Mobile Ad-hoc NETworks (MANETs) depends on network cooperation schemes to work properly. It assume that mobile nodes voluntary cooperate in order to work properly. Nevertheless, if nodes have a selfish behavior and are unwilling to cooperate, the overall network performance could be seriously degraded. The use of watchdogs is a well-known mechanism to detect selfish nodes. Sometimes watchdogs can fail, generating false positives and false negatives that can induce to wrong operations. In this paper, the authors propose a collaborative contact-based watchdog approach, which is based on the fast diffusion of selfish nodes awareness when a contact occurs.