Highly contagious and constantly evolving, infectious bronchitis virus (IBV) is notoriously difficult to control. According to the University of Georgia’s Mark Jackwood, PhD, vaccination is critical to keeping flocks protected — but vaccinating properly is key.
“We have three main things to think about [with IBV vaccination],” Jackwood told Poultry Health Today at the 2019 World Veterinary Poultry Association Congress in Bangkok, Thailand. “One is surveillance to know what’s circulating in the field. Then we have to look at available vaccines and…make sure we’re picking the right ones. Third, we need to make sure we’re applying vaccines properly.”
The difficulty in controlling IBV has to do with how the virus replicates, Jackwood explained. As a single-stranded RNA virus, it has limited “proofreading abilities,” causing mutations that over time result in genetic drift and the emergence of new and more virulent field strains.
When it makes a mistake, it’s not good at going back and fixing it, he said. Those changes may give the next generation a fitness advantage, allowing it to replicate faster and cause disease even in the face of vaccination.
According to Jackwood, vaccination plays an important role in keeping birds protected, but the vast genetic diversity of the virus means that not all vaccines are cross-protective.
“We have hundreds and hundreds of viruses circulating in the field but only a handful of vaccines that we’re able to use against them,” he explained.
“If we don’t [have homologous vaccines available], we have to start looking at other vaccine types that may have some cross protection against circulating field virus.”
Going back to the bird
The best way to achieve cross protection is by using more than one vaccine type, Jackwood said, adding, “This way we can get a broader immune response that can hopefully protect at least somewhat against the circulating field viruses where we have no homologous vaccine.”
However, he stressed, the real challenge is picking the right combination of vaccines.
Several tools are available for testing cross-protective efficacy, including virus-neutralization studies and genetic tests that help determine how related various strains are to one another. But these tests can be expensive and are not always accurate, Jackwood noted, and the only reliable way of determining whether any given vaccine strains will cross-protect is through surveillance.
“The only true way of determining whether we’re going to get protection is to vaccinate the bird, and then when we challenge the bird, look at whether it’s protected or not,” he said, adding that molecular diagnostic advances such as [polymerase chain reaction] testing now allow scientists to accurately assess the efficacy of any given vaccine strain.
Proper application essential
While surveillance and diagnostics are key to selecting the right vaccine or vaccine combination, applying them correctly is essential, the expert stressed.
According to Jackwood, vaccination by spray in the hatchery is the most efficient and easiest to monitor. But while this may sound simple, he cautioned, it’s easy for things to go wrong.
As an enveloped virus, IBV is very fragile, the expert said. It’s easily affected by temperature and shearing forces and can lose titer quickly, so the vaccine should be kept cold, mixed with cold diluent and used before it warms to room temperature.
Proper dosage is also important, he added. When disease pressure is lower in summer, he said, producers sometimes cut doses to reduce vaccine reaction. However, if the dose is too low — or if the vaccine is not a good match for circulating field strains — it may not induce adequate immunity. This allows field virus to replicate at a low level “under radar,” potentially giving rise to new and more virulent strains.
Another important consideration is vaccine volume, Jackwood said.
“For years, 7 ml is what was sprayed on 100 chicks, but what we found [at that volume] was that little vaccine is actually getting to chicks. We found that if you increase volume to 14 or even 21 ml [per 100 chicks], you get a lot more vaccine to the level of the chicks,” he explained, adding that producers should also pay attention to droplet size.
“The finer the droplet size, the easier the vaccine [can] float off on [air] currents in the hatchery. By increasing the size of the droplet, [more vaccine] will get down to the level of the chick and we’ll get a much higher percentage of our chicks vaccinated.
“Ideal situation…within 7 days post vaccination, we want to have 95% to 100% chicks [vaccinated], with good high titers of that vaccine.”