Epidemiologic Monitoring Of Newcastle Disease And Exploration Of Immunization Programs

Newcastle disease (ND) is a kind of highly contagious disease of poultry caused by Newcastle disease virus (NDV). Outbreak of ND was firstly reported in Java, Indonesia in 1926. Currently, the disease has a worldwide distribution and causes tremendous economic losses to poultry industry. To date, vaccination is a common practice to protect chickens from ND. However, many atypical ND cases emerged from vaccinated chickens in China each year which were led by many factors, such as the backyard model for raising chicken in rural areas, the contaminated poultry farms and the wrong immunization programs. In this study, chicken farms were monitored, and NDV isolation and identification were performed. The factors for immunization programs were systematically studied in order to provide reference for the ND control.1. Epidemiological monitoring of Newcastle diseaseDuring 2008, 4800 serum samples and 2400 cloaca swab samples were collected from 4 layer and 2 broiler farms in Jiangsu, Anhui and Shanghai. 3 NDV strains were isolated from these swab samples. According to the analysis of the F gene by reverse transcriptase-PCR (RT-PCR) and sequencing, all three viruses were lentogenic and originated from LaSota vaccine strain. The average titer of hemagglutination -inhibition (HI) antibody against to NDV in the layers was 7.9 log2, and the qualified rates were higher than 90%, while that of HI antibody against to NDV in the broilers was 5.4 log2, and the qualified rates were from 70% to 80%.Seven virus strains were isolated from Xuzhou, Hai'an, and Yangzhou of Jiangsu province. These isolates were classified as NDV by hemagglutination (HA) and HI test. In order to determine the phylogenetic relationships among these NDV isolates, the variable regions of F glycoprotein gene were amplified by RT-PCR, cloned and sequenced. Sequencing analyses of F gene showed that the seven isolates shared homology from 96.8% to 99.7%, and compared with NDV LaSota, F48E9 and ZJ1 strains, shared with homology of 78.1% to 80.5%, 80.7% to 82.6%, 94.7% to 99.5% respectively. All NDV isolates possessed Lys at position 101 and Val at position 121 which were the characteristic amino acid of the genotypeⅦNDV. The deduced amino acid sequences of F gene at the cleavage sites of all seven isolates were 112R-R-Q/R-K-R-F117, which is the typical motif of the virulent NDV strain. The phylogentic tree based on of F gene sequences revealed that the seven strains belonged to subgenotypeⅦd. The results indicated that subgenotypeⅦd virus was responsible for the NDV prevalence in Jiangsu province.2. Exploration of Immunization Programs of Newcastle diseaseOne hundred 1-day-old chickens with the maternal HI antibody against to NDV of 9.4 log2 were divided into 5 groups randomly. The chickens were challenged with 105 ELD50 F48E9 at the NDV-HI maternal antibody titer of 8 log2, 6 log2, 4 log2, 2 log2, 0, respectively. Birds were observed daily for signs of disease. The cloacal swabs were collected on 3 and 5 days post infection (p.i.) for detection of virus shedding. Complete protection against from lethal challenge with HI antibody titer of≥4 log2, virus almost could not be detected from cloacal swabs. In contrast, the chickens with HI antibody titer of <4 log2 were only partially protected against challenge and virus shedding were detected in majority of infected chickens.Sixty chickens were primely immunized with inactivated vaccine at 7-day-old, and then boosted vaccination with both inactivated and attenuated LaSota strain vaccine at 27-day-old, respectively. Three groups of twenty chickens were challenged with 107 ELD50 F48E9 at the NDV-HI antibody levels of 6 log2, 4 log2, 2 log2 approximately post vaccination, respectively. Same as before treatment, chickens were observed daily for signs of disease. The cloacal swabs were collected on 3 and 5 days post infection (p.i.) for detection of virus shedding. The chickens with HI antibody level of≥4 log2 were completely protected from lethal challenge and almost have no virus detection in cloacal swabs. In contrast, chickens with HI antibody level of <4 log2 were only partially protected against challenge and virus shedding were detected in majority of infected chickens.Two groups of twenty 34-day-old chickens with no NDV-HI antibody were vaccinated with inactivated or attenuated LaSota strain vaccine, respectively. Birds were detected antibody weekly. The antibody in the inactivated vaccine group, have not detected on the first week post vaccination, were increased at titer of 5.8 log2 on 2 week post vaccination, and reached the peak titer (9.2 log2) on the 6 week post vaccination, and then slowly descended to 4.2 log2 on 21 week post vaccination, maintained at least 20 weeks period at the level of≥4 log2. However, the antibody in the attenuated vaccine group were increased to 4.4 log2 on the first week post vaccination, 6.6 log2 on 2 weeks post vaccination, and then decreased to 4.5 log2 on the 5-week post vaccination and 4.0 log2 in 15 weeks post vaccination, maintained 15 weeks period at the level≥4 log2.As can be seen from the above experiment, the threshold value of NDV HI antibody of immune protection is 4 log2; ND inactivated vaccine produced antibodies slowly with relatively high antibody levels after immunization, while ND attenuated vaccine produced antibodies fast with relatively low antibody levels after immunization. The time for maintaining the NDV HI antibody level of effective protection of both the vaccines was longer than 15 weeks.