Exodus 8:17
And they did so; for Aaron stretched out his hand with his rod, and smote the dust of the earth, and it became lice in man, and in beast; all the dust of the land became lice throughout all the land of Egypt. And they did so; for Aaron stretched out his hand with his rod, and smote the dust of the earth, and it became lice in man, and in beast; all the dust of the land became lice throughout all the land of Egypt.
The plague followed Pharaoh's renewed refusal to release Israel once relief from the frogs had come, for when the pressure lifted he hardened his heart and would not let the people go. Then at God's command Aaron stretched out his hand with his rod and struck the dust of the ground, and all the dust throughout the whole land of Egypt turned into lice upon people and animals. Those who number it call it the fourth plague, a stroke in which the smitten dust spread as lice afflicting man and beast everywhere in Egypt.
The change of dust into lice was accomplished by the power of God alone working through Moses and Aaron, with no human power, influence, or science of their own producing it. It was sent as a sign to convince Pharaoh that the great I AM had sent Moses and that it was Pharaoh's duty to let Israel go to serve the living God, giving him sufficient evidence that he could have believed if he had been willing. The stroke also fell upon Egyptian idolatry, since the lice rendered the people ritually impure so they could not approach the altars, and that interruption of worship exposed the inability of Egypt's idols to protect their worshipers and declared Jehovah supreme.
Moses in breaking the bondage prefigured Christ's first advent to break Satan's power over the human family and set captives free. Satan stood upon the ground to oppose God's work by aiding the magicians to counterfeit it, aiming to destroy the influence of Moses' miracles, to harden Pharaoh against God's power, to stagger the faith of Moses and Aaron, to keep Israel in servitude, and to reach down through all ages to destroy faith in Christ's future mighty miracles. In that conflict Moses and Aaron acted as servants of God working by His command, while the magicians acted as servants of Satan working by his command, not by their own science alone but by the power of their god the devil, whose enchantments can accomplish only a few things.
Pharaoh called his magicians to do the same by their enchantments, but they could not bring forth the lice, and their failure proved God's work superior to Satan's power and brought their imitative power to an end. God would not suffer Satan to go further, and His controlling power cut off the channel through which Satan had been working. Convinced by their own inability, the magicians were compelled to confess to Pharaoh that this was the finger of God. Yet even after that acknowledgment Pharaoh's heart was hardened, he remained unmoved and would not listen, just as the Lord had said.