other causes of abortion in the animals.
“The conclusion my colleagues and I came to was that modified-live IBR vaccines occasionally cause abortion when given to pregnant cattle,” O’Toole says.
Vaccinating for IBR
In order to control IBR, producers have the option of using two major types of vaccines – a modified live virus and killed virus. Recently, changes in vaccination licensing have increased the use of modified live vaccines.
“Several vaccine manufacturers received licenses from the USDA so that ranchers can vaccinate pregnant cows,” explains O’Toole of the modified live vaccinations. “It is a big advantage for producers to be able to vaccinate year-round – not just when cows are open.”
However, O’Toole notes that several researchers have concerns about utilizing the vaccines on pregnant cows.
“Sporadically, we see abortions occurring one to two months after vaccination with modified live IBR products. This is the same time interval that wild type IBR virus causes abortion after a natural infection,” he explains, referencing the episode in UW’s beef herd. “They have seen this at Colorado State University, in South Dakota and in Texas.”
“It looks like these abortions may be linked to the vaccination,” O’Toole adds.
An article was published in the Journal of the American Veterinary Medical Association pointed out an increased number of IBR abortions have been seen since the introduction of modified live vaccines.
“A total of 85 fetuses have tested positively for BHV-1 from Jan. 13, 2002 through May 14, 2009,” reads a narrative by the Colorado State University Veterinary Diagnostic Laboratory, referencing the organism that causes IBR. “In contrast, one IBR abortion was diagnosed in 1998, and none in the years of 1999-2001.”
Licenses for use of modified live vaccines in pregnant cattle emerged beginning in 2003.
Until very recently, the problem has been that it was impossible for laboratories to distinguish between wild type IBR abortions and a potential vaccine-induced abortion.
“It could have been a coincidence – the cattle could have been infected with wild type IBR around the time of vaccination,” he says. “However, we are seeing more IBR abortions in the years since these products came on the market for use on pregnant cattle. The episodes are sporadic, and we don’t understand what the variable is.”
While O’Toole says there may be a link between abortions and the use of modified live vaccines in pregnant cattle, he also notes that off-label vaccine use may also be a factor.
“Producers need to follow the label instructions,” he comments. “Cattle must have several shots of that same vaccine line first, before it is safe to give modified live IBR virus to pregnant cattle. The modified live vaccines shouldn’t be given to pregnant animals unless they have received several shots before.”
Exposure to the vaccine prior to pregnancy is important, says O’Toole, who adds, “Folks are busy. They sometimes forget what products they’ve given previously. We’ve definitely seen producers and veterinarians inadvertently use these products off-label and cause abortion as a result.”
In order to determine whether IBR abortions result from the modified live vaccines or a wild type infection, Oklahoma State University professor Bob Fulton has developed a method to identify vaccinal IBR virus in tissue.
“Until now, all we could say was that the abortion was due to IBR, period,” O’Toole mentions. “We weren’t able to distinguish wild type from vaccine virus.”
At the same time, due to questions being asked by researchers in South Dakota, Texas, Colorado and Wyoming, Pfizer conducted a study to see if the incidence of abortions increased since modified live vaccine use in pregnant cattle had increased.
“It was a pretty good study,” comments O’Toole. “They did detect that IBR abortions have gone up slightly, but they had not gone through the roof.”
O’Toole partnered with a dairy practitioner, Robert Corbett of Utah, and sent a letter to the Journal of Veterinary Diagnostic Investigation about high rates of cows returning to service after vaccination with a modified live vaccine.
In their letter, O’Toole and Corbett note that to determine whether vaccination with the modified live vaccine is positively associated with reproductive losses, there are two options: testing aborted fetal tissue or making observations after discontinuing use of the vaccine.
In one particular dairy herd that was vaccinated with modified live vaccines, they write, “The rate of abortions decreased after live [vaccine use] discontinued.”
While concerns have been seen, O’Toole encourages producers that there is no proof of a link between abortion and modified live vaccines that are used properly.
He also cautions, however, that producers who vaccinate with modified live vaccines and see IBR abortions should be aware of testing options through Oklahoma State University and Fulton’s innovative technique.
“We want to get the word out to diagnosticians and veterinarians,” O’Toole comments. “The real safety test of any vaccine is what happens when it is used under commercial conditions.”
Etiology of IBR
Infectious bovine rhinotracheitis (IBR) is due to a herpes virus that infects cattle. The two main complications from IBR are respiratory disease and abortion.
“The IBR virus belongs to the same family of agents that cause cold sores in people. Some ranchers know it as Rednose,” explains O’Toole. “It causes a respiratory tract infection in cattle, typically feeder animals. It can kill cattle due to direct effects on animals’ airways or as a result of secondary infections leading to pneumonia.”
O’Toole continues, “Another form of the disease occurs when it crosses the placenta and causes abortion or infects ovaries leading to fetal resorption early on in gestation. This used to be relatively common in the 1970s and 80s, until people began vaccinating for IBR when cows were open.”
With several cases and herd outbreaks each year, O’Toole notes that cattle abort one to two months after exposure to the virus.
“We see typical changes in the fetus,” he explains. “It is usually dead and rotten, but has characteristic liver damage. Laboratories use several tests to confirm that abortion was due to the IBR virus.”