Infectious Laryngotracheitis (ILT) - A Chink in the Armour of Biosecurity?

By DuPont Animal Health Solutions - In the 75 years since it was first identified, infectious laryngotracheitis (ILT) has proved to be a persistent cause of concern in poultry, especially commercial layers.
calendar icon 20 February 2005
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Technical Bulletin: Poultry News - The problem

Much of that concern is through the fear of introduction of infection into an area or onto a site, since once contamination is established it is notoriously difficult to remove. Due to the highly significant role of fomites (inanimate objects) such as egg trays, trolleys, equipment, people and vehicles in the spread of the virus, introduction of ILT onto a site is usually an admission that the general biosecurity strategy has failed.

The disease

The disease is caused by a herpes-virus, which, like the herpes cold sore virus in humans, can lie dormant in infected hosts, and be re-activated later, especially when birds are stressed. Re-activated virus can then spread to other birds on the site and cause a severe flare-up of the disease, particularly on multi-age sites. As the name implies, the main organ affected by infectious laryngotracheitis is the trachea. The severity of the disease varies considerably with the strain of virus active in an area, but essentially falls into one of two broad variations.

  1. Acute form: Here there is bleeding from the mouth and nose from sudden damage to the trachea with haemorrhages. Birds cough up blood and frequently die very suddenly from suffocation.

  2. Chronic (or sub-acute) form: Here the damage to the trachea is more chronic with cheesy deposits developing in the lining of the trachea and larynx. This leads to morbidity, loss of condition and generally low mortality.

Clinical signs

In the acute form, there may be severe respiratory distress, craning of the neck and coughing up of blood. Birds may also show conjunctivitis and slightly swollen heads. Meanwhile, in the same flock, or in flocks only mildly affected, less severe signs may be seen of coughing, sneezing and conjunctivitis. Disease may be exacerbated where other infections are present, notably in combination with Mycoplasma gallisepticum.

In the chronic form, there may be simply sudden death of poor birds, or low grade respiratory signs.

On mixed age laying sites, chronic infection can persist, leading to an age-specific mortality in different houses, which may be significant if the flock is highly stressed, while the occasional acute outbreak with mortality is often self-limiting.

Infection of birds in lay can cause drops in egg production, mainly as a reflection of sick birds in the house not feeding and, hence, not laying. However, infection has been linked to some egg quality problems, and specifically, to the so-called 'white egg' syndrome seen in brown egg layers.

In geographical areas where a high level of infection in commercial layers is coupled with a widespread use of live vaccine, there has been "overspill" into broilers, causing mild respiratory lesions, and into broiler breeders, leading to some egg production problems.


Accurate diagnosis is needed to ensure that infected sites are identified promptly. Strong suspicion can be aroused by gross post-mortem lesions. Confirmatory laboratory tests will demonstrate the presence of the ILT virus in the affected tissues. Blood tests are of limited use in the acute stages of an outbreak, but may be useful retrospectively in monitoring the spread of infection.


As indicated above, introduction of ILT virus onto a site is a sign that biosecurity measures have failed, with infection gaining access via birds or fomites.

Prevention of this situation must always be better than cure, and the biosecurity strategy employed at all sites should be periodically re-assessed, particularly in the light of the risks in specific geographical areas.

To maintain site biosecurity, the following steps should be taken:

  • Keep all visitors to sites to a minimum.

  • Provide full protective clothing including boots, overalls and hats.

  • Provide hand washing facilities and instant hand sanitisers.

  • Ensure all equipment, including egg trays and trolleys, are cleaned and disinfected prior to being brought on-site.

  • Ensure all vehicles visiting sites are clean, and that wheels and wheel arches are sprayed with an appropriate disinfectant.

  • Purchase stock from reputable sources, preferably single age rearing sites.

  • Maintain regular diagnostic and monitoring service of birds through clinical and post-mortem examination, with strategic blood sampling, to identify the appearance of infection at the earliest possible stage.


Where infection does gain access to a site, or there is significant risk of introduction from contaminated farms in the vicinity, vaccination may help to crowd out or exclude the clinical effects of the virus. One live ILT vaccine is available in the UK (ILT Vaccine, Fort Dodge Animal Health Animal Health). This should ideally be administered by eye dropping of individual birds to ensure all are vaccinated. However, mass vaccination methods tend to be favoured for ease of administration, usually as coarse spray or via the drinking water. Such methods may be successful if all birds are covered effectively. Inefficient vaccination can lead to a poor "take" or excessive vaccine reactions, the risk of the latter being particularly high if the vaccine is administered as too fine a spray.

Poor response may also occur due to interference by maternally-derived antibody from the parent bird if pullets are vaccinated too early in the year. They could then require revaccination prior to lay.

Breaking the cycle

  1. ILT remains a potent threat to poultry production, notably in commercial laying stock.

  2. Although some mild infections may be self-limiting, unfortunately infection tends to persist on contaminated sites until they are totally de-populated.

  3. Following de-population, terminal disinfection - the thorough cleansing and disinfection of all buildings and equipment - is essential to break the cycle. Once the flock has been removed from the housing concerned, it should be emptied of equipment, dry cleaned thoroughly, then cleaned with a detergent sanitiser such as Antec HD3 or Antec DSC1000 (DuPont Animal Health Solutions). After cleaning, the housing should be disinfected using a broad spectrum product such as Antec Virkon® S (DuPont Animal Health Solutions). All the removed equipment must be similarly cleaned and disinfected before being replaced.

  4. Once birds are re-introduced, strict adherence to a structured biosecurity programme must be observed to prevent subsequent re-infection of the site. The measures listed above should be rigorously followed. In addition, DuPont Animal Health Solutions have produced a series of helpful biosecurity programmes for use in various types of poultry production, describing efficient methods of both continual and terminal disinfection procedures.

To sum up, prevention is better than cure, and the best protection against the virus lies in good management practices, with the importance of site biosecurity impossible to over-emphasise.

To ensure regulatory compliance in your country please read all product labels carefully.

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Source: DuPont Animal Health Solutions - February 2005

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