Abstract: A Newcastle disease vaccine and method of using the vaccine for protecting a poultry animal from Newcastle disease is provided. The vaccine consists of a naturally attenuated live Newcastle disease virus which has the identifying characteristics of ATCC No. VR 2239. The vaccine is capable of conferring solid immunity against Newcastle disease and causes minimal respiratory or other undesirable reactions in inoculated animals.

Abstract: Mutogenesis of the Clemson University (CU) vaccine strain of P. multocida with N-methyl-N-nitro-N-nitrosoguanidine resulted in temperature sensitive (TS) mutants which grew at 37.degree. C. but not 42.degree. C. Two mutants, PM#1 and PM#3, were found to provide turkeys with a level of protection against challenge with a virulent serotype 3 P. multocida strain (P-1059) comparable to the protection provided by the CU strain. Intravenous (IV) inoculation of PM#1, PM#3, and CU was used to determine differences in virulence. PM#1 and PM#3 resulted in lower mortality and lameness than the CU strain. Histopathological evaluation of spleens at 24, 48, and 72 hours post IV inoculation demonstrated that the CU strain induced 2 times more splenic necrosis than either PM#1 or PM#3.