Poultry producers across Europe need to focus on testing and vaccination to ensure flocks are properly protected against evolving strains of infectious bursal disease virus (IBDV), according to an expert.

Sjaak de Wit, DVM, PhD, immunologist and senior researcher at GD Animal Health, said data from across Europe shows the traditional very virulent DV86 strain of IBDV has been mostly replaced by new field strains.

Because the strains do not cause mortality or other clinical signs of disease, some producers are choosing to halt their IBDV vaccination strategies — a move that “underestimates” the virus reassortment and puts flocks’ long-term health and productivity at risk, he said.

Speaking at an industry seminar, de Wit said new field strains have emerged in Europe over the last 10 years, with studies showing the virus had mutated several times to become less virulent.

However, while producers reported lower mortality — prompting some to stop IBDV vaccination programs altogether — studies show infection depletes the bursa for at least 3 weeks, causing long-term immunosuppression.

“We know immunosuppression is costly,” de Wit said, citing a study in Northern Ireland that shows immunosuppression associated with IBDV reduces incomes by 11% to 14%.

Early challenge by IBDV can also make birds more susceptible to E. coli and Campylobacter infections — pathogens that can cause consumer illnesses, he added.

To protect young birds from the risk of infection, de Wit said it is vital producers implement a robust control plan incorporating good biosecurity, testing and vaccination.

“IBDV is a resistant virus; proper cleaning and disinfection has to be at the start of any management strategy,” he said.

“The second part is [ensuring] sufficient levels of maternal antibodies to prevent early infection, as early infection is the most damaging to the immune system.”

As well as a proper vaccination program for breeder flocks, de Wit said it’s critical to vaccinate progeny and to check the quality of vaccinate application to make sure it’s working as expected.

The final step is to identify what field strains are present in flocks to create a full picture of what’s happening on each farm. Ideally, sample bursas should be collected from 2 to 5 weeks, with genetic testing carried out on samples from any underperforming flocks to determine if a field strain is present.

“I think one of the reasons this reassorted immunosuppressive virus has spread so successfully over Europe is [it is] being underestimated,” de Wit said.

“Farmers and veterinarians are used to [detecting IBDV] based on clinical signs…but if you don’t have [them], people don’t know where to look.

“The ideal situation is to vaccinate based on serology,” he added. “If it’s not available or practical then you might need to vaccinate more times, but if you don’t use serology then it’s getting more difficult to have a successful vaccination program.”

For farms where there are high levels of field challenge, de Wit said it’s important to have a clear idea about the antibody levels present on the farm, as it can have a significant impact on vaccine success.

“Antibody levels [make] the vaccination moment critical. If you apply it too early it will be neutralized; if it’s too late then the field strain [will reach birds] first. You really need to have an idea about the level of antibodies you’re dealing with.

“That, together with good maternally derived antibodies, cleaning and disinfection should mean your IBDV-control program is quite successful,” he said.