Why is Egypt described as a beautiful heifer that has been bitten by a biting fly?

Egypt is described as a beautiful heifer, probably referring to her prosperous and fertile agricultural economy. But a biting fly from the north has come upon her. A biting fly is not just a minor irritation, but an insect whose bite would cause cattle to be totally overwhelmed with agitation. The invasion from the north would cause chaos in Egypt but would not be fatal.¹ Again, Babylon is not named but is clearly implied.