Six-week-old inbred male BALB/c mice (vivarium of FRSI SRC VB "Vector"). Animals were placed to individual cages with food and water available ad libitum. To determine the MLD 50 and MID 50 , mice were anaesthetized by diethyl ether inhalation and infected intranasally with 50 µl 10-fold serial dilutions of allantoic fluidin PBS (pH 7,2). Each group contained 10 animals. Animals were observed daily for 15 days for mortality (MLD 50 ) or sacrificed on day 5 after the challenge with following virus detection in the lungs by inoculation of 10 days-old embryonated chicken eggs (MID 50 ). MLD 50 and MID 50 were calculated by the method of Reed and Muench. Animals from group where 1MLD 50 had been observed were taken to determine virus titers in lung, spleen, kidneys, and liver and brain tissues. Mind time to death (m.t.d) was calculated as previously described [13] . Pathogenicity to chickens was determined by IVPI test [14] . All animal studies were performed according protocols approved by Animal Care & Use committee of FSRI SRC VB "Vector".