Research shows recombinant HVT vaccines in broiler flocks can curb post-vaccination reactions in dual ILT strategies

A university challenge study has examined how recombinant HVT-ILT vaccines used at the hatchery affect the response to live, chicken-embryo-origin (CEO) ILT vaccines applied later as part of a dual infectious laryngotracheitis (ILT) vaccination program.

ILT is a serious respiratory disease that typically peaks in the colder months, causing mortality spikes. It affects both broilers and broiler breeders, though the effects are more severe in broilers. Biosecurity interventions help reduce outbreaks, but vaccination is standard practice in regions where the risk of ILT is endemic.

Producers have vaccine choices

Live ILT CEO vaccines are commonly used to curb mortality during outbreaks. However, prevention strategies have begun to rely on recombinant HVT-ILT vaccines at the hatchery.

In high-challenge areas, the use of a CEO or tissue culture origin (TCO) ILT vaccine has been added to the recombinant ILT vaccination program. Live vaccine choices are often dictated by biosecurity and vaccine regulations across the US, with some states restricting the use of ILT CEO vaccines due to potential increased virulence in flocks and adverse bird reactions.¹

“The aim is to mitigate issues with ILT in flocks over time,” said Alexandra Reilley, PhD, MS, of Merck Animal Health. “ILT CEO vaccines are good at reducing virus shedding and symptoms, but they can cause post-vaccinal reactions and regain virulence if not properly administered to the birds.

“If administration of the ILT CEO vaccine does not achieve complete coverage, birds can experience rolling respiratory reactions, which can be very aggressive,” she explained.  “Using a recombinant ILT vaccine as the program foundation becomes more important if live vaccines are included for additional protection.”

Testing broiler performance under challenge

Gel drop administration of the ILT CEO vaccine at day of age has seen increased use in the industry in recent years as a method to improve early coverage of chicks against ILT challenge in the field. However, questions have been raised about the effectiveness of this method of vaccination with ILT CEO in birds this early.

A study led by Reilley aimed to put the dual ILT vaccination program to the test. Her team used five groups of broiler chicks:²

1. Vaccinated with Innovax®-ILT-IBD in ovo, challenged
2. Vaccinated with gel drop live ILT CEO vaccine at day of age, challenged
3. Vaccinated with Innovax-ILT-IBD in ovo and live ILT CEO vaccine gel drop, challenged
4. Unvaccinated, challenged
5. Unvaccinated, unchallenged

Birds were challenged at 30 days of age with Genotype VI strain ILT virus, via ocular and nasal routes.  Seven days post-challenge, they assessed clinical signs, survival, body weight and body weight gain.

Dual vaccination program cuts clinical signs

The researchers found that birds vaccinated with the ILT CEO vaccine via the gel drop method alone had severe clinical signs at 6 to 10 days post-vaccination and experienced 20% mortality.

In comparison, the vaccinated group receiving only the Innovax-ILT-IBD vaccine showed no clinical signs, as expected. The dual vaccination approach (Innovax-ILT-IBD + ILT CEO gel drop) had mild to moderate signs. Neither the Innovax-ILT-IBD group nor the dual vaccination group had any mortality.

The dual vaccination program resulted in the lowest viral load among challenged groups at 3 and 5 days post-challenge. This vaccination program also had the highest body weight gain over the course of the study.

The reliability of the Innovax-ILT-IBD vaccine comes from its high stability and consistent titers, Reilley noted, as well as the fact that full immunity is achieved by 28 days, complementing the earlier immunity from the ILT CEO vaccine, creating a more diverse immune response.

“Using Innovax-ILT-IBD vaccines, you can reduce respiratory issues related to the post-vaccination reactions observed with ILT CEO vaccines,” she said. “Those reactions are costly, so reducing their severity can help support flock performance.

“The gel drop method has been adopted as a way to strengthen early protection,” Reilley said. “What this study suggests is that timing and program design matter.

“When the recombinant vaccine is used as the foundation, and the ILT CEO vaccine is layered on, we saw less severe post-vaccination reactions while maintaining protection under challenge. That balance is important from a performance standpoint.”

1 García M. 2017. Current and future vaccines and vaccination strategies against infectious laryngotracheitis (ILT) respiratory disease of poultry. Vet Microbiol. 206:157–62.
2 Mendoza-Reilley A, Zamora A, Paudel R, Ellington C, Maekawa D, Alvarado IR, Garcia M. Infectious laryngotracheitis control: Assessment of a rHVT-ILT vaccine administered alone or in combination with a CEO vaccine. 2025. XXIII WVPA Congress, Malaysia.

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