Dana Simpson DVM MRCVS, St David’s Poultry Team
What is ILT
Infectious laryngotracheitis (ILT) is a highly contagious, acute respiratory disease of chickens and pheasants caused by Gallid Herpesvirus type 1 (GAHV-1). Once infected the bird becomes a life-long carrier of the disease and can shed virus during times of stress thus becoming a potential source of infection to susceptible birds. Latently infected chickens are most often the primary source of ILT virus outbreaks.
ILT is distributed globally and is geographically endemic in GB. The commercial poultry industry in regions where the disease is endemic is faced with high losses because of mortality, egg production losses and decreased bird growth.
Current situation in Northern Ireland
Multiple cases have been confirmed in Northern Ireland since May 2021 and the ILT strain has been identified as a field strain. The use of live vaccine is prohibited in Northern Ireland unless administered via eye drop. The last ILT outbreak in Northern Ireland was in 2013 and there have been sporadic outbreaks in both commercial and hobby flocks since. Hatchery vaccination has continued in some sectors since 2013.
Signs of infection and viral replication
ILT manifests primarily as injury to the upper respiratory tract. Early signs include weepy eyes, conjunctivitis, snicking, sneezing, depressed feed consumption and drop in egg production. Eyes can become swollen and the breathing more laboured. Coughing and rales can often be heard, and some birds may have a bloodstained beak and mouth from the tracheal haemorrhage. In severe cases birds can be seen in respiratory distress with an outstretched neck and gasping. Sudden death with no clinical signs is caused by asphyxiation, as a result of a complete tracheal and laryngeal obstruction.
In chickens, two main forms of ILT have been described under field conditions which include the severe acute form characterized by severe clinical signs accompanied by high mortality reaching up to 70% and a milder form characterised by mild to moderate clinical signs mortality which usually range between 0.1 and 2%. Production drop can be up to 30% in affected laying flocks.
Signs of infection can be noticed 5-12 days after natural exposure and can range from subclinical to severe. Clinical presentation of ILT within a flock will vary depending on the virulence and pathogenicity of the viral strain, the age of the affected birds and the presence of other diseases such as Infectious Bronchitis (IB) and Mycoplasma. Co-infections with other respiratory pathogens and environmental factors adversely affect the respiratory system and prolong the course of the disease.
The clinical signs usually subside after approximately 2 weeks in the absence of coinfections or secondary bacterial infections.
After successful multiplication of the virus it can become resident in the Trigeminal Ganglion of the brain. This is called the latent infection stage during which the birds will not exhibit signs of disease, nor will they shed virus. Stress events such as transfer, onset of lay and vaccination can trigger reactivation of the virus and cause re-emergence of disease and viral shedding putting susceptible birds at risk of infection. In much the same way as cold sores (caused by a type of human herpes virus) can suddenly reappear following periods of stress.
Diagnosis of ILT
Infectious laryngotracheitis in chicken can be tentatively diagnosed based on the clinical signs such as conjunctivitis, gasping, open mouth or extended head respiration, expectoration of bloody mucous, dyspnoea, and finding lesions including catarrhal to haemorrhagic tracheitis, fibrinopurulent to caseous exudates or cheesy or caseous plugs in the larynx and trachea on necropsy. It is highly important to differentiate ILT from other respiratory diseases of which the main differential diagnoses are Newcastle Disease, Avian Influenza, Fowl Pox, Fowl Adenovirus and Infectious Bronchitis. Diagnosis is confirmed by PCR of swabs or histopathology of trachea sections. PCR and quantitative real-time PCR (qRT-PCR) are the widely used and preferred molecular assays for confirmation and quantification of viral load in biological samples due to their higher diagnostic sensitivity and accuracy.
Virus transmission and spread
Transmission between farms can occur by airborne particles, birds and fomites such as equipment, vehicles and clothing. Once a facility is infected, transmission occurs primarily on a bird-to-bird basis. Further spread is facilitated by biosecurity breaches, during transport and spread of contaminated litter.
