I will sever ... - This severance constituted a specific difference between this and the preceding plagues. Pharaoh could not of course attribute the exemption of Goshen from a scourge, which fell on the valley of the Nile, to an Egyptian deity, certainly not to Chepera (see the last note), a special object of worship in Lower Egypt.
Related Commentaries of Exodus 8:22
Exodus 8:22
22 And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the LORD in the midst of the earth.