Well, I didn't have the resources to do a proper dissection of the infected animal, but it appeared as if the central nervous system was heavily affected by a parasitic growth - it had mushroom caps growing along its neck and cranial hull, and up onto the antlers, and it was much more aggressive than the other deer I saw out there.

Honestly, if it's not something that reverted after the Fissure stabilized, I'm concerned about the risk the infection might pose to anyone going back to the forest. Especially if people are inclined to use the deer as a food source.