Jamieson, Fausset and Brown on Exodus 9:6

6. all the cattle of Egypt died--not absolutely every beast, for we find (Ex 9:19, 21) that there were still some left; but a great many died of each herd--the mortality was frequent and widespread. The adaptation of this judgment consisted in Egyptians venerating the more useful animals such as the ox, the cow, and the ram; in all parts of the country temples were reared and divine honors paid to these domesticated beasts, and thus while the pestilence caused a great loss in money, it also struck a heavy blow at their superstition.