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According to the Merck Manual the incubation period for ILT is 5-12 days. If you have had the birds longer then that then they could have been infected by wild birds or other creatures that they have come in contact with. Another possibility is that they have survived the disease when young and the stress of the new home made it reoccur.
This is from the Merck Manual.
Infectious laryngotracheitis (ILT) is an acute, highly contagious, herpesvirus infection of chickens and pheasants characterized by severe dyspnea, coughing, and rales. It can also be a subacute disease with lacrimation, tracheitis, conjunctivitis, and mild rales. It has been reported from most areas of the USA in which poultry are intensively reared, as well as from many other countries.
Clinical Findings:
In the acute form, gasping, coughing, rattling, and extension of the neck during inspiration are seen 5-12 days after natural exposure. Reduced productivity is a varying factor in laying flocks. Affected birds are anorectic and inactive. The mouth and beak may be bloodstained from the tracheal exudate. Mortality varies, but may reach 50% in adults, and is usually due to occlusion of the trachea by hemorrhage or exudate. Signs usually subside after ~2 wk, although birds may cough for 1 mo. Strains of low virulence produce little or no mortality with slight respiratory signs and lesions and a slight decrease in egg production.
After recovery, some birds remain carriers for extended periods and become a source of infection for susceptible birds. The latent virus can be reactivated under stressful conditions. Infection also may be spread mechanically. Several epidemics have been traced to the transport of birds in contaminated crates.
