Ex 9:1 Then the LORD said to Moses, "Go in to Pharaoh and tell him, 'Thus says the LORD God of the Hebrews: "Let My people go, that they may serve Me.

2 "For if you refuse to let them go, and still hold them,

3 "behold, the hand of the LORD will be on your cattle in the field, on the horses, on the donkeys, on the camels, on the oxen, and on the sheep-a very severe pestilence.

4 "And the LORD will make a difference between the livestock of Israel and the livestock of Egypt. So nothing shall die of all that belongs to the children of Israel."'"

5 Then the LORD appointed a set time, saying, "Tomorrow the LORD will do this thing in the land."

6 So the LORD did this thing on the next day, and all the livestock of Egypt died; but of the livestock of the children of Israel, not one died.

7 Then Pharaoh sent, and indeed, not even one of the livestock of the Israelites was dead. But the heart of Pharaoh became hard, and he did not let the people go.

God slapped many pagan gods of Egypt in the face here. Pagan gods associated with the animals that were hit by the plague like Apis, Ra, Hathor, Anubis, Bakis, Mentu and Khnum were all discredited. 

Only all the animals "in the field" were struck. It should not surprise as later on in the chapter that there were other animals which survived. Apparently, there were other Egyptians who took Moses' warning seriously who secured their animals from the field at this point in time. 

This was a direct hit to the economy of Egypt. The cattle, for example, was considered as the most important wealth of Egypt. Oxen were used as "equipment" to do heavy labor in agriculture. Camels, horses, donkeys were for transportation. This was an all-around economic crises here in Egypt. The stock market was down! 

The timing, duration and the limitation of the plague prove the supernatural element of this disaster. Someone supernatural was obviously controlling the timing, the duration and the extent of the plague.