The same reason we have different Antibiotics some work better than others for different infections. moxidectin and fenbendazole kill parasites in multiple larvae stages that abenza and ivermectin don't. moxidectin is like ivermectin but much stronger and for serious infections in animals moxidectin is NOT what you start off with. Thats why ICUS protocol is safe and effective. But now that I have my infection down to a minimum I started taking fenben. after a few series of fenben I will try moxidectin . As sick as I was after steroids and the extremely high worm burden moxy or fenben might have been to strong killing too many worms at one time causing septic shock. fenben is what my farm doc wanted me to switch to but only after I had my worm burden to a minimum. fenben is strong but moxidectin is the strongest (according to farm doc) and should never be used in serious parasitic infections but once the serious infection is controlled this is what doc switches to(fenben 1st and moxy 2nd).