Since 2003, nineteen human cases of highly pathogenic avian influenza (HPAI) have been reported in Cambodia, eight of them, all fatal, occurred in 2011 in people <19 years old. The source was mainly sporadic infection with direct exposition to sick poultry but this sudden increase of incidence was alarming for the national health authority of Cambodia. In order to assess the sensitivity of the national surveillance system of HPAI H5N1 in Cambodia, scenario tree modelling was used to describe and compare the components of the surveillance system in human and animal populations in order to make recommendations to enhance early detection of outbreaks. The human surveillance consists of two main components: one based on syndromic surveillance with weekly case reporting but with low capacity for laboratory confirmation and one, more research based, with a high sensitivity but low coverage. For the animal surveillance, several components were implemented but few were sustainable. The most established one is the passive surveillance based on the network of 12,000 Village Animal Health Workers. The sensitivity of this component was estimated to be 0.54 (95% CI 0.18-0.80) but with large variation between provinces. This study showed that some components in the animal surveillance system, like the market surveillance, need to be redefined in order to meet the objective of early warning and that there is a great effort needed to integrate human and animal surveillance together. (Texte intégral)