When your vaccination program holds a particular disease at bay, it's safe to say the vaccine provided adequate resistance relative to the disease challenge — if there was one. Or, you just got lucky.
Conversely, when you get into a wreck with a disease you vaccinated against, isolating the cause can be like pitching darts at a flea's belly. The vaccine may have worked for its designed purpose, but the disease challenge was too great.
Or, perhaps the vaccine didn't have an adequate chance to work because of other management constraints, such as poor nutrition, improper product administration, or vaccinating too late to provide protection.
“There's no situation where vaccination is 100% effective all the time,” says Larry Hollis, Kansas State University Extension beef veterinarian. There are too many strains of disease organisms and other extenuating variables. Plus, he adds, every vaccine can be overwhelmed by disease challenge if it's severe enough.
Producers can increase their odds of success by working with their veterinarian to select the correct vaccine to provide the desired disease protection.
The correct tool for the job
“When you look at the vaccines available for a particular disease, it's tempting to view them as equal, but there may be vast differences in what they actually do or are designed to do,” Hollis says.
Many producers are well versed in the tradeoffs between using killed (inactivated) and modified-live virus (MLV) vaccines. Generally, killed vaccines work by stimulating humoral immunity — where viral or bacterial antigens induce an immune response. The result is the production of antibodies that circulate in the bloodstream and bind with the disease-causing bacteria or virus and neutralize them.
MLV vaccines, on the other hand, stimulate cell-mediated immunity (CMI), as well as humoral immunity. In simple terms, CMI works at the cellular level to destroy viruses that take over normal cellular function in order to replicate themselves.
Depending on how a particular disease organism works, killed vaccines may be sufficient. In other cases, MLV vaccines may be required to offer adequate protection. Hollis says a classic example is infectious bovine rhinotracheitis (IBR). Managing the disease requires CMI attacking and killing the virus within the cells; humoral immune response isn't effective by itself.
But even vaccines of the same type can provide different levels of protection. In fact, vaccine labels describe the level of protection.
Vaccine manufacturers can make one of four standard USDA-approved claims (see sidebar on page 22). At one end of the scale are vaccines with approval to claim “prevention of infection.” By USDA definition, this label claim can only be made if the vaccine prevents colonization of animal tissues by particular organisms.
In other words, using the vaccine prevents particular organisms from infecting cells, replicating and causing a particular disease. That doesn't mean vaccinates are totally immune to the disease. It means clinical trials have proven specific strains of the organism cited on the label can't replicate in the presence of the vaccine.
On the other end of the spectrum are products with approval to claim, “aid in disease control.” By USDA definition, vaccines with this designation have been proven to reduce disease duration or severity, or delay onset of disease.
Obviously, there's a fair gap between preventing disease and helping control it. Either can be an effective tool, depending on the situation and vaccination goals, Hollis says.
However, even vaccines of the same type with the same label claim don't necessarily provide the same level of protection.
USDA's minimum standards for challenge studies — the level faced by the vaccine in receiving USDA approval — represent the middle of the road.
“Vaccines are designed to protect you against routine, everyday levels of disease exposure,” Hollis says. He says some biological companies challenge their products at the lowest allowable level; others challenge their products against some of the most virulent strains of a particular disease organism. “Some meet minimum USDA standards, and some far exceed them. And, there's nothing on the label to tell you which is which,” Hollis says.
Dig beyond the broad label claims described earlier, though, and you can begin to get a feel for how robust a particular product might be.
For instance, Hollis says he trusts data a lot more than people, especially data that comes from a third-party source. Check to see how much testing was done, who did it, and whether the results were corroborated by other sources.
Other information carried on some vaccine labels can be a clue, too — “days of immunity,” for instance. If USDA allows a company to include this information on the label, it means the company has jumped through the hoops to demonstrate the duration of protection defined by the label.
“If it's on the label, it says the company went the extra mile to prove the product does what they say it will do. That should provide users with an added level of confidence,” Hollis says.
There are the usual commonsense barometers, too, such as a company's reputation and the reliability of its other products over time.
Walking a fine line
Though easier said than done, vaccinating against a particular disease is a game of juggling disease resistance with disease challenge. In healthy herds, Hollis explains the resistance line exists above the challenge line (Figure 1). Whenever the challenge line creeps above the level of resistance, disease occurs.
That's even true when you've done everything right — maintaining disease resistance and boosting it ahead of a seasonal spike in disease challenge. The challenge can still overwhelm the resistance (Figure 2).
Incidentally, vibriosis and calf scours serve as different examples of how timing can knock the good intentions out of any vaccination program.
Because vibrio spreads by venereal transmission, the greatest challenge will be at breeding time. If you vaccinate for it at preg-check time, though — thinking you're getting a leg up on next spring's breeding — Hollis has some bad news for you.
“If you vaccinate for vibrio in the fall, disease resistance will probably drop below the challenge line before breeding season. You need to vaccinate cows for vibrio shortly before each breeding season,” he says.
Likewise, killed vaccines for calf scours require two injections. Labels on these products typically recommend two injections three weeks apart. What the labels don't tell you, Hollis points out, is the second injection needs to be given at least a week before calves will be exposed to the disease. Otherwise, there isn't enough time to mount an immune response.
That's why using killed products to vaccinate for BRD in un-weaned, commingled calves is like trying to douse a grassfire with a water pistol. By the time calves are vaccinated, the damage is done; they can't build immunity in time.
“You can't buy health management in a bottle. If you have an animal disease problem in your herd, you need to identify the causative agent and the management factors that are required to assist in beating the disease,” Hollis says.
“Some producers expect the vaccines to do all of the work. Vaccines are only one of the tools in the fight against infectious diseases. It takes a combination of vaccination plus management practices to get the results producers are looking for,” he adds.
Definition of label claims
Prevention of infection — Approved for products able to prevent all colonization or replication of the challenge organism in vaccinated and challenged animals. If such a conclusion is supported with a very high degree of confidence by convincing data, a label statement such as, “for the prevention of infection with (specific microorganism),” may be used.
Prevention of disease — Allowed only for products shown to be highly effective in preventing clinical disease in vaccinated and challenged animals. The entire interval (95%) estimate of efficacy must be at least 80%. If so, a label statement such as, “for the prevention of disease due to (specific microorganism),” may be used.
Aid in disease prevention — Allowed on products shown to prevent disease in vaccinated and challenged animals by a clinically significant amount that may be less than required to support a claim of disease prevention (see above). If so, a label statement such as, “as an aid in the prevention of disease due to (specific microorganism),” may be used.
Aid in disease control — Claim exclusive to products shown to alleviate disease severity, reduce disease duration or delay disease onset. If so, a label statement such as, “as an aid in the control of disease due to (specific microorganism),” or a similar one stating the product's particular action, may be used.
Other claims — Products with beneficial effects other than direct disease control, such as the control of infectiousness through the reduction of pathogen shedding, may make such claims if the size of the effect is clinically significant and well-supported by the data.
Source: USDA Center for Veterinary Biologicals