This is Ignatius in full rant and it perfectly conjures the image of New Orleans as, in his eyes, some kind of human swamp. The use of the word 'jade' is particularly apt given Ignatius's interest in the medieval world
The OED defines it as a bad-tempered woman or a worn-out horse. It has a Middle English origin and, in its horse sense at least, was used by Chaucer. There's a brief disussion on its meaning here.