Cysteine protease inhibitor which inhibits members of the peptidase C1 family (By similarity). Does not inhibit asparaginyl endopeptidase (PubMed:15664654). In the vector stage, expression starts at the L2 larval stage and continues throughout the L3 infective larval stage. After infecting the host, expression rapidly decreases and is absent from the L4 larval stage and in adults (PubMed:9233676, PubMed:18249028). Not expressed in microfilariae (PubMed:9233676). Belongs to the cystatin family.