| Newcastle disease (ND), caused by the Newcastle disease virus (NDV), is an acute, violent and contagious avian disease. It is also one of the List A animal infectious disease classed by Office International Des Epizooties (OIE). It has caused worldwide outbreaks for four times as yet, and poultry productions of many countries have suffered considerable economic losses. Waterfowls such as geese and ducks were considered to be the natural reservoir for viruses. However, since the year 1997, the geese flocks in some regions of the East and South China have suffered from an acute disease, whose characteristic is the pathological lesions of alimentary canal and immune organ. The disease has spreaded quickly and has become a nationwide threat to Chinese goose farming. The pathogen has been identified to be the Newcastle disease viruses, whose F gene belongs to genotypeâ…¦.The main measure to control the epidemic of ND is preventive inoculation. Theoretically, classical ND vaccines are serviceable enough to prevent the infection of any virulent NDV strains, because NDV has only one serotype. Nevertheless, one certain vaccinal strain, which has experienced replication again and again or long-term use, may strengthen its virulence or weaken its immunogenicity increasingly. This would undoubtedly add difficulties to the prevention of diseases. The emergence of untypical ND, the occurrences of new virulent NDV strains which could infect geese, the existence of different genotypes may be caused by the long-time and extensive use of live vaccines. Therefore, any low virulent strains, whose immunogen is excellent enough to serve as vaccinal strain, deserve to be researched.A natural avirulent NDV strain was isolated in Jiangsu province from a healthy goose flock. Hemagglutination (HA) tests, Hemagglutinin inhibition (HI) tests, electron microscopy and biological experiments were used to identify this NDV strain. The virions were pantomorphia, enveloped by peploses displayed from electron microscope. The results revealed that the Minimum Lethal Dose (MLD) was 10-5/0.1ml, the Mean death time (MDT) of chicken embryos was 148.8h and the Intracerebral Pathogenicity Index (ICPI) in 1-day-old chicken was 0.375. The numerical data were consistent with the standard of NDV avirulent strains, and it was named after G0405.Fusion (F) gene of the virus strain was amplified by RT-PCR, and then cloned and sequenced. The nucleotide sequence of F gene and the amino acids sequence that was encoded were analyzed and a phylogenetic tree was constructed based on obtained sequences of reference NDV strains from the domestic and other countries. The results revealed that the F gene of G0405 strain belonged to genotypeâ… . Its cleavage site region of amino acid sequences of F protein was 112G-K-Q-G-R-L117, which was the characteristic motif of avirulent strains. The homologies of nucleotide sequence of the coding region of F gene between G0405 and the other NDV strains were from 87.8% to 96.0%, and the homologies of the N-terminal 551 amino acids sequence were from 89.5% to 98.4%.The immunological characteristic of G0405 was researched by immunogenicity test. The result proved that its strong immunogenicity could stimulate high HI antibody. The HI titers could be observed 5 days after immunization, and achieved the top level (7.2 Log2) 12 days after immunization. There is no significant difference between immunized and control geese who were feed together. The virus of G0405 strain that was used to immunize geese was amplificated in geese embryos and chick embryos. The result showed that the virus from geese embryos origin had better effect than the other. G0405 strain obtained higher HI titers than V4 strain, but lower than Lasota strain. All challenge experiments showed that each G0405 group as well as Lasota and V4 group could resist virulent NDV strain belonged to genotypeâ…¦.In conclusion, G0405, the natural avirulent NDV strain of goose origin, has potentiality to be a new candidate of NDV vaccine strains. |
