| Newcastle Disease is an acute and highly contagious disease of poultry caused by Newcastle disease virus. The disease was first reported in Indonesia's Java and New England's Newcastle in 1926. Up until now, the disease has been spread to many countries and regions of the world and has always being one of the most important diseases threatening poultry industry. It is an A-list disease of OIE and I-list disease according to Ministry of Agriculture. Newcastle Disease is a member of Rubulavirus, which is a subfamily of Paramyxovirinae. It is a single-stranded, nonsegmented, negative-sense RNA virus. The full length genome is about 15kb containing 6 functional genes (3′-NP-P-M-F-HN-L-5) coding six proteins(NP protein, P protein, M protein, F protein, HN protein and L protein). No effective drugs and cures are available for the disease. Vaccine inoculation is still the principal mean to prevent outbreaks and spread of the disease both in and aboard.The hemagglutination titer, viral content and the stability of the vaccine strain virus (La Sota) of our lab decrease, causing a decline in the quality of vaccine. Based on this situation, plaque purification technology and End-point dilution method were used to purify the current vaccine strain. Homogeneous plagues formed 60-72h after inoculation with the diameter of 1.0～2.0mm were chosen. After being purified for 3 generations, chicken embryo end–point dilution was applied. Allantonic fluid with the highest dilution rate and hemagglutiantion titer was chosen for further end-point dilution. After being purified for 5 generations, one strain with high hemagglutination titer and stable quality was obtained. Death time inoculated with this strain of virus is comparatively uniform, mainly in the range of 96 to 117 hours. The purified strain is a good fit for vaccine production. Compared with the original strain, the hemagglutination titer the purified strain was 11log2 compared to 9log2 of the original virus, the EID50 was 10-9.5/0.1mL compared to 10-9.1/0.1mL of the original virus ,and the stability of the purified strain were also improved. The homology of F gene and HN gene between the purified virus and the original virus was 99.5% and 98.6% respectively, which indicates no significant mutation was found during the purification and screening.At the same time, immunogenicity test, safety test and the test of immunogenic efficacy indicated that the purified strains were safe, with better immunogenicity and effectiveness which posed no adverse effects to the chickens, fit the Requirements of "State Veterinary Biological Products criterion" . Test chickens inoculated with the purified vaccine strains can effectively resist the attack of virulent Newcastle disease virus, the result suggests that the purified strain can be used as a candidate vaccine strain of Newcastle disease for the development of new effective vaccines, as well as to provide a better master seed of vaccine for better prevention the outbreak of Newcastle disease.
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