Derivation of an isolate of low virulence from the Essex '70 strain of Newcastle disease virus
Reeve, P.; Alexander, D.J.; Allan, W.H.
Veterinary Record 94(2): 38-41
1974
ISSN/ISBN: 0042-4900 PMID: 4818678 Document Number: 72234
ND virus of low virulence was derived from the virulent 'Essex '70' strain by passage to limiting dilutions in chick embryos and the selection of the last egg to die or infected embryos still alive six or seven days after infection. The intracerebral pathogenicity index of the virus was shown to diminish with passage from an initial 1.65 to 0.44 by the 15th passage. The latter virus, designated 'Scrubs '72', had advantages over the La Sota vaccine when administered by aerosol in that less respiratory distress was caused although similar Hl titres were induced; these were felt sufficient to justify large scale evaluation of the virus as a potential vaccine.