ILT is caused by a virus belonging to the herpes group. Only one serotype is known.
Field infection occurs from bird to bird by the respiratory route. Most outbreaks of ILT on farms are traced to transmission by fomites (visitors, shoes, clothing, egg boxes, used feeders, waterers, cages, crates etc.) The incubation period varies from 4 to 12 days.
Respiratory distress is usually quite pronounced due to the build-up of blood, sloughed tracheal lining and even caseous exudates in the larynx and trachea. When a caseous plug occludes the larynx or trachea, the affected chickens will have extreme difficulty breathing ("pump handle"breathing) and will frequenly die from suffocation.
Mortality is approximately 1% per day in a typical ILT outbreak. Milder forms of ILT outbreaks occur where less virulent strains of ILT virus are involved. Conjunctivitis and respiratory rales (wheezing) can be observed, with little or no mortality in such cases.
The disease spreads through a chicken house more slowly than either IB or ND. Egg production in laying flocks will usually decrease by 10% to 50%, but will return to normal after 3 to 4 weeks.
In a flock, signs of respiratory distress, with possible coughing-off of blood and mortality are indicative of ILT. Bloody mucous and cheesy exudates can be found in larynx and trachea. In the laboratory, a definitive diagnosis can be made by histological examination of tracheal tissues or virus isolation from tracheal mucous with embryonated chicken eggs.
Treatment and control
Prevention of ILT by vaccination with mild eye-drop vaccine is by far the best control method. Sometimes such vaccines are applied by drinking water or spray methods with variable success. Even when an outbreak of ILT has been detected in a chicken flock, immediate vaccination is advisable to stop the spread of infection.