Infectious Laryngotracheitis11 March 2014
On overview of this viral respiratory disease of poultry published in 'Drumstick' magazine from the New South Wales Department of Primary Industries.
Infectious laryngotracheitis (ILT) is a contagious respiratory disease that is characterised by gasping, neck extension and conjunctivitis (inflammation of the membrane around the eye).
ILT is a notifiable disease because it is similar to the early stages of virulent Newcastle disease. If owners, vets, laboratory staff or others suspect this disease, they must report it to their local biosecurity response agency in their respective state or territory.
ILT is caused by a virus that can live for eight to 10 days in droppings and up to 70 days in carcasses; hence correct disposal is essential. The virus may survive for up to 80 days in tracheal exudate (throat exudate) if not disturbed. This demonstrates the importance of sound cleanup procedures and high-pressure hosing.
ILT affects fowls, pheasants and turkeys. Water fowls (ducks and geese) show no signs but ducks have been known to carry ILT for up to two weeks. Wild birds may act as carriers.
Early signs may include bouts of hard swallowing, ruffled feathers on the back of the head, squinting and the watering of one or both eyes (conjunctivitis).
After the incubation period of three to 14 days - though five to 12 days is most common - increased mucus forms in the trachea, often followed by tracheal haemorrhage (bleeding). This causes the bird to cough and extend its head in a characteristic manner to breathe. In some cases, only mild respiratory signs are visible but one eye may completely close.
The classic signs are gasping, coughing and extending the neck forward and upwards with each breath to clear the mucus in the trachea (windpipe). In fact, many birds die from this disease due to suffocation, as the windpipe becomes completely blocked. There has been up to 70 per cent mortality in acute cases.
There is a marked variation in the pathogenicity (potency) of various virus strains. Three major forms are known:
- Peracute. This has a high mortality of up to 70 per cent. Severe respiratory signs of rales (rattles in the throat), gasping, coughing with expulsion of blood or blood-stained mucus are visible, and the bird is very depressed. On post mortem, acute haemorrhagic inflammation of the trachea and larynx is present, and the lumen (centre) of the trachea is blocked by mucoid blood clots and, sometimes, yellow caseous exudate (cheesy plug-hard pus). Death is normally by suffocation
- Subacute. This has a high morbidity (sickness) rate but a lower mortality rate of 10 to 30 per cent. There are less severe respiratory signs of rales, coughing with expulsion of caseous matter, mucoid nasal discharge, gasping and infra-orbital sinus swelling. There is often conjunctivitis with severe lacrimation (eye discharge) and eyelids matted together. On post mortem, mucus, which may be bloodstained, is found with membranes in the upper respiratory tract. Death is normally by suffocation
- Mild or chronic. This has a low morbidity (sickness) rate of five per cent. The bird is drowsy with signs of conjunctivitis, squinting eyes and bronchitis combined with a cough. There is often a concurrent infection with coryza. Egg production may drop 10 per cent. On post mortem, false membranes are seen in the upper respiratory tract, which may cause death.
The ILT virus is released from the respiratory tract and followed by rapid airborne transmission among birds in close contact, such as cage or pen mates. The virus enters the bird through the eye, nose or mouth. The coughed-up mucus and blood contains the virus and is another way that the disease quickly spreads.
In the past, most outbreaks have been traced to the movement of poultry, people and equipment. However, if environmental conditions are suitable, windborne spread may also be a factor.
The virus depends on a transporting agent to spread. The virus is not transmitted through the egg, so chickens are not infected at hatching. The virus can be spread via the following means:
- The introduction of infected birds. This includes the introduction of affected birds, carrier birds or birds that are incubating the disease at the time of introduction. Carriers of the wild strains of ILT can shed the virus at times of stress, infecting susceptible in-contact birds
- People and contaminated equipment. These can introduce infection into any flock. Contaminated crates and feed trucks are known sources of infection. People who are in contact with infected birds and, on the same day, with susceptible flocks may transfer the disease if they do not take suitable precautions
- Airborne spread. ILT’s airborne spread depends on the prevailing conditions. There is rapid airborne transmission among birds in close contact. The virus often requires mechanical transfer to cove r even short distances, such as from one building to another. However, birds in sheds close to roads may even be infected by diseased birds being transported down the road. Under conditions such as cloud cover, humidity, showers and gusty winds, the ILT virus can easily cover 500 metres and possibly much further. Small feathers and shed dust are ideal transporting agents
- Litter and manure. The ILT virus can survive in the birds´ environment for periods of time, and transmission may occur when susceptible birds are placed in a recently contaminated but uncleaned environment.
Eye-vaccinated birds usually start to show signs on days 3 to 5 and normally finish shedding virus by 11 or 12 days.
Wild or field strains cause the birds to shed virus over a longer period than vaccine strains. Again, the period of shedding depends on the incubation time, which is usually longer than for vaccine strains (up to 14 days). The length of the shedding period depends on when the last birds in the building became infected.
This situation can be short-circuited by vaccination; however, a resultant carrier state, where the bird appears normal but may shed virus when stressed, is established in many birds.
The virus can survive for 10 days or more in droppings and up to 70 days in carcasses. The virus lasts longer in winter due to the cool temperature. The virus may survive up to 80 days in tracheal mucus on non-conductive material such as wood. One per cent lysol or three per cent cresol will inactivate the ILT virus in less than a minute.
Sunlight, heat and desiccation (drying) are the three natural enemies of the ILT virus.
Diagnosis and Control
Acutely affected birds show free blood in the trachea, which is generally associated with a mucus plug that inhibits normal breathing. The symptoms rapidly spread throughout the flock.
Birds with subacute and mild infections may show only slight difficulty in breathing and perhaps a mild watering of one or both eyes. However, the disease can still be easily transmitted from one bird to another. A mild ILT infection may look like any other respiratory or viral infection.
Laboratory diagnosis will always be required to determine whether the ILT virus is present.
ILT may be controlled by:
- management practices, including quarantine, the isolation of introduced birds and no introduction of stock to farm (not practical for show poultry). However, this does not guarantee protection
- vaccination. Annual vaccination protects all birds. Eye-drop vaccination is normally carried out at seven to 10 days and again at eight to 12 weeks.
Similar diseases include:
- Newcastle disease
- infectious bronchitis
- fowl cholera
- infectious coryza
- fowl pox (wet pox)
- chronic respiratory disease
- vitamin A deficiency
- avian influenza
Treatment and Prevention
Antibiotics have no effect on the virus. Vaccination and the vaccine’s short incubation period is used to halt an outbreak.
The disease is prevented by vaccination. Because ILT is similar to early exotic Newcastle disease, it is maintained as a notifiable disease.
The virus is not transmitted through eggs, so chickens are not infected at hatching.
Find out more information on Infectious Laryngotracheitis (ILT) by clicking here.
Adapted from the Queensland Department of Agriculture, Fisheries and Forestry. For a link to the article on its website, click here.