We consider the problem of failure detection in dynamic network such as MANET. Unreliable failure detectors (FD) are classical mechanisms providing information about process failures. They allow to solve consensus in asynchronous network. However, most of implementations consider a set of known processes fully connected by reliable links. Such an assumption is not applicable in dynamic environments. Furthermore, the majority of current failure detector implementation are timer-based ones while in dynamic networks there is not an upper bound for communication delays. This paper presents an asynchronous implementation of a failure detector for dynamic environments. Our implementation is an extension of the query-response algorithm previously proposed by Mostefaoui, Mourgaya and Raynal [MMR03] which does not rely on timers to detect failures. We assume that neither the identity nor the number of nodes are initially known. We also proof that our algorithm can implement failure detectors of class <>S when some behavioral properties are satisfied by the underlying system. Simulation results have validated our approach.