Within-flock mortality during the high-pathogenicity avian influenza (H7N7) epidemic in the Netherlands in 2003: Implications for an early detection system

Daily within-flock mortality data, from a few days before until a few days after onset of increased mortality, from H7N7-infected flocks were analyzed with nonlinear regression for layer (organic and free-range or caged), broiler, and turkey flocks. The following notification thresholds were recommended for the Netherlands: 1) organic layer flocks, broiler flocks, and turkey flocks <= 11 wk of age: >= 0.5% mortality/day for two consecutive days; 2) layer flocks with birds housed in cages: >= 0.25% mortality/day for two consecutive days; 3) turkey flocks >= 16 wk of age: >= 1% mortality/day for two consecutive days. Notification of increased mortality to the veterinary authorities should take place on the second day of increased mortality. Interpretation of mortality thresholds should be on the level of the poultry barn in which clinical problems arise. Because of nonoptimal specificity of proposed thresholds (mortality possibly caused by other diseases), use of PCR-diagnostics (results within 24 hr) without costs to the individual farmer should be promoted to exclude avian influenza in suspect clinical situations in order to minimize negative economic consequence for farmers and stimulate notification by farmers and veterinary practitioners.