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Virulence in chickens. Ten 8-week-old chickens were infected oculonasally with 104 PFU of R65 (black diamonds) or R65-PB2K627E (white circles) and scored daily for clinical symptoms. The birds were observed for 10 days for clinical signs and scored as healthy (0), ill (1), severely ill (2), or dead (3); the daily clinical score was calculated from the sum of individual clinical scores from all birds divided by the number of animals per group.
