Effectiveness of control measures on the transmission of avian influenza virus (H7N7) between flocks

On 28 February 2003 an epidemic of fowl plague started in The Netherlands, caused by a highly pathogenic avian influenza virus (HPAIV) of type H7N7. The epidemic started in the ‘Gelderse Vallei’, spread to adjacent areas and to the province of Limburg. During the epidemic, 255 flocks were diagnosed as infected. The epidemic was combated by stamping out infected flocks and pre-emptive culling of flocks within a 1-km radius. Moreover, screening and tracing activities were implemented to enhance the detection of infected flocks. In addition, a transportation ban was enforced. In a further stage of the epidemic, poultry-free buffer zones were created, contacts between different parts of the country were reduced by compartmentalization and large areas were depopulated of all poultry. In all, 1,000 commercial flocks were pre-emptively culled, in addition to over 17,000 flocks of smallholders. In this study we quantified the transmission of HPAIV between flocks during different phases of the epidemic. To stop an epidemic, infected flocks should be detected and depopulated before they have infected on average more than one other flock. This average number of secondary infections caused by one infectious flock is called the reproduction ratio (Rh). Upon the implementation of the control measures in the Gelderse Vallei, Rh dropped from 5.0 (95% confidence interval (CI): 2.9-8.6) to 0.91 (95%CI: 0.39-2.13). Moreover, in Limburg, Rh dropped from 2.5 to 0.86 (95%CI: 0.28-2.68) after the control measures came into force. Apparently, the measures significantly reduced the transmission of the virus. However, because the 95% confidence intervals of Rh after the implementation of control measures include one, it is uncertain whether the implemented measures are really sufficient to eliminate the virus in an area with a high poultry density. Consequently, additional control measures should be considered