The distinction is now locked in. Targum Pseudo-Jonathan on Exodus 9:4: the Lord will work wonders between the flocks of Israel and the flocks of the Mizraee, that not any of those which belong to the sons of Israel shall die.
The word the meturgeman uses is porishan — distinctions, or wonders of separation. By the fifth plague, the divisions have become almost legal. A property line runs through the middle of every pasture in Egypt. On one side of the line, the cattle fall. On the other side, they live. The same air, the same grass, the same water — and entirely different outcomes.
This is not a natural epidemic. Epidemics do not check owners' registries. Only God does. And the meturgeman wants Pharaoh — and the listener — to notice that the plague is not a disease. It is a court summons with names on it.
For the Israelites watching, the distinction is a promise as much as a protection. Their animals' survival is the visible token that they are being counted separately in the divine books. Long before they walk out of Egypt, their cattle are already walking as free cattle while Egyptian cattle are dropping in the next field.
The takeaway: God's redemptions begin before His redemptions. The line between exile and freedom is drawn through our possessions long before it is drawn through our lives. By the fifth plague, Israel's livestock are already out.