Filarial nematode parasites such as Dirofilaria immitis (Dim) and Brugia Malayi (Bma) enter their definitive host via the bite of an insect vector at the developmentally arrested iL3 stage. This step is critical for establishment of the infection and development to adulthood. Previously, we have shown that the nuclear receptor DimDAF-12 is central for the development of D. immitis iL3. Here, we identified and cloned DAF-12 from B. malayi and found high sequence identity with DimDAF-12. Interestingly, Dim and Bma DAF-12 share a remarkably higher sensitivity to their natural ligands such as Δ4-dafachronic acid (4-DA) compared with DAF-12 from the non-filarial nematode species, Haemonchus contortus and Caenorhabditis elegans. We observed that Dim and Bma DAF-12 were specifically activated by mammalian sera. Remarkably, DAF-12 natural ligand Δ4-DA has been identified in mammalian sera at level above its EC50 for filarial DAF-12. Moreover, we showed that depletion of 4-DA from serum by charcoal stripping impairs significantly the development of D. immitis iL3 in vitro. Finally, analysis of RNAseq data from B. Malayi revealed that, at the time of infection, the parasites do not seek to produce their own dafachronic acids. These observations suggest that filarial nematodes have evolved to sense the mammalian host environment directly through DAF-12 to resume their development and open the route to novel therapies against filarial infections.