Though building herd immunity is neither predictable nor precise, it's a whole lot easier than measuring it.
Sure, you could bleed cattle and check for antibodies to specific antigens, but antibodies indicate exposure, not protection, says Chris Chase, DVM, PhD, a South Dakota State University professor in veterinary science.
“Unless you measure cell-mediated immune response, it's difficult to get a good handle on overall herd immunity,” Chase says. Such tests exist, but they're expensive and impractical for day-to-day beef production.
If a producer was bent on being more scientific, Chase says he could use antibody testing to narrow the window of when cattle might most likely respond to a particular vaccine, and to determine which antibodies exist for which antigens. Such a program could be applied to a sample of cattle fairly economically — $150 or so. But, he says, that only tells you part of the story.
“If you were to use antibodies to project when to vaccinate, you'd miss a large part of the picture,” agrees Jim Roth, DVM, PhD, director of the Center for Food Security and Public Health in Iowa State University's college of veterinary medicine. Antibodies get at humoral immunity but say nothing about cell-mediated immunity (CMI), which has as much or more to do with overall immunity.
CMI increases the activity of phagocytic cells and certain types of lymphocytes which increase an animal's resistance to infection.
Conversely, humoral immunity revolves around antibodies that fight against particular antigens. By and large the active agents in vaccines are antigens that hopefully induce the immune system to build antibodies and CMI against them.
So, Roth says, “Rather than spend money to measure immunity, my advice is to follow an effective management and vaccination program that allows an animal's immune system to develop and work.”
Set the stage for immunity
With that in mind, Chase and Roth explain that while vaccines are a key to building herd immunity, the only way they can work is if the immune system is developed and functioning properly.
That means making sure pre-vaccinates are on a positive nutritional plane, including vitamins, trace minerals and no mycotoxins (from mold) in the feed. It also means ensuring the animal's immune system is free to concentrate on the antigens in the vaccine.
“As we've learned more about CMI and regulation of the immune system, we've found when an animal is parasitized (internal or external), their T-cells (key to CMI) shift to respond to the parasites,” Roth explains. “They do so in a different way than what's needed to respond to viruses. So, you might not get the vaccination response you want. Ideally, get the parasites under control first, then vaccinate a couple weeks later.”
Similarly, Roth believes working with your veterinarian to identify and remove animals persistently infected (PI) with bovine viral diarrhea (BVD) virus is an essential step in allowing immunity in the rest of the herd to develop and function.
“Testing for and removing PI animals will do more good than anything,” he says. That's because the BVD virus clobbers immune systems to the point they can't respond to other infections. And, PI cattle shed the infection throughout their lives.
Incidentally, Chase favors using two or three different BVD virus vaccines and a non-cytopathic one prior to breeding in order to increase cross-protection provided by the strains in various vaccines.
Keep in mind, anything adding stress to an animal decreases the odds of vaccination success — including administering the vaccine.
“Any time we stick a needle in a calf there is a negative associated with it,” Chase says.
Whether in the form of reduced performance, or increased stress, the cost of vaccination runs beyond the dollars paid for the vaccine and the labor to apply it. That's why Chase believes in vaccinating strategically for health challenges that either exist in the herd or in a particular geographic region.
Roth emphasizes vaccination in itself is a stress.
“Optimally, you wouldn't be vaccinating animals during other high-stress times in their lives, such as at branding or weaning time,” he says. If you do, you should expect less response than if you vaccinated during a low-stress time.
Likewise, Roth says producers' best intentions can interfere with the steps to build herd immunity. For instance, since producers want to do things the best way, they may give all of the vaccinations at the same time, thinking it's the most effective way to provide broad protection quickly. But stress can reduce response.
“It's better to give vaccines a few at a time and to spread them out,” Roth says.
Chase agrees, “I think we have a tendency to over-vaccinate rather than vaccinating strategically for a problem within the herd. I only want to strategically vaccinate for the risks I see in my herd.”
Besides avoiding stress, Chase explains an animal's immune system needs time to recognize antigens, respond to them with antibodies, and mature their immune response against the antigen. That's why he emphasizes booster shots be given no sooner than two or three weeks after vaccination.
After all, he explains, boosters have nothing to do with increasing the immunity of animals whose immune systems have already responded to the antigen. Instead, boosters are aimed at increasing the number of animals whose immune systems do respond.
The reasons cattle may not respond to vaccination at a particular time are multi-fold. It could be the vaccine's antigen is a different strain than what ends up infecting the animal. Perhaps the animal's immune system is unable to respond due to any of the reasons mentioned earlier, or a calf is still carrying sufficient maternal antibody that it won't respond to vaccination.
For that reason, Chase is against vaccinating pregnant cows even if the label says it's okay. On one hand, he says it can make it difficult to know when the calves will respond to vaccination. He explains advocates of the practice often cite the need to increase passive immunity — the immunity a cow will pass on to its calf — but Chase says in young calves the most common health challenges are digestive.
Moreover, Chase is afraid vaccinating pregnant cows can offer producers a false sense of security — believing they're getting some protection when the damage may already be done.
In short, Roth says the best time to vaccinate animals is when they're healthiest and the most stress-free.
Replacements a prime target
With the basics in place, both veterinarians say replacement heifers should be a focus of building herd immunity.
“Develop a nutrition and vaccination program so you're doing the best job you can to develop their immune and reproductive systems,” Chase recommends.
Though providing the same care to non-replacements increases their odds of developing effective immune systems, if any class of cattle in an operation is already being pampered nutritionally, it's the replacements. Plus, upfront investments made in preventive health get cheaper the longer a replacement remains in the herd.
“If you can get started with the heifers, you're not playing catch-up,” Chase says.
Finally, Roth points out producers can do everything exactly right to build herd immunity, then sabotage their protection by ignoring biosecurity.
For example, when testing and removing PI animals, it makes little sense to buy and bring in replacements without screening first. It also makes little sense to let your “clean” cattle have fence-line contact with the neighbor's cattle. Roth suggests coordinating grazing periods with neighbors so such contact is avoided.
When any new cattle are procured, Roth advises quarantining for at least 30 days and testing them at least for the BVD virus.
Chase explains biosecurity also means knowing more about the source supplying your animals. Even if the source screens for particular diseases, knowing the program's health history adds to buying confidence.
There's nothing fancy to this. As Roth says, “Building immunity boils down to all the basics most producers alreay know but don't necessarily think about…Vaccines can't overcome management problems.”