Download Infectious Period

January 2013
Contact: Margaret Sexton (08) 8207 7866
Poultry Diseases - Infectious Laryngotracheitis (ILT)
Infectious Laryngotracheitis (ILT) is a highly contagious respiratory disease
characterised by gasping, neck extension and conjunctivitis.
In most States, ILT is a notifiable disease.
The Causative Agent
ILT is caused by a Herpes virus. It may retain infectivity for 8 to 10 days in droppings
and longer in carcasses, hence correct disposal is essential. It is believed that the virus
may survive for up to 80 days in coughed up mucous.
Species affected
The virus can cause disease in fowls,
peahen, pheasants and turkeys.
Water fowl (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.
After an incubation period of three to
14 days, there is increased mucus
formation in the windpipe (trachea),
often followed by tracheal haemorrhage and discharge from the eyes due to
conjunctivitis. Meat chicks will show a marked reduction in feed intake and growth rate
while layers will suffer a drop in egg production.
The classical signs are gasping, coughing and neck extension with each breath in an
effort to clear the mucus and debris build
up in the windpipe. Many birds die from
suffocation, the windpipe becoming
completely blocked. In severe cases,
there has been up to 70% mortality.
Post-mortem findings include blood
stained mucus or clots of blood and
dead tracheal lining that has fallen off
and found in the upper respiratory tract.
Death is normally by suffocation.
Transmission
The ILT virus is released from the respiratory tract and there is rapid airborne
transmission among birds in close contact. 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 have been traced to the
movement of poultry, people and equipment. If environmental conditions are suitable,
windborne spread must also be considered.
The virus depends on a transporting agent to get around. It is not transmitted through
the egg so chickens are not infected at the time of hatching.
Introduction of infected birds; a major means of spread of the disease is by the
introduction of affected birds, carrier birds or birds incubating the disease at the time of
introduction. Carriers of ILT can shed virus at times of stress thus infecting susceptible
in-contact birds. In show bird flocks, it is common to get outbreaks after birds have
been transported to, and returned home from, a show.
People and contaminated equipment; these can also carry infection into any flock.
Contaminated crates and catching crews have been known to spread 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 another
but there is anecdotal evidence for movement between farms on strong prevailing
winds. Diseased birds being transported may infect birds in sheds close to 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 dander are ideal transporting agents.
Litter and manure; ILT virus may survive in the birds' environment for some time,
transmission occurring when susceptible birds are placed in a contaminated and
unsanitised environment.
Infectious Period
Individual birds are infectious from about the end of the incubation period and up to
another two weeks. Even the vaccine strains can be shed for this period after
inoculation and may cause disease in susceptible birds, although usually milder than
field strains. The viral shedding period depends on when the last birds in the building
became infected.
Many birds develop a carrier state with the virus hiding away in the facial nerves until
the bird immunity wanes. If some stress occurs, many of these birds can shed
infectious virus and some may even show disease, again usually a milder form but
posing a risk to susceptible birds in close contact.
Diagnosis
The clinical signs of bloody tracheal discharges or diphtheritic membranes on the
epiglottis, conjunctivitis and the high pitched wheezy cough are usually diagnostic but
the milder forms need laboratory testing to make sure.
Differential Diagnosis
Vitamin A Deficiency
Fowl Pox
Trichomoniasis
Newcastle Disease
Avian Influenza
Infectious Coryza (Avibacterium paragallinarum)
Infectious Bronchitis
Treatment
There is no treatment but vaccinating early in an outbreak can slow the spread and
reduce the clinical effects in the rest of the flock.
Prevention
Vaccinating birds, first with a mild vaccine strain then about four to six weeks later with
a more virulent vaccine strain produces an immunity that lasts at least a year. Moulted
flocks should be done again at the end of the moult.
There should also be proper biosecurity on commercial farms, i.e. strict quarantine with
control of people and materials entering the farm and particularly the poultry sheds.
Author: Dr Kim Critchley, Poultry Health, PIRSA
FOR FURTHER INFORMATION:
Contact your local veterinary practitioner.
IF YOU HAVE SIGNS OF UNUSUAL OR SERIOUS ANIMAL DISEASE*, phone
Biosecurity SA Animal Health at Glenside on 82077900, contact your local PIRSA
Animal Health officer, call your local veterinary practitioner
OR RING THE DISEASE WATCH HOTLINE: 1800 675 888.
*Signs such as mass mortality events, sudden onset of multiple ill birds, rapid spread of
a problem or a noticeably unusual illness in birds.