New Zoetis study highlights late infectious bursal disease (IBD) challenge on immune suppression in broilers

25 June 2025

6 minute read

By: Sarah Mikesell

North America

Dr. Kalen Cookson, Director of Clinical Research with Zoetis, recently spoke to The Poultry Site’s Sarah Mikesell at the 2025 International Processing and Production Expo (IPPE) in Atlanta, Georgia about IBDV and immune suppression. Dr. Cookson is continuing Zoetis research to answer the question: When does IBDV cause significant immune suppression and when does it not?

Editor’s note: This interview has been edited for length and clarity.

We're going to talk about one of the studies that you presented at the International Poultry Scientific Forum (IPSF) this year looking at the impact of a late IBD challenge on immune suppression. This topic may sound familiar because you had a similar study that we talked about at IPPE last year.

Yes, last year the study was a little bit different. It was our first look at immune suppression from an IBD challenge, and it was a three-week variant AL2 challenge in broilers. The test for immune suppression was looking at infectious bronchitis protection from a standard day of age spray program, Mass plus GA08. We challenged at three weeks of age with the IBD AL2. Then we came back four days later with the DMV/1639 bronchitis challenge. Then six days later we measured protection. To frame that, 10 days after the IBD challenge we were measuring bronchitis protection.

What we saw is the AL2 challenge did in fact reduce the immunity that the birds had from the Mass-GA08 day of age vaccination. It reduced it from like 90-95% to about 55%, so it was significant. The kicker to that is we also had a treatment group that received our recombinant HVT IBD vaccine in ovo. When they received the 21-day challenge with IBDV there was no loss of IBV immunity at 31 days, so they were 90% protected.

Basically, we demonstrated that the three-week IBD challenge caused a depression of immunity to bronchitis. And protecting against the IBD challenge with the vaccination prevented the immune suppression from occurring – not just IBD and bursal protection but protection from the immune suppressive effects.

Let's talk about the new IBD research you conducted and recently presented.

A good study should probably bring you some surprises and some unanswered questions which we had. One of those, and we mentioned in the interview was, what if we did a four-week challenge? Would we see the same type of effect and a ding to the immune system? So, we devised a round two study. This time in specific pathogen free (SPF) Leghorns, we vaccinated them with our recombinant vaccine again at day of age to see what effect it would have. This time we challenged ourselves in three different windows:

• 11-day challenge – inside of two weeks when we know there's going to be significant and permanent immune suppression
• Three-week challenge (21-day) – like the last study
• Four-week challenge (28-day) – with the AL2 challenge

To measure immune response, what we did this time was administered an inactivated killed SE-ND-IB bacterin at four weeks of age. Then we waited four weeks for the birds to seroconvert to that vaccine and measured bursa and serology.

Like the previous study, we did see that there was an immunosuppressive effect. We looked at Newcastle serology and Salmonella serology to their response to the kill vaccine. What we found was the Newcastle serology was impaired by the 11-day challenge as we would expect. The 21-day was not, but then the 28-day was. The 28-day challenge caused a significant reduction in Newcastle titer response. Conversely, in the vaccinated birds, there was no depression of the Newcastle response. Where we measured the immune suppression, it was restored by protecting against the AL2 challenge by the vaccine.

On the Salmonella side, it was as I expected. The 11-day challenge caused the most suppression of Salmonella titers, followed by the 21-day challenge which was still significant but not as a greater reduction in the titers. The four-week challenge was noticeably lower, like 50% reduction, but it wasn't significant. If we had larger bird numbers, it would probably have been.

It demonstrated the greatest immune suppression inside of two weeks, still getting immune suppression at three-weeks and even seeing some at four weeks. Obviously with the Newcastle response, we saw a significant impact.

In all those cases, the recombinant vaccination not only gave 90+% protection against all the challenges, but it also absorbed any hits to the immune system. The birds responded normally just like the non-challenge controls.

How did those findings measure up to what you expected?

The Salmonella serology was what I was expecting. The three-week late challenge was not going to be as great as the 11-day but was still significant. The 28-day challenge wouldn't be quite as much of an effect as the three-week.

What was unexpected was with the Newcastle three-week challenge - their seroconversion was fine. It was the four week that was surprising.

We've always said with late infections, “timing is everything.” When is that temporary window of immune suppression and what is happening during that time? Perhaps with bacterial challenges, it has more of a linear response curve. The further out with the IBD challenge, the safer you're going to be from those mitigating problems. But maybe with viruses, it's more about timing.

For the 21-day challenge, we're asking the birds to respond to the antigen a week later, and they're going to optimally respond about 10 to 14 days after. Maybe during the temporary immune suppression that was created at three weeks, they got past that window when they still had time to respond to the Newcastle antigen. Bacteria are more complicated antigenically, so maybe it's more challenging if a bird is immune suppressed. If there's a temporary immune suppression, maybe it lasts longer to get an optimal response to the bacteria.

Is there anything that you wanted to add?

Yes, one other thing was interesting. In most IBD challenge vaccination challenge studies, you don't carry out the study much beyond about seven to 10 days after the IBD challenge because you're looking at the bursal protection. Because we waited for seroconversion, it gave us an opportunity to follow the bursal atrophy from the challenge, whether there was bursal recovery, and the challenge control versus the vaccinates. I mentioned the vaccinations had 90+% protection. So, you can't do much better than that.

What was interesting was in the 11-day challenge, at eight weeks of age when we terminated the study, there was still significant bursal atrophy. An early challenge that bursa is not recovering even six or seven weeks later. Even in the 21-day challenge, we saw little evidence of the bursa bouncing back.

One last thing, with the recombinants, just like with other recombinants, they don't completely prevent infections. They're best at limiting the amount of infection and the tissue damage that results. So, they don't prevent infections, but they definitely reduce the infectious load. The vaccinated birds got infected with IBD. We saw this with PCR samples that we took over time courses, but they cleared the virus out much faster than birds that weren't vaccinated. That immune response was still positive. It probably means less shedding back into the environment of the IBD virus which is another positive.