Month: February 2024

Choosing Modified-Live or Killed Vaccines For Calves

By Heather Smith Thomas

There are many commercial vaccines available to protect cattle against infectious agents. Some are modified-live vaccines and others are killed products. It’s not always easy to decide which are best when vaccinating calves.

Dr. Donal O’Toole, a pathologist at Wyoming State Veterinary Laboratory, University of Wyoming, says producers use many different products but his preference, at least for viral agents, is to use a modified-live vaccine—when it is safe to use modified-live products.

“If you give a vaccine by a natural route, such as an intranasal vaccine for respiratory viruses, the modified-live product has advantages. With a killed product, the amount of antigen an animal receives is basically just what is in the injection. When using an attenuated modified-live vaccine the antigen will expand in the animal’s body, and also moves to some of that agent’s target sites, such as the lungs. It essentially gives animals a mild and generally subclinical infection,” he says. This triggers more response within the body, and stronger immunity.

“Another advantage, especially when using intranasal viral respiratory agents, is that it by-passes the effect of maternal antibodies,” says O’Toole. In young calves, many vaccines do not stimulate a strong immune response because they still have passive immunity from the dam (antibodies in colostrum). Those antibodies in the calf neutralize most or all of the antigens in the vaccine, so the calf’s immune responses are minimal.

Most vaccines are ineffective in calves until colostral antibodies (passive transfer) wanes, but the intranasal modified-live vaccines can be given at a younger age, with good results. If a producer is having respiratory disease in young calves, this would be the best option, to try to prevent problems.

“In terms of using modified-live vaccines, the only ones I have reservations about are the IBR products against bovine herpes virus one (which causes ‘red nose’) just because we have seen abortions caused by this virus. It may be shed by vaccinated calves and the vaccine virus gets back to pregnant cows if those calves are still nursing their mothers. Most of these abortions occur at 30 days with some strung out as late as four months after exposure. This makes it harder to figure out an association between the vaccine and the abortion, unless we ask if the producer used modified-live IBR vaccine,” says O’Toole.

“If there is a chance that the pregnant cow does not have robust immunity to IBR, it’s a questionable practice to give nursing calves modified-live IBR vaccines. “We published several papers on this. I don’t have a good feel for how or why we see bovine herpes virus one abortions associated with vaccine strains on such a sporadic basis. There must be additional factor(s) that we don’t understand. Abortions due to vaccine IBR strains are relatively rare, but at our laboratory we see one or two each year,” he says.

Killed vaccines are safer, but need more booster shots to be fully protective. “When producers use killed products, they give a primary shot followed by a booster two or three weeks later. This involves handling the cattle again.” If producers know they won’t be able to give a booster, it’s probably best to use a modified-live vaccine. They should talk with their veterinarian when making a plan for a vaccination program.

Vaccinations given to calves during the first months of life often serve as a priming vaccine. Vaccinations given at weaning then act as a booster to further stimulate the immune system to ensure adequate protection.

“On cow-calf operations pneumonia in young calves is common. Most respiratory diseases are due to viral agents. These are BRSV, PI-3, IBR (bovine herpes virus one) and BVDV. Some producers vaccinate against these. There is variability among producers in whether they also use bacterins for bacterial agents such as Mannheimia and Pasteurella multocida. Personally I prefer that people vaccinate for those as well as viral agents, since bacterial agents complicate most cases of calf pneumonia and are often the actual cause of death,” says O’Toole.

Most modified-live vaccines against respiratory agents are for viruses, but there are a few modified-live bacterial vaccines. Your veterinarian can guide you on these choices, and whether to use the killed or modified-live products.

Trying to select a “best” vaccine is difficult because so many products are available. “Veterinarians can advise you, but their experience varies, and they will differ in their advice to clients. If a veterinarian has a wreck with a certain product, especially if it happens more than once, he or she generally won’t touch that product again. I teach a course to veterinary students, and they often ask which products to use. They want me to tell them which vaccines are the most reliable ones on the market, but given the large number of licensed products for individual agents, it’s not that simple,” says O’Toole.

