In March 1999 a syndrome characterized by depression, anorexia, fever, and respiratory and enteric signs appeared in many flocks of turkeys and, to a lesser extent, chickens in the densely populated poultry-rearing regions of Northeastern Italy. Initially the disease was characterized by sinusitis, tracheitis, peritonitis, and pancreatitis. The responsible agent was identified as low-pathogenicity (LP) avian influenza (AI) of H7N1 subtype. Concerning the light layers, the mortality was variable, from 1.7% to 9.5%, whereas egg production decreased by 10% to 40%. According to the epidemiologic data, chickens seemed to be less sensitive to the virus than were turkeys. Nine months later, the AI virus changed to a highly pathogenic (HP) AI virus and affected, besides turkeys, a great number of pullet and layer flocks, with high mortality (80%–100%) in a few days. However, the course of disease was more prolonged in pullets. Within 3½ mo, over 100 outbreaks were reported. Following the HPAI outbreaks, in late 2000 and early 2001, LPAI reemerged, but only one flock of layers was affected.
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