Dual HVT recombinant and live attenuated vaccine programs enhance overall protection against ILT and ND

Research has highlighted the health and production benefits of using recombinant HVT vaccines at the hatchery alongside live vaccines against infectious laryngotracheitis (ILT) and Newcastle disease (ND).

Controlling ILT is a priority for broiler producers in regions where the virus is endemic, particularly during the fall and winter periods, when lower temperatures and concurrent respiratory challenges increase disease pressure.

The clinical presentation of ILT in long-lived birds can pose challenges when managing flocks in the field, Ivan Alvarado, DVM, PhD, associate director at Merck Animal Health, explained.

“A particular characteristic of ILT is that when you realize your birds are sick, those birds have been spreading the virus for many days,” he said. He noted that infected but clinically asymptomatic flocks can enable the spread of the disease through movement of personnel or equipment to susceptible farms.

Changes in how birds are handled and moved during high-risk periods can help curb the spread of disease, but vaccines are needed in endemic areas.

In broilers, preventative programs often use recombinant HVT products at the hatchery, such as Merck Animal Health’s Innovax® range, alongside live chicken-embryo-origin (CEO) vaccines, with some variation depending on US state regulations and producer preferences. Although HVT vaccines are known for their safety and efficacy, CEO vaccines have been associated with reversion to virulence and adverse effects on performance in some cases.¹

For longer-living birds, such as commercial layers or broiler breeders, tissue culture origin (TCO) vaccines are available, providing good protection without the potentially negative effects associated with CEO vaccines.

Enhancing protection of long-life birds against ILT

Research conducted at the Poultry Diagnostic and Research Center at the University of Georgia focused on using Innovax® HVT-ND-ILT and LT-IVAX® vaccines, both alone and in combination, against ILT challenge in longer-living birds.² Chickens in the study fell into five groups:

1. HVT-ND-ILT alone
2. Live TCO ILT vaccine alone
3. HVT-ND-ILT and Live TCO ILT vaccine
4. Unvaccinated, challenged
5. Unvaccinated, unchallenged

The researchers administered HVT-ND-ILT subcutaneously at day of age and performed live TCO ILT vaccination by eye drop at 10 weeks. They challenged the birds at 15 weeks via intratracheal and eye-drop routes with a highly virulent strain. To assess the efficacy of the dual program, they evaluated clinical signs, shedding of the challenge virus in the trachea, mortality and ability to transmit the virus to naive birds.

They observed no mortality in the group vaccinated with both the HVT-ND-ILT and the LT-IVAX vaccines, compared with 95% survivability in the group vaccinated with the HVT-ND-ILT vaccine alone and 87.5% in the group receiving the TCO ILT vaccine alone. Survivability was only 41.7% in the unvaccinated, challenged group, corroborating the virulence of the challenge virus.

Striking results on viral shedding

The researchers also saw the lowest clinical signs in dually vaccinated birds, while viral shedding was lowest in the dually vaccinated and TCO groups.

The shedding recorded in the dually vaccinated group was comparable to that of the unvaccinated, unchallenged control, and the introduction of the contact-naive, unvaccinated birds post-challenge further confirmed this. Here, the team recorded no infection of naïve birds from the group that received the dual program.

“We have recommended that our customers with long-lived birds use the recombinant HVT-ND-ILT vaccine at the hatchery and the TCO ILT vaccine at about 10 to 12 weeks of age by eye drop. When implementing a dual vaccination program, outstanding protection has been observed in endemic areas time and time again,” Alvarado said.

“Running this study under controlled conditions not only supported our field observations but also showed us where the dual program makes a difference. We found that it not only better controls clinical signs associated with the challenge virus, but also essentially eliminated viral shedding.

“There was no transmission of the virus after challenge because birds vaccinated with the dual program had solid local and systemic protection, preventing the challenge virus from replicating to levels compatible with horizontal transmission to naïve birds.”

Tackling the evolving threat of Newcastle disease

ND remains a concern for managers of US broiler flocks in some states due to its potential role in complicated respiratory conditions, elevated mortality and the possible presence of virulent strains.

A second study, conducted at the National Poultry Research Center (SPRL), evaluated combinations of Innovax HVT-ND-IBD administered subcutaneously at day of age with a commercial Newhatch-C2® live vaccine administered by coarse spray.³] The groups were:

1. HVT-ND-IBD at day 1
2. HVT-ND-IBD and live C2 vaccine at day 1
3. HVT-ND-IBD at day 1 plus live vaccine C2 at day 11
4. Live C2 vaccine at day 1 and 11
5. Unvaccinated

The study also explored the use of two challenge times: 3 and 4 weeks of age. For the challenge, the researchers used the California 2018 strain of ND virus, a highly virulent strain that causes severe illness and mortality in the field.

HVT and live vaccine at hatchery bring best results

All vaccinated birds had 100% survival when challenged at 3 weeks of age, with a slight drop-off at 4 weeks in the group receiving two live vaccines. In contrast, 100% mortality was observed in the non-vaccinated challenged group, with birds dying before day 5 post-challenge. Similar to the ILT study, the team recorded the most notable results in viral shedding.

The data suggested that the best combination overall was the use of the recombinant HVT-ND-IBD vaccine plus the live Newhatch-C2 vaccine at day one. This approach resulted in no clinical signs, a higher antibody response and reduced viral shedding across time points in the study.

Strong challenges call for dual approach

Alvarado noted that his colleagues have seen particular success using dual vaccination approaches in areas where ILT and ND are considered endemic, or in flocks placed on farms with previous outbreaks. “My advice is, when you have a strong challenge, think about adding a live vaccine to your recombinant vaccine,” he said.

“This elicits local immunity, which is critical for the protection against respiratory diseases such as ILT and ND. It doesn’t have to be a very strong live vaccine; it can be mild. You just need that synergy, which is going to give you the best performance during the whole production cycle.”

“By protecting broilers and long-life birds against ILT and ND, they can achieve their genetic potential, optimizing performance,” he added. “The cost of the program is largely insignificant if you can prevent the adverse effects associated with these potentially devastating diseases from occurring.”

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 Becerra R, Maekawa D, García M. 2023. Protection efficacy of recombinant HVT-ND-LT and the live attenuated tissue culture origin vaccines against infectious laryngotracheitis virus when administered individually or in combination. Avian Dis. 67(2):145–52.
3 Ferreira HL, Reilley AM, Goldenberg D, Ortiz IR, Gallardo RA, Suarez DL. 2020. Protection conferred by commercial NDV live attenuated and double recombinant HVT vaccines against virulent California 2018 Newcastle disease virus (NDV) in chickens. Vaccine. 38(34):5507–15.

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