The present invention provides for filariid nematode cysteine protease proteins; to filariid nematode cysteine protease nucleic acid molecules, in particular, Dirofilaria immitis L3 larval cysteine protease nucleic acid molecules and Onchocerca volvulus L3 larval cysteine protease nucleic acid molecules; to antibodies raised against such proteins, and to compounds that inhibit filariid nematode cysteine protease activity. The present invention also includes methods to obtain such proteins, nucleic acid molecules, antibodies and/or inhibitors. The present invention also includes therapeutic compositions comprising such proteins, nucleic acid molecules, antibodies and/or inhibitors, and the use of such compositions to protect an animal from disease caused by parasitic helminths.