Infectious laryngotracheitis (ILT) is a respiratory tract disease affecting chickens around the world. The disease generates severe production losses due to increased mortality, decreased egg production, delayed body weight gain, and a predisposition to other respiratory pathogens causing enormous economic losses to the intensive poultry industry. Two licensed vaccines, comprising live or vectored ILT, are available to control the disease. The present trial was conducted to determine changes in chicken interferon gamma fold expression levels in chicken spleens and to evaluate tracheal lesion scores before and after vaccination with live and vectored infectious laryngotracheitis vaccines. Broilers were kept under commercial conditions until 35 days of age. Spleen and trachea samples were taken at 14, 28, and 35 days of age. Tracheas were stained with H&E, histopathology was performed, and the INF-γ fold expression level in spleen samples was analyzed. In the spleens of birds vaccinated with the live ILT vaccine, the increase in INF-γ expression levels was statistically significantly (p < 0.01) lower at 28 days of age, or 7 days post vaccination, (trial 1) and higher at 35 days of age, or 14 days post vaccination, (both trials). At 7 days post vaccination with the live ILT vaccine, the mean tracheal lesion score was significantly higher (p < 0.01) than it was in the control group (both trials). At 14 days post vaccination with the live ILT vaccine, the mean tracheal lesion score was significantly higher (p < 0.05) than it was in the control group 3 (trial 1). No significant differences were found between the control group 3 and group 2, which was vaccinated with the vectored HVT/LT vaccine (both trials). The results of the histology of trachea lesions indicate that vaccination with live ILT may have induced early local immunity. The INF-γ analysis results might indicate that the birds in group 2 (HVT/LT) did not develop local immunity until 35 days of age From the practical point of view, the vectored ILT vaccine is more beneficial because of its ease of administration in the hatchery, lower labor cost, and the absence of clinical signs post vaccination. Its potential drawback, however, is poor local immunity and the slow onset of optimal immunity in commercial broiler chickens.
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