There are many commercial vaccines available to protect cattle against infectious agents. Many producers use a combination product for BVD (bovine viral diarrhea) and IBR (infectious bovine rhinotracheitis) and there are inactivated (killed) and modified-live virus (MLV) vaccines.
Dr. Chris Chase, professor in South Dakota State University’s Department of Veterinary and Biomedical Sciences, says that when a producer has naпve open females (that have not been vaccinated before) or heifers to go into the breeding herd, it’s best to use MLV vaccines because they mimic the actual infection. “The virus replicates in the body so the immune system must interact with it,” he explains.
“When we use a vaccine that’s attenuated (modified), the virus isn’t as strong as it is in the field (and therefore shouldn’t cause the actual disease), but the immune system becomes fully activated. When we use inactivated (killed) vaccines they can’t mimic the natural infection.
“There are certain proteins that are only expressed when viruses divide and multiply. Though the killed vaccine products are good, with a lot of antigen, they may not contain all the intermediate proteins. Whatever response the immune system produces during an actual infection won’t happen with a killed vaccine,” says Chase.
“The MLV vaccines are less costly than inactivated vaccines because they contain less antigen. They depend on the animal making antigen from the MLV in the vaccine. With killed vaccines the antigen is already all there. There are adjuvants included, to protect the vaccine against pre-existing antibodies. If the animal already has an immune response, there is oil or some other adjuvant to protect the antigen so it can still interact with the immune system and get a booster response,” he says.
We generally use MLV vaccines to get young animals off to a good start because they are very effective, but there can be safety issues for breeding-age females.
“The vast majority of cows in a herd that have been previously well vaccinated, if you boost them with MLV vaccine, they have an immune response already and the virus doesn’t get a chance to replicate so it doesn’t get to the bloodstream—and therefore does not get to the ovary or uterus. But if there’s a cow that wasn’t properly vaccinated or her immune system isn’t working very well (perhaps due to stress or nutritional issue) she may be at risk from the vaccine virus. We vaccinate cows to protect them while they are pregnant, but if the virus can reach the ovary or the fetus, it may cause problems.”
The IBR virus is notorious for getting into the bloodstream. “If the cow is not yet pregnant and the virus gets into the ovary, it can damage the corpus luteum and change that particular cycle. She won’t settle because there is no corpus luteum to maintain the pregnancy. In open animals that you’re trying to get bred, don’t use an MLV vaccine very close to the time you’d put a bull with them or synchronize for AI. Give these vaccines at least a month before these animals would be bred,” Chase says.
By contrast, studies show that a killed vaccine that’s well adjuvanted can be given within a week prior to artificial insemination, with no adverse effect on conception rate. In one of his studies he had a control group that was given MLV, and their conception rate was only 48 percent. “We knew this would happen, but wanted to show that an adjuvanted killed virus did not have a negative affect when administered as part of an estrus synchronization program. A new finding was that naпve heifers receiving MLV vaccine had a high number of abnormal cycles and were less likely to get bred on the next estrous cycle.” This fact reinforced the practice of vaccinating at least a month or two before breeding.
“If you vaccinate a month or two before you put the bulls in, if there are a few animals in the herd that don’t already have some immune protection and MLV vaccine virus reaches the reproductive tract, they’ll have a chance for their reproductive cycle to recover.” The ovaries will be back to normal; they can conceive and keep a pregnancy.
When vaccinating for viral reproductive disease, he says that use of MLV vaccines is absolutely imperative in developing heifers. “After that, you can decide what to give them after they’ve calved,” he says.
“Vaccination of the cow herd with MLV is hopefully done after they’ve calved. But in some herds the only time they are handled may be for preg checking, so that’s when they are vaccinated. We generally see little to no issues with MLV vaccines in pregnant animals that have been well vaccinated,” Chase says.
If you are using these vaccines for BVD protection against producing persistently infected (PI) calves, there’s no question that vaccination must be done prior to breeding. “When using the combination vaccine (IBR/BVD) this is the only time it should be given to prevent PI calves. Vaccinating pregnant animals doesn’t do any good for preventing PI in the fetal calf for that pregnancy.”
Some people have had problems using MLV vaccines in pregnant cattle, with a higher-than-normal rate of abortions. “If every heifer got vaccinated at a young age like they were supposed to, they probably wouldn’t have any trouble,” says Chase.
There is a safety claim for MLV viral vaccines for pregnant animals. “These studies were done under optimal conditions. Those animals were on a good vaccination program and a good nutritional plane. Every animal used in those studies was verified that they were vaccinated earlier—before they were vaccinated during pregnancy. So yes, under these conditions I think the MLV vaccines are safe,” says Chase.
Under field conditions, not all animals mount the same immune response to vaccination, and some don’t respond. Those animals may not be safe to vaccinate with MLV vaccines, even if you’ve followed label directions, and vaccinate all the animals the same. If for some reason a few animals in the herd have less immunity, they may be at risk if you give them the MLV vaccine during pregnancy. — Heather Smith Thomas, WLJ correspondent
