Pathotyping
Intracerebral pathogenicity index (ICPI) was determined following European guidelines (CEC, 1992). Ten one-day-old SPF chickens were inoculated intracerebrally with 0.05 ml of 10-1 diluted virus stock (HA > 32) and examined daily for 8 days. The ICPI is calculated by scoring each bird every day from 0 (normal), 1 (sick) and to 2 (dead). Based on this score NDV-isolates are considered lentogenic (-0.7), mesogenic (>0,7-1.3) or velogenic (>1,5) (Alexander, 1998). Animal experiments were carried out in accordance with the legally approved protocol (MV-LALLF- /7221.3-1.1-030/13). Specific pathogen free White leghorn chickens purchased from Lohmann Animal Health, Cuxhaven, Germany, were raised in isolation units at FLI until age of inoculation. Infection and immunization experiments were performed in in separated cages at animal biosafety level-3 (BSL-3). Feed and water were provided daily.