Infectivity for chicken embryos of tissue-culture-modified infectious laryngotracheitis virus

The present study shows the infectivity for chicken embryos of the SPL strain of tissue-culture-modified infectious laryngotracheitis (ILT) virus. The modified virus formed small pocks on the chorioallantoic membrane (CAM) and gave no adverse reaction after administration to chickens. For 1- and 6-day-old embryonated specific-pathogen-free (SPF) eggs, the SPL strain showed higher infectivity than virulent strain NS-175 or wild strains of ILT virus. A high percentage of embryos of day-old eggs inoculated with strains SPL via the yolk sac died within several days, and virus was recovered from yolk and embryos with high titer. In the embryos inoculated at 6 days of age, histopathological lesions characterized by syncytium formation with intranuclear inclusion bodies were observed mainly in the CAM, liver cells, epithelial cells of the oral cavity, and esophagus and rarely in epithelial cells of the uriniferous tubule, bronchus, trachea, choroid plexus, or feather follicles. In the 10-day-old embryonated SPF eggs inoculated via the chorioallantoic cavity, however, the SPL strain demonstrated low pathogenicity compared with strain NS-175 and the two wild strains.