Characterization of Newcastle disease viruses isolated from chickens and ducks in Tamilnadu, India.

During 1993, outbreaks of Newcastle disease occurred on many farms in Tamilnadu, India. Six Newcastle disease virus (NDV) isolates were obtained from the chickens on five different farms and from the birds on one duck farm during outbreaks of the disease. All the isolates were characterized as velogenic, based on the mean death time, intravenous pathogenicity index, intracerebral pathogenicity index (ICPI), stability of haemagglutinin at 56 degrees C, agglutination of equine erythrocytes, haemagglutination elution pattern and adsorption of haemagglutinin by chick brain cells. The isolate obtained from ducks resembled a group D strain, based on its ICPI and its reaction with a panel of monoclonal antibodies. The other five NDV isolates obtained from chickens were placed in groups B(1), C1(2) and D(2) on the basis of their binding patterns with the panel of monoclonal antibodies. In challenge experiments, it was found that LaSota vaccine provided 100% protection against each of these field isolates and against a local NDV strain obtained from the Institute of Veterinary Preventive Medicine, Tamilnadu, India, while unvaccinated chickens succumbed to challenge. The possible origin of epizootic viruses causing outbreaks in vaccinated flocks is discussed.