(ILT) is a contagious respiratory disease which is characterised by gasping,
neck extension and conjunctivitis
(inflammation of the membrane around the eye).
caused by a virus which may live for 8 to 10 days in droppings and up to 70 days
in carcasses, hence correct disposal is essential. It is believed that the
virus may survive for up to 80 days in tracheal exudate (throat exudate) if not
disturbed. This shows the importance of sound clean-up procedures and high
Fowls, pheasants and
turkeys. Water fowl, such as ducks and geese, show no signs, but ducks are known
to carry ILT. Wild birds may act as carriers.
symptoms may include bouts of hard swallowing, ruffled feathers on the back of
the head, squinting and watering of one or both eyes, known as conjunctivitis.
After the incubation
period of 3 to 14 days, there is increased mucus formation in the trachea,
frequently followed by tracheal haemorrhage. This causes the bird to cough
and to extend its head in a characteristic manner in order to breathe. In
some cases only mild respiratory signs are seen but one eye may completely
The classical signs are gasping, coughing and sticking the neck
forward and upwards with each breath in an effort to clear the mucus which
builds up in the trachea (windpipe) – in fact, many birds die from the
disease due to suffocation, ie, the windpipe becomes completely blocked.
In acute cases, there has been up to 70% mortality.
There is a marked
variation in the pathogenicity (potency) of various strains of the virus.
Three major forms – the peracute, the subacute and mild or chronic forms
Three broad forms are
Peracute form – high mortality
of up to 70%. Severe respiratory signs of rales (rattles in the throat),
gasping, coughing with expulsion of blood or blood-stained mucus are seen.
The birds are 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 form – A high morbidity (sickness) rate but
a lower mortality (10-30%) occurs. 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 the eyelids matt 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 form
– This shows as a low morbidity (sickness) rate (5%), the birds are 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%. 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 there is rapid
airborne transmission among birds in close contact such as cage or pen
mates. The virus enters the bird through the eye, the nose or the
mouth. The coughed-up mucus and blood contains virus and is another means
of quick spread of the disease.
Most outbreaks in the past have been
traced to the movement of poultry, people and equipment, however if
environmental conditions are suitable windborne spread must be considered.
The virus depends on a transporting agent to get around. The virus is
not transmitted through the egg so chickens are not infected at the time of
Introduction of infected birds – A
major means of spread of the disease is by the introduction of affected birds,
carrier birds or birds which are incubating the disease at the time of
introduction. Carriers of the wild strains of ILT can shed virus at times
of stress thus infecting susceptible in-contact birds.
and contaminated equipment – these can also introduce infection
into any flock. Contaminated crates and feed trucks are known sources of
infection. People in contact with infected birds and on the same day
contacting susceptible flocks may transfer the disease if suitable precautions
are not taken.
Airborne spread – ILT airborne
spread depends on the prevailing conditions. There is rapid airborne
transmission among birds in close contact. The virus often requires
mechanical transfer to cover even short distances such as from one building to
However, it is recognized that birds in sheds close to roads
may be infected by diseased birds being transported down the road. Under
conditions of cloud cover, humidity or showers and gusting winds, it would
appear that the ILT virus can easily cover 500 metres and possibly much
further. Small feathers and shed dust are ideal transporting agents.
Litter and manure – ILT virus may survive in the
birds’ environment for periods of time, and transmission may occur when
susceptible birds are placed in a recently contaminated but uncleaned
Eye vaccinated birds
usually start to show signs on days 3 to 5 and have normally finished shedding
virus by days 11 or 12.
It is generally accepted that wild or field
strains cause the birds to shed virus over a longer period than vaccine
strains. Again the period of shedding would depend on the incubation time
which is usually longer than for vaccine strains (up to 14 days). The
length of the shedding period will depend on when the last birds in the building
This situation can be short circuited by vaccination
however a resultant carrier state (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 when it is cooler. It appears the virus may survive up to
80 days in tracheal mucus on non-conductive material such as wood. One
percent lysol or three percent cresol will inactivate ILT virus in less than a
Sunlight, heat and desiccation (drying) appear to be the three
natural enemies of the ILT virus.
(severely) affected birds will show free blood in the trachea, generally
associated with a mucus plug which inhibits normal breathing. The symptoms
will 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. Mild ILT infection may look like any
other respiratory or virus infection.
Laboratory diagnosis will
always be required to determine whether ILT virus is present.
ILT may be controlled
management practices: quarantine, isolation of
introduced birds, no introduction of stock to farm (not practical for show
poultry). Does not guarantee protection.
vaccination will result in protection for all birds. Eye drop vaccination
is normally carried out at 7-10 days and again at 8-12 weeks.
Antibiotics have no effect against the virus.
Vaccination and the short incubation period of the vaccine is used to halt an
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