But the hand of Yahweh was heavy on them of Ashdod, and he destroyed them, and struck them with tumors, even Ashdod and the borders of it.
Returning to the great swarm of rats which was a key element in the judgment of God against the Philistines, that would have been a double plague.  The rats not only carried the bubonic plague infection but devastated the fields and ate up the crops as well.