Comparison between active and passive surveillance within the network of epidemiological surveillance of animal diseases in Chad

A comparative study between passive and active surveillance based on herd visits (villages) was conducted over a period of 24 months. It included 106 surveillance stations of the animal disease epidemiological surveillance network in Chad distributed randomly into 52 stations of active surveillance and 54 stations of passive surveillance. Nine diseases of various vaccination and expected prevalence status were monitored. The active surveillance stations carried out four herd visits monthly to look for the diseases under surveillance and organised four farmers awareness-raising meetings to stimulate them to make disease notifications. The passive surveillance stations held each month four farmer awareness-raising meetings. The suspicions recorded by the stations were consigned to a suspicion form specific to each disease, indicating whether a call from the farmer, a visit to the herd or a awareness-raising meeting was the source. The results showed that, irrespective of surveillance type, all diseases under surveillance, except the rare diseases (Rinderpest and Rift Valley fever) were reported by the surveillance agents. However, suspicions recorded following farmer calls are significantly more important than suspicions carried out during herd visits or meetings. Nevertheless, a considerable number of suspicions is recorded during awareness-raising meetings. Finally approximately 83% of the herd visits realised by the active surveillance stations showed negative results (no suspicion identified). Passive surveillance stimulated by awareness-raising meetings appears to be better adapted to Chads conditions and less expensive for the surveillance of existing diseases. However, for the rare diseases, other methods of specific active surveillance (such as for example sentinel herds) remain important to complete passive surveillance.