Key takeaways

  • Current ILT outbreaks in the U.S. are predominantly associated with field strains in the cases studied.1
  • Proper application protocols are important for optimal ILT vaccination. 
  • An effective ILT prevention strategy entails both biosecurity and vaccination. 

Infectious laryngotracheitis (ILT) remains one of the poultry industry’s most disruptive respiratory diseases. It can spread rapidly through a flock, causing high morbidity and produce average mortality of 10% to 20%. When an outbreak occurs, the immediate response often includes tighter biosecurity, adjusted vaccination programs and renewed debate about whether vaccination was the source of the problem.

Generally considered a seasonal disease, ILT is a disease challenge we’re starting to see year-round, according to test results from our Zoetis diagnostics lab in Durham, North Carolina.

Are ILT outbreaks caused by vaccination?

For years, many ILT field cases were presumed to be vaccinal in origin, particularly those associated with chicken-embryo-origin, or CEO, vaccines. Contemporary genotyping, however, shows a different pattern.

More than 200 ILT isolates were genotyped in 2024, and approximately 87% were identified as Group 6, a nonvaccinal field strain. Data from January through May 2025 were similar, with nearly 89% identified as Group 6. These findings indicate that the great majority of viruses circulating in the cases studied were field viruses, not vaccine strains.1

That does not mean CEO vaccines should be used indiscriminately. Vaccination decisions should be based on current diagnostic evidence indicating which strains are present paired with appropriate control measures.

How should ILT vaccines be applied for optimal flock coverage? 

A modified-live CEO vaccine, such as Laryngo-Vac®, can be an important part of an ILT control program. As with any live vaccine, success depends on detailed planning and careful application.

Before field vaccination begins, teams should confirm that medicators, pumps and water systems are working and that clean mixing equipment is available.

For drinking-water application, the objective is for every bird to receive vaccine. Water-withdrawal time should be adjusted for flock and environmental conditions. A vaccine stabilizer with dye can help buffer chlorine, minerals, and pH and confirm that lines have been drained and filled correctly. Lines should be flushed until dye reaches the ends, lowered together, and birds should be walked regularly so all have access. When there is reason to believe birds were missed, such as finding birds without visible dye on their tongues immediately following vaccination, the vaccination protocol may need to be repeated.

Application problems can leave gaps in flock coverage and contribute to rolling vaccine reactions later in the flock. Those process failures should be corrected, and they are not necessarily evidence that vaccination caused the original field outbreak.

How can ILT outbreaks be prevented or controlled?

Vaccination is only one layer of ILT control. Biosecurity remains the foundation. Farms should document visitors, maintain dedicated clothing and footwear and prevent employees from returning in contaminated clothes or shoes. It’s recommended that personnel use disposable protective gear, foot baths for entry into poultry houses, clean and disinfect vehicles, treat tires before entry and after exit, and move from younger to older flocks. No one should travel to another farm when disease is suspected.

The industry also needs to look beyond external threats. In one field investigation, Group 6 ILT was detected in clinically normal broiler breeder pullets. Spike males from those flocks were moved to multiple breeder farms, and approximately half of the recipient flocks were also positive without showing clinical disease.2Accounting for traffic and bird movements within a system, not just risks entering from the outside, is an important ILT prevention measure.

As you consider what an effective ILT strategy should prioritize, we advise customers to use routine genotyping and data-driven vaccination decisions with close review of internal traffic and bird-movement practices. ILT control requires disciplined biosecurity, accurate diagnosis, coordinated outbreak response and precise vaccine application. When we identify the virus that is actually present and manage the risks, we can make better decisions for flock health.

The information provided in this article is general guidance and does not constitute veterinary advice. Zoetis recommends consulting your veterinarian for all animal health decisions.