The dust was not fit matter to produce lice, and therefore shows this work to be Divine and miraculous. All the dust of the land, i.e. a great part of it, the word all being commonly so understood in Scripture.
Related Commentaries of Exodus 8:17
Exodus 8:17
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.