Isolation, identification, biological characterization and patho-epidemiology of genotype-XIII Newcastle disease virus outbreak in commercial vaccinated broiler farms

Newcastle disease (ND) is one of the most destructive diseases of poultry and has the potential to cause huge economic losses in the poultry industry. There were several outbreaks of ND recorded in and around Anand area of Gujarat despite routine vaccination programs demanded a scientific investigation. Isolation and identification of the Newcastle disease virus (NDV) were done by propagation of NDV in specific pathogen-free (SPF) embryonated chicken eggs of 9–11 days, Haemagglutination (HA) and Haemagglutination Inhibition (HI) tests. Biological characterization was done by Intracerebral Pathogenicity Index (ICPI). During the field study, mortality due to ND was documented in 10 broiler farms in spite of routine vaccination which contain Genotype-II strain of the NDV. The incidence of ND was mostly noticed in 23–34 days of age and during severe hot months of May and June, and average mortality rate in affected farms was 28.11% for the period of 14 days. The disease resulted in substantial reduction in body weight and feed intake. Greenish diarrhoea was generally seen in birds that survived in early infection and birds with torticolis showed lingering mortality. Gross lesions were characterized by wasting and dehydrated carcass with deep congestion of breast musculature, multifocal to diffuse haemorrhages around the proventricular gland and necrotic (diptheretic) haemorrhagic ulcers throughout the intestine and caecal tonsils. The microscopic lesions include marked atrophy of the lymphoid organs leading to immunosuppression in vaccinated chickens. All the 10 field samples along with F and R2B vaccine strain were found positive for HA and HI activity and the values of ICPI were 2.0 for all the field samples which were suggestive of velogenic NDV strains. The confirmation of Genotype-XIII NDV was supported by F gene sequencing and whole genome sequencing. Isolation of the novel genotype from the field outbreaks confirms the limit of presently used live (Genotype-II Lentogenic/LaSota/B1) and inactivated vaccines (Mesogenic/R2B vaccines) to protect the birds against genotype XIII NDV infection. This wide gap in the genetic makeup could be an important reason for the vaccine failure. More and more field isolates from various regions of the country should be genotyped and research should be directed towards identifying suitable live candidate vaccine virus.