“There are probably 90 products available for IBR, and another 150 for BVD. There is no way I can recommend a specific product. Each veterinarian has to try to keep up with what is going on with biological products and use his or her best judgment. I think it’s wise to consult with a veterinarian who knows which diseases circulate in his/her practice area and ask which products give the best results over time. It would be nice if we only had to choose between five or 10 products, with different strains of the pathogens, and formulations, but that’s not the case. It’s hard for producers to make these choices. It’s hard enough for clinicians to try to figure it out. My suggestion is to ask a veterinarian with a few gray hairs, who has seen many things, for that advice.”

Dr. Russ Daly, Extension Veterinarian/ Professor, Department of Veterinary and Biomedical Science, South Dakota State University says there are some vaccines that have to be modified live (not killed) such as the intranasal vaccines. “You don’t have a choice there, because there are no killed intranasal vaccines. And if you are giving toxoids like seven-way or eight-way clostridial vaccines (or C. perfringens C and D toxoid) these are only killed vaccines,” he says.

“Where we do have a choice for calves is with the viral diseases like IBR, BVD, BRSV or PI-3. There are some killed and some modified-live vaccines for those. When talking about bacterial pneumonia, and pathogens like Mannheimia and Pasteurella there are some live versions of those vaccines as well as killed,” says Daly.

Any time a vaccine is approved for sale and use in the U.S. it has to go through a testing and approval process. The manufacturer must be able to show that the vaccine actually has some benefit and is safe. “The vaccine has to stimulate the immune system. For some vaccines, a good deal of research exists to show that this stimulation results in disease reduction in vaccinated animals. For other vaccines, the research may simply show that vaccinated animals have a higher antibody titer than the unvaccinated animals and this is enough to get USDA approval,” he explains.

“Any vaccine should be expected to stimulate the immune system of the calf, if administered appropriately as per label directions. There are several different things to consider however, when trying to decide between modified-live and killed vaccines.

First, we want to look at efficacy, and what we expect that vaccine to do in the calf. Modified-life vaccines tend to stimulate the immune system in a different way than killed vaccines do. The immune system responds to modified-live vaccines similar to the way it responds to the actual viral infection. The immune response is directed more toward cell-mediated immunity, which is very important in protection against viral infections. From an efficacy standpoint, it makes sense to consider using modified-live vaccines when trying to prevent a viral infection,” says Daly.

“This does not mean that killed vaccine can’t do the same thing in terms of prevention and stimulating the immune system, but the body’s response to killed vaccines is weighted more towards antibody production in the animal. Antibodies focus more on the pathogens outside the body’s cells, such as bacterial infections or early viral infections. This difference can help us understand a little more in terms of which choices we should make, from a standpoint of effectiveness,” he says.

“The next thing to consider would be convenience. Because modified-live vaccines often don’t have a strict requirement for boosters, you can use them in some situations where a killed vaccine might be difficult to implement.” If you only have one opportunity to vaccinate calves and won’t be able to run them through again later for a booster, or might be delayed on getting them in again (during the proper time frame) for their next shot, it might be wise to use a modified-live vaccine.

“If killed vaccines were used pre-weaning, a booster is critical to provide for a good immune response. If modified-live vaccine were given, the need for (and timing of) that booster is not as important,” Daly explains. Thus the modified-live vaccines tend to give a little more flexibility.

Another factor related to flexibility in vaccine choice has to do with handling and storage requirements of modified-live vaccines, which are less forgiving than killed vaccines. While both types must be kept at refrigerator temperatures up to the time of administration, modified-live vaccines must be used promptly after being mixed.

“The next thing we worry about is safety. Killed vaccines, by their nature, can never cause the disease we are trying to prevent. With live vaccines, though they have been attenuated, there is a chance that the virus can replicate in the animal’s body and cause the disease we are trying to prevent. Approved modified-live vaccines have been tested to assure that, in normal calves, this won’t be a concern. The vaccine company tests the vaccine by administering it to calves at a dose that is several times higher than label dose, to make sure it can’t revert to the disease form. This safety factor means that in most cases we don’t have to be concerned about modified-live vaccines causing the illness they are trying to prevent,” Daly says. This level of comfort may not be present, however, when modified-live vaccines are given to animals that are stressed or have a compromised immune system for some other reason.

“The final thing to consider is cost. The modified-live vaccines tend to be less expensive than the killed vaccines. Each producer must look at all the pros and cons to determine which vaccine best fits his own situation. A conversation with their veterinarian can guide them the best way possible. There are choices, and it’s not a situation where one is always better than the other. Either type of vaccine should be expected to be helpful in stimulating the calves’ immune systems.”