Infectious bursal disease (IBD) remains a significant challenge for poultry producers because early infection in a bird’s life can compromise immune function and create opportunities for additional diseases and performance problems.
Kalen Cookson, DVM, MAM, DACPV, director of clinical research for the U.S. segment of Zoetis Poultry, discussed the evolving IBD virus landscape and vaccination strategies during the 2026 World’s Poultry Congress in Toronto, Ontario, Canada.
His ongoing research has examined about 1,500 IBD virus sequences collected over 15 years, providing insight into the diversity of viruses circulating in North America and how commercial vaccines protect against them.
The findings point to the importance of protecting chicks during their first weeks of life, beginning with breeder vaccination, while also considering the antigenic diversity represented in an IBD vaccination program.
Early IBD infection can have lasting consequences
One of the challenges with IBD is that the timing of infection can significantly influence its impact on the bird.
“It’s important to remember that it's an immune suppressive virus,” Dr. Cookson said. “The timing of when birds get infected is crucial.”
Birds infected within the first two weeks of life are particularly vulnerable to significant immune suppression, according to Dr. Cookson. The consequences can extend beyond IBD itself, potentially resulting in poorer performance and leaving birds more susceptible to secondary infections and other viral diseases.
That makes preventing early infection a critical objective of an IBD control program. Dr. Cookson said one of the most effective approaches begins before the chick reaches the farm.
“Because the first two weeks are crucial for protection, the single most effective and efficient way to prevent those infections is from the breeder side,” he said.
Proper breeder vaccination, particularly with killed vaccines, allows hens to provide passive immunity to their progeny. That maternal protection can help shield chicks during the period when an early IBD infection could cause significant immune suppression.
Surveillance reveals a diverse IBD virus population
Developing effective vaccination strategies also requires understanding the viruses circulating in commercial poultry.
Dr. Cookson and his colleagues have conducted extensive surveillance of IBD viruses in North America. Dr. Cookson said the surveillance database continues to grow.
“We're now close to about 1,500 sequences that we've surveyed over the past 15 years,” he said.
Researchers placed the sequences into a phylogenetic tree to evaluate the relationships among circulating viruses. From that work, they identified approximately 10 to 12 antigenically unique IBD viruses across the spectrum present in the United States (Figure 1).
That diversity has implications for vaccine selection. Historically, bursal-derived vaccines were designed to provide strong protection against classic IBD and Delaware variants. However, the IBD landscape has continued to evolve, and additional variant types are now circulating more frequently in U.S. poultry.
To understand how existing vaccine options perform against that broader diversity, researchers selected viruses representing different clusters within the phylogenetic tree and evaluated how four commercial vaccines protected against them.
Antigen diversity matters in vaccine programs
The research identified differences among the bursal-derived vaccine formulations available in the United States (Figures 2-5).
Dr. Cookson said two of the vaccines evaluated were primarily driven by formulations designed to protect against Delaware variants, while two others included additional variant types.
When researchers conducted fractional-dose challenge studies, vaccines containing greater antigenic diversity tended to provide better protection against a wider range of variants included in the study panel.
“This tells us in order to get the most coverage against the range of IBD viruses that our birds face, the antigen diversity in your vaccination programs matters,” Dr. Cookson said.
The finding is relevant beyond U.S. poultry production. Three of the 12 variant viruses included in the research panel also represented the top three variant types identified in Canada, Dr. Cookson said.
For breeder managers, that means vaccination strategy is not simply about generating antibodies. The diversity and quality of the antibodies that hens produce and subsequently pass to their chicks can influence how well progeny are protected against the spectrum of IBD viruses they may encounter.
“The better and more diverse the quality of antibodies that are expressed by hens and passed to the progeny, the fewer gaps they will have in early protection,” Dr. Cookson said.
Extending passive immunity buys valuable time
The primary target is strong immunity during the first two weeks of life, but Dr. Cookson said an effective breeder vaccination and live priming program can potentially extend passive immunity closer to three weeks. That additional time can be valuable because it gives vaccines administered to broilers more opportunity to engage the immune system before field virus exposure occurs.
“The longer we can extend that passive immunity, the more successful the rest of our program will be,” Dr. Cookson said.
