Highly pathogenic avian influenza A H5 virus outbreaks in broiler farms in the Netherlands – Clinical signs, transmission and identification of reporting thresholds

Background For a successful control of highly pathogenic avian influenza virus (HPAIV) epidemics in poultry, early detection is key and it is mostly dependent on the farmer’s prompt identification of disease and reporting suspicions. The latter could be further improved by providing references to farmers for triggering suspicions. Methods Here we report observations on clinical signs of HPAIV H5N1 and H5N8 infected broiler farms in the Netherlands and analyze their daily mortality and feed and water intake data in order to identify thresholds for reporting suspicions. In addition, mortality data was used to characterize the transmissibility of these viruses, which could help estimate how fast infection spreads within the flock and when infection was likely introduced. Results The most frequently observed clinical signs in affected flocks were sudden increase in mortality, cyanosis of wattles comb and/or legs and hemorrhagic conjunctiva. Analysis of mortality data indicate that daily mortality higher than 0.17% is an effective threshold for reporting a HPAIV-suspicion. Reliable thresholds for food or water intake could not be stablished. The estimated within-flock transmission rates ranged from 1.1 to 2.0 infections caused by one infectious broiler chicken per day. Conclusions We identified effective mortality thresholds for reporting suspicions of HPAIV infections. The estimated transmission rates appear to indicate a slow progression of a H5 HPAIV outbreak in affected broiler flocks. The information here provided can be used to improve syndromic surveillance and guide outbreak response.