ILT cannot be transmitted to humans and poses no food safety risk to consumers.
The virus is usually shed in respiratory secretion for at least 6-8 days following initial infection and continue at a reduced level for as long as 10 days post infection. Wind-borne transmission of ILTV has been demonstrated between commercial poultry operations.
In respiratory exudates and chicken carcasses, the virus can remain infective for 10 days to 3 months at a temperature range of 13-23 °C. Dogs and cats retrieving dead bird carcasses from affected poultry houses can also be vectors for transmission.
In deep litter, the ILTV survives for 3-20 days at 11-24.5 °C, in the droppings of battery cages for 3 days at 11–19.5 °C and at least for 3 weeks in buried carcasses. The studies demonstrate that the viability of the virus in litter reduces while applying heat at 38 °C for 24 hrs or composting.
Darkling beetles (litter beetles) and mealworms also act as a source of infection to the birds and the live virus has been demonstrated in darkling beetles even 42 days after the disease outbreak.
Recent studies demonstrated that ILTV can persist in the biofilm of drinking water lines and spread to susceptible birds.
Control of the disease
Good biosecurity practices combined with vaccination are the practical methods to control ILTV in the absence of any effective treatment.
The virus gets readily destroyed by common disinfectants however, the presence of organic matter reduces the efficiency of disinfectants.
Enhanced biosecurity measures must be taken to mitigate the spread of disease:
- Ensure staff or personnel do not have contact with live poultry or other captive birds.
- Only allow essential personnel and visitors onto the site and ensure all vehicles and equipment are cleaned and disinfected coming onto the site.
- Staff and visitors should wear designated clothes and shoes, additional Personal Protective Equipment (PPE), disposable gloves and wash hands after contact with poultry.
- Provide disinfectant footbaths at the entrance and exit to each poultry house and clean and replenish regularly with a DEFRA approved disinfectant.
- Ensure yards and outdoor areas are kept clean and free from build-up of feed, debris and stagnant water.
It has been the experience in Northern Ireland that the movement of litter from infected premises is a major means of spread.
Exercise vigilance and contact your Private Veterinary Practitioner (PVP) immediately if you notice a decrease in production or suspect signs of disease.
A large number of birds are currently vaccinated in the hatcheries against ILT with a HVT vector vaccine.
The HVT vector vaccines are characterised by inability to transmit to other birds and are therefore very safe to use.
The chicks are vaccinated either in-ovo or at day old by subcutaneous (under the skin if the neck) injection. Protection against ILT starts 4 weeks after vaccination and lasts for at least 60 weeks.
When the vaccine is given to chickens, the animals’ immune system recognises the virus as ‘foreign’ and makes antibodies against it. In the future if the birds are exposed to a similar virus the immune system will be able to respond more quickly. This will help protect the chickens against ILT.
This method of vaccination is safer in contrast to the use of live ILT vaccine which if it passes from bird to bird following vaccination. The vaccine virus strain can become more virulent with each passage and can potentially become a disease threat. For this reason, it is safest to apply by eyedrop as this prevents the spread from bird to bird.
ILT remains a significant threat to the poultry industry worldwide and serious attention must be given to control the ILT in poultry-dense areas not only to prevent the economic loss but also to enhance the poultry welfare and health.
A coordinated plan including rapid diagnosis, implementing of strict biosecurity, the vaccination programme, use of GIS technology, proper cleaning, disinfection of poultry houses, responsible movement of litter from affected flocks and increased communication between government and industry will be the most effective approach in controlling ILTV.
Infectious laryngotracheitis: Etiology, epidemiology, pathobiology, and advances in diagnosis and control – a comprehensive review. Vasudevan Gowthaman et al. (2020)
Innovax-ILT | European Medicines Agency (europa.eu)
MSD Animal Health Hub – Innovax ILT
The Merck Veterinary Manual 11th edition
Originally written for The Ranger magazine.