"Your cat came in here yesterday with a fever of undetermined origin," Franklin told me. Then she broke into what seemed an inappropriate smile of joy. "But I know what's wrong. You found it. And I can fix it right now! Your cat chased a rabbit into its hole."
JXN had a "cuterebra grub" which is toxic to cats, Franklin said, but once it's removed an adult cat will quickly recover. JXN's fever and accompanying slightly elevated white blood cell count were his immune system's response to the grub's toxins.
Half an hour later, I was taking a woozy JXN home. He had a shaved spot on his neck, and the hole was a bit larger where Franklin had used tiny tweezers to gently remove the entire grub.
A bit of research on the Michigan Department of Natural Resources website helped me understand that JXN was the victim of Cuterebra horripilum. After a pair of botflies' summer mating, gravid females deposit eggs, usually along runways or at entrances to the host animal's burrow. C. horripilum tends to seek out the throat region in cottontail rabbits (or overly curious marauding cats). JXN probably picked his up chasing a rabbit into its lair.