Albert Barnes Commentary Exodus 9:7

"And Pharaoh sent, and, behold, there was not so much as one of the cattle of the Israelites dead. But the heart of Pharaoh was stubborn, and he did not let the people go." — Exodus 9:7 (ASV)

Was hardened – See Exodus 4:21. Pharaoh probably attributed the exemption of the Israelites to natural causes. They were a pastoral race, well acquainted with all that pertained to the care of cattle, and dwelling in a healthy district probably far more than the rest of Lower Egypt.