Studies of infectious laryngotracheitis vaccines: immunity in layers

Ten-week-old layer chickens obtained from a commercial source were eye-drop vaccinated with chicken-embryo-origin (CEO) or tissue-culture-origin (TCO) vaccines for infectious laryngotracheitis (ILT). Controls were not vaccinated. Approximately one-third of the layers were challenged with virulent ILT virus at 21, 40, or 60 weeks of age. Serum samples taken from the layers before challenge were used in a virus neutralization (VN) test to determine vaccination titers at those three ages. Both vaccines induced low VN titers (geometric mean titer GMT] less than 6). At 21 weeks of age, the titers produced by the two vaccines were not significantly different, but at 40 and 60 weeks of age the VN GMT of the CEO-vaccinated group was significantly greater than that of the TCO-vaccinated group. The VN GMTs did not drop over time in either group and actually rose between 21 and 60 weeks of age in the CEO group. Both vaccines protected layers against severe challenge with virulent ILT virus, neither being significantly better than the other under these experimental conditions. Unvaccinated sentinel chickens were maintained in contact with the vaccinated layers during three intervals between 1 day and 6 weeks post-vaccination. Diagnostic tests performed on the sentinels to detect lateral spread of vaccine virus from vaccinated to unvaccinated chickens showed scattered positive results.