Incubation period
Clinical disease occurs 6-12 days after natural exposure to the ILT vinus, although experimentally a shorter incubation period of 2-4 days is often seen

Clinical signs
ILT vs causes an acute respiratory disease in chickens centred on the trachea. Typically, affected chickens have a nasal discharge and a mild rattling noise on breathing that is often accompanied by gasping and coughing in severe forms the breathing can become very laboured and blood stained mucus is often expelled
In recent years a milder form of the disease has become more widespread and this is typically manifested as unthriftiness, depressed egg production, conjunctivitis (watery eyes) sinusitis and a mild tracheitis and a persistent nasal discharge of variable duration in the acute disease morbidity is 90-100% and mortality is typically 10-20%, but can reach more than 60% in the mild form morbidity is often as low as with mortality rarely over 1-2%

In ILT lesions can be found throughout the respiratory system but are most commonly found in the trachea and voice box (laryn Tissue charges can range from mild, in which there is just an excess of mucus present, to severe, in which there can be a severe mucoid tracheitis characterised by structural degeneration, necrosis and haemorrhage.
Often mucoid/semi caseous casts can be seen which can extend the length of the windpipe in the severest forms there can be severe haemorrhage into the traches resulting in blood cast/clots in the trachea in the midst forms of LT the gross lesions may only comprise oedema and congestion of the conjunctiva and infraorbital sinuses and a mild mucoid tracheitis
At the histopathological level intranuclear inclusion bodies can be found in respiratory epithelial cells as early as three days post infection. These inclusion bodies are typically seen early in the disease and may be hard to find later on

ILT virus infection results in viral replication multiplication) in epithelial cells of the trache and larynx and sometimes in these cells in the conjunctiva, respiratory sinuses and lower respiratory tract. Infectious virus usually remains in tracheal tissues and secretions for about a week. The ILT virus can spread to the trigeminal ganglia parts of one of the facial nerves) and LT vinus can be reactivated from these ganglia for some time-at least 15 months. This reactivation can occur when chickens are rehoused pre-lay

Various respiratory diseases can present clinical signs and pathological lesions similar to those seen in ILT and so laboratory tests are required to confirm a dugnosis of this disease. The presence on histopathology of intranuclear inclusion bodies in the trachea is confirmation of ILT