If broilers receive recombinant vaccines, live vaccines or a combination of approaches, stronger passive immunity can provide additional time for those vaccines to become effective in controlling IBD virus.
Vaccination practices have also changed. Historically, maternal antibody levels could influence the timing of field vaccination. A flock with a higher maternal immunity barrier, for example, might be vaccinated at 18 days rather than 12 days.
Today, much of the vaccination occurs in the hatchery, either in ovo or subcutaneously. That shift changes how producers and veterinarians can think about combining maternal immunity with live and recombinant vaccine technologies.
Protecting the most susceptible chicks
Live vaccines are influenced by maternal antibodies, but Dr. Cookson said they can still have an important role in hatchery vaccination.
The objective of administering a mild live or intermediate vaccine in the hatchery is not necessarily to vaccinate every chick effectively at the same moment. Instead, the vaccine can provide protection to the subset of chicks entering the flock with lower maternal antibody titers.
A flock overall may have strong maternal immunity lasting two or three weeks, but a small percentage of chicks may have lower titers and become susceptible earlier.
“If you can vaccinate that 5% of the flock with low titers, you can buy time where the field virus can't come in earlier and start infecting before the rest of them become susceptible,” Dr. Cookson said.
In this situation, the live vaccine effectively helps protect the flock’s most vulnerable birds while buying time for immunity in the broader flock.
Recombinant vaccines offer a different advantage because they are not affected by maternal antibodies.
According to Dr. Cookson, whether chicks have high or low maternal antibody levels does not influence when immunity generated by the recombinant vaccine begins. Immunity generally starts developing at approximately 10 to 14 days and reaches its maximum level by 3 1/2 to 4 weeks.
That creates an opportunity for complementary strategies.
“The two work well together,” Dr. Cookson said. “You have the live vaccine that can cover the most susceptible chicks, even if it's a small percentage, but it buys time for when that recombinant comes in.”
Immune complex vaccines can influence field challenge
Immune complex vaccines provide another option. Dr. Cookson said these vaccines can provide lasting immunity after maternal antibodies disappear while also functioning similar to conventional live vaccines by protecting more susceptible, low-titer birds by approximately 10 to 14 days of age.
Beyond individual flock protection, Dr. Cookson said immune complex vaccines may also influence the amount of IBD virus circulating in a poultry operation over successive flock placements.
That distinction is important because recombinant vaccines do not prevent infection, meaning field viruses can continue cycling and potentially challenge subsequent flocks. Immune complex vaccines, by comparison, can compete with field viruses.
“Immune complex vaccines will actually compete with the field viruses, and you can get a competitive exclusion situation,” Dr. Cookson said.
Dr. Cookson said this approach can block a field virus or delay its entry by up to a week or more. Over multiple placements of immune complex-vaccinated flocks, producers may be able to progressively lower the level of field challenge on an operation.
That ability also opens the door to more strategic vaccination programs over time.
For example, Dr. Cookson said an operation could use immune complex-vaccinated flocks to reduce the level of IBD field challenges going into winter respiratory season.
“You can lower the challenge going into the winter season and then come back in with a recombinant,” he said. “It's more effective after you've settled the challenge down, and it can coast through the winter season with the rest of your program.”
Ultimately, Dr. Cookson’s research highlights the importance of viewing IBD control as a coordinated program rather than relying on a single point of vaccination. Strong breeder immunity, antigenic diversity, maternal antibody protection and the strategic use of live, recombinant and immune complex vaccines can each play a role.
As the diversity of circulating IBD viruses continues to challenge poultry flocks, matching vaccination strategies to the viruses that birds face while protecting chicks during their most vulnerable first weeks of life remains central to controlling the immune-suppressive effects of the disease.
References:
Data on file, Additional Study No. 040322-KC-70AQO-KC-6121, Zoetis Inc.
Data on file, Additional Study No. RM-0Q0-KC7034, Zoetis Inc.
Data on file, Additional Study No. 040322-KL-70AQO-KC6533, Zoetis Inc.
Data on file, Additional Study No. 040322-KC-70AQO-KC-6431, Zoetis Inc.
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