Infectious laryngotracheitis (ILT) has proved to be a persistent cause of concern in poultry, especially commercial layers, for over 75 years. Once the virus is established on a site, 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 farm’s general biosecurity strategy has failed.
THE DISEASE
The disease is caused by a herpesvirus which, like the common herpes cold sore virus, can lie dormant in infected hosts and be reactivated later, especially when birds are stressed. This virus can then spread to other birds on the site and cause a flare up in the disease. As the name implies, the main organ affected by ILT is the trachea, or windpipe. The severity of the disease varies considerably with the strain of virus active in an area, such that some flocks may show only a small drop in egg production and quality, whilst others will experience high sudden mortality.
CLINICAL SIGNS
In the acute form of the disease, 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 as coughing, sneezing and conjunctivitis. Disease may be made worse 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, whilst the occasional acute outbreak of sudden 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 the so called "white egg" syndrome of brown egg layers. In geographical areas where there is a high level of infection in commercial layers coupled with the more widespread use of live vaccine, there has been "overspill" into broilers causing mild respiratory lesions in broiler breeders leading to some egg production problems.
DIAGNOSIS
Accurate diagnosis is needed to ensure that infected sites are identified promptly. Strong suspicion can be reached through gross post mortem lesions. Confirmatory laboratory tests will demonstrate the presence of the ILT virus in 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.
SPREAD OF INFECTION
This may be by direct contact from birds or indirect contact via egg trays, trolleys, people etc. Biosecurity and general site hygiene are important in preventing the introduction of infection onto a site. Infected or vaccinated birds may shed virus again later after clinically returning to normal if they are subsequently stressed, and remain a potent reservoir of contamination on multi age sites.
CONTROL
As indicated above, introduction of ILT virus onto a site is an indication that biosecurity measures have failed with infection gaining access via birds or fomites. On the basis that prevention is better than cure, all sites should periodically reassess their biosecurity strategy in light of risks in specific geographical areas.
For example:
· Keep all visitors to sites to a minimum.
· Provide full protective clothing including boots, overalls and hats.
· Provide handwashing facilities and instant hand sanitisers.
· Ensure all equipment including egg trays, trolleys etc are cleaned and disinfected prior to entry onto a site.
· Ensure all vehicles visiting sites are clean and wheels and wheel arches 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 introduction of infection at an earlier stage.
VACCINATION
Always consult your veterinary surgeon before embarking on a vaccination programme for ILT, as this may not always be the most appropriate strategy for your farm. Where infection does gain access to a site or where 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 licenced in the UK (ILT vaccine, Fort Dodge). This should ideally be administered by eyedropping 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 poor "take" or excessive vaccination reactions, the latter especially if vaccine is administered as too fine a spray. Poor response to vaccination may be due to interference from maternally derived antibody from the parents if pullets are vaccinated too early in rear. They could then require a further revaccination prior to lay.
POINTS TO REMEMBER:
· ILT remains a potent threat to poultry production, notably in commercial laying stock.
· Although some mild infections may be self limiting, unfortunately, infection tends to persist on contaminated sites which remain contaminated until they are totally depopulated.
· Following depopulation, thorough cleansing and disinfection of all buildings and equipment is essential to break the cycle.
· Strict adherence to a structured biosecurity programme must then be observed to prevent subsequent reinfection of the site.
