Virulence of three vaccinal lentogenic strains (La Sota, Ulster and VG-GA) of the Newcastle disease virus (NDV) was determined by morphometric analysis of tracheal thickness, in 45-day-old SPF chickens (n=12), free from NDV antibodies. Tracheal thickness was evaluated 1, 2, 3, 4, 6, and 8 days after intratracheal vaccination. La Sota strain induced higher tracheal swelling than the others, during most of the time of the experiment. Maximum swelling of tracheal mucosa occurred at the thirdday after vaccination. At that time, La Sota and Ulster had the same virulence, and both caused higher swelling of tracheal mucosa than VG-GA strain.
Chicken; virulence; lentogenic vaccines; Newcastle disease
