Newcastle disease virus: an evolving pathogen?
Australia experienced outbreaks of virulent Newcastle disease (ND) in chickens in the state of New South Wales in the years 1998, 1999 and 2000. The disease had occurred previously in Australia in 1930 and 1932 but the country was free of it until the recent outbreaks. Avirulent strains of Newcastle disease virus (NDV) were detected in 1966 and, during the next two to three decades, strains (so-called lentogenic strains) able to induce mild respiratory disease equivalent to that induced by vaccine strains such as LaSota were also detected. Nucleotide sequence analysis of the genes encoding the haemagglutinin and fusion proteins of Australian isolates of the virus during this time demonstrated that Australian chicken strains of NDV could be differentiated from NDV isolated elsewhere. Analysis in this way demonstrated that NDV isolates causing the recent outbreaks of virulent disease were Australian viruses that were so closely related to a recognized Australian lentogenic strain, termed the Peat's Ridge strain, that it was considered to be the precursor of the virulent virus. The outbreaks of virulent disease in 1998 and 1999 were controlled by an official "stamping out" eradication campaign. This was subsequently replaced by strategic use of ND vaccines when virulent virus was again detected on some farms that had been restocked following depopulation. The national situation with regard to ND is now being assessed through a structured national survey of ND viruses, particularly to determine the distribution of the precursor strain. No new outbreaks of virulent ND have been recognized since February 2000, although immunization of flocks in areas where the disease was recognized has occurred.
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Australian Animal Health Laboratory, CSIRO Livestock Industries, PO Bag 24, Geelong, Victoria 3220, Australia.