Between January 6 and April 23, 1997, 11 outbreaks of Newcastle disease were confirmed in Great Britain, four in broiler chickens and seven in turkeys. Although the viruses isolated gave intracerebral pathogenicity indices in day-old chicks between 1.65 and 1.95, the clinical signs of disease in field infections were variable and not always associated with high mortality, especially in turkeys. Epidemiological investigations indicated that the majority of the outbreaks occurred as a result of secondary spread by human agency from two or more primary infected flocks. The presence of similar outbreaks in Scandinavian countries in 1996 and the unusual patterns of movement of migratory birds at the end of 1996 and beginning of 1997 suggest they may have been responsible for the primary introduction of the causative virus into Great Britain.
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