Why new IBV variants keep emerging — and what producers can do about it

The poultry industry has spent decades chasing infectious bronchitis virus (IBV), adjusting vaccination programs and developing new vaccines as new variants emerge. Yet despite those efforts, the cycle continues.

According to Mark Jackwood, PhD, a consultant for Ceva Animal Health and one of the world’s leading IBV researchers, that’s not a sign that vaccination has failed. It’s simply the nature of the virus.

“IBV is constantly evolving,” Jackwood said. “As long as the virus is replicating, genetic change is occurring.”

The question isn’t whether new variants will emerge, but how quickly. Scientific evidence suggests that the answer depends on how effectively producers can limit viral replication in commercial flocks — a factor that may influence not only disease control today but the emergence of future variants as well.

That may sound like a subtle distinction, but it has important implications for how the industry approaches infectious bronchitis control. Although vaccines remain the cornerstone of prevention, Jackwood said their value extends beyond reducing clinical disease and production losses. Effective vaccination also helps slow the emergence of future variants by reducing opportunities for the virus to replicate and evolve.

A costly and persistent challenge

IBV remains one of the most economically important viral diseases affecting poultry worldwide. The avian coronavirus primarily targets the respiratory tract, causing signs such as coughing, gasping and tracheal rales, but its impact can extend well beyond respiratory disease.

Depending on the strain involved, IBV may affect the kidneys, reproductive tract and overall flock performance. In laying hens, infections can lead to drops in egg production, poor shell quality and reduced egg quality. Some strains can damage the kidneys, while infection of young pullets may permanently damage the developing oviduct, resulting in so-called false layers.

“The virus affects much more than the respiratory system,” Jackwood said. “The production losses associated with IBV can be substantial, particularly when secondary bacterial infections become involved.”

Those losses may include poorer feed conversion, reduced weight gain, increased condemnations at processing and lower reproductive performance. In severe outbreaks, the economic consequences can quickly become significant.

Yet what makes IBV particularly challenging isn’t simply the damage it causes. It’s the virus’s remarkable ability to change.

Why IBV keeps changing

Like other coronaviruses, IBV carries its genetic information as RNA rather than DNA. Every time the virus replicates, it must copy that genetic material. The enzyme responsible for making those copies is prone to mistakes, creating mutations throughout the viral genome.

“Those mistakes generate genetic diversity,” Jackwood explained. “Most of the time, the mutations don’t matter. But occasionally, one mutation provides an advantage that allows the virus to survive or spread more effectively.”

Over time, those accumulated mutations can produce meaningful changes in the virus. This process, known as genetic drift, is responsible for the emergence of most new IBV variants.

Many of those changes occur in the spike protein, the structure the virus uses to attach to and enter host cells. The spike protein is also a primary target of the bird’s immune response.

“When mutations occur in key regions of the spike protein, the immune system may recognize the virus differently,” Jackwood said. “That’s how new antigenic variants emerge.”

IBV can also evolve through recombination, a process in which two different virus strains infect the same cell and exchange genetic material. Although recombination has contributed to the appearance of some new variants, Jackwood said most significant IBV types arise through the steady accumulation of mutations over time.

“The challenge is that IBV replicates extraordinarily efficiently. A single infected bird can transmit the virus to many others, creating countless opportunities for additional mutations to occur,” Jackwood said. “The more replication you have, the more opportunities the virus has to change.”

An unexpected role for vaccination

That relationship between replication and evolution has led researchers to think differently about vaccination.

For years, poultry health professionals viewed vaccination primarily as a way to prevent disease. Although that remains true, Jackwood said vaccination can also influence how quickly new variants emerge.

The reason lies in a concept that can be difficult for producers to fully appreciate: IBV vaccination does not produce sterile immunity.

“Even when birds are very well protected, field virus can still infect and replicate at low levels,” Jackwood said. “Vaccination dramatically reduces replication and transmission, but it doesn’t eliminate them entirely.”

At first glance, that may sound discouraging. But in reality, it highlights why strong vaccination programs remain so important.

When a flock is well protected, virus replication is greatly reduced. Fewer infected cells mean fewer opportunities for mutations to occur, fewer opportunities for transmission and a lower chance for the virus to spread through the population.

“In a fully protected flock, you’re slowing the virus down,” Jackwood said. “You’re reducing the amount of replication that’s occurring and reducing the opportunities for new variants to emerge.”

Partial protection ‘worst-case scenario’

Interestingly, research suggests that the greatest evolutionary pressure may occur not in fully susceptible flocks or fully protected flocks, but somewhere in between.

According to Jackwood, partially protected populations can create conditions that favor the selection of viral variants capable of escaping existing immunity.

“The worst scenario for the emergence of new variants is often partial protection,” he said. “The virus is under pressure to survive, but it’s still able to replicate. That combination can favor the selection of viruses with advantageous mutations.”

That doesn’t mean vaccination causes new variants. Rather, it underscores the importance of achieving consistent, high-quality protection throughout the flock.

“The goal is to reduce replication as much as possible,” Jackwood said. “The more effectively you control the virus, the fewer opportunities it has to evolve.”

Staying ahead of a moving target

Because IBV continues to evolve, vaccination alone is not enough. Producers also need a clear understanding of which virus types are circulating in their region and how those viruses are changing over time.

Modern molecular diagnostic tools have transformed the industry’s ability to monitor IBV. Tests such as reverse transcriptase-polymerase chain reaction (RT-PCR) and sequencing can rapidly identify circulating strains and help veterinarians determine whether existing vaccination programs remain appropriate.

“Surveillance is critical because it tells us what’s happening in the field,” Jackwood said. “You can’t make informed vaccination decisions if you don’t know which viruses are circulating.”

For Jackwood, that combination of surveillance and effective vaccination is central to Ceva’s approach to infectious bronchitis control. Through its Ibron® portfolio, the company emphasizes using surveillance data to help veterinarians tailor vaccination programs to local IBV challenges as the virus continues to evolve.

That information becomes increasingly valuable as the virus continues to evolve. Although researchers have made tremendous progress in understanding IBV, Jackwood believes producers should expect change to remain a constant feature of the disease.

“This virus isn’t standing still,” he said. “New variants will continue to emerge because that’s what RNA viruses do.”

The good news is that producers are not powerless against that process. Effective vaccination, strong biosecurity and ongoing surveillance can all help reduce viral replication and limit opportunities for change.

“We may never completely stop IBV evolution,” Jackwood said. “But by reducing replication and transmission, we can slow it down and stay ahead of the next challenge.”

Want to learn more?

Explore Ceva’s Infectious Bronchitis Knowledge Hub for additional technical resources, practical guidance and expert insights on infectious bronchitis vaccination. 

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