Use of virulence index tests for avian influenza viruses.

The intravenous and intracerebral pathogenicity index tests normally used for Newcastle disease virus isolates were used to measure the virulence of 13 avian influenza viruses. The tests allowed quantitative measurements of the virulence of the avian influenza viruses, and the results confirmed the range in virulence, between the two extremes, of the avian influenza viruses and demonstrated the lack of correlation between virulence and antigenic type.