The story about Louis brought up some memories for me that I thought I should
share as a warning to other cat slaves.  The cause was different but the
symptoms were the same.  

Several years ago our Bengal, Tigger, stopped begging for her treats in the
morning.  I noticed she hadn't been eating her food either and when I picked
her up she seemed warm.  She had gotten her distemper and rabies vaccines a
week earlier.  I took her into the vet that afternoon and she had a very
elevated temperature.  I asked if the vaccine could be the cause and was told
a reaction would have happenned immediately not a week later.  I paid $400.00
there for tests and then had to pick her up and take her to an overnight care
center since they didn't have people there at night.  For three days I had to
pick her up in the morning and take her to the vet and then pick her up in
the evening and take her back to the overnight place.  All in all it cost
about $1500.00.  She came very close to dying.  The whole time my DH and I
were asking "are you sure it couldn't be the vaccine" since the tests had all
come back negative.  Their response was rude, as if we were accusing them of
not giving her proper care.  She finally came home and was fine but we had no
idea what it was.

About a week later I get a message left at 4:55 (vet closed at 5:00) at work
that I needed to call back immediately about Tigger.  Of course when I called
back they were no longer there.  I will never forgive them for that sleepless
night wondering what they had found and what was wrong with my baby.  When I
finally reached them the next day, not until 11:00 they took their sweet time
returning my call, I found out that it WAS a vaccine reaction.  They had
borrowed vaccine from another clinic because they ran out and didn't even
check until another cat came in with the same symptoms.  When I got upset
with them, they said "well we didn't have to tell you".  They promised to
refund our money for the tests but it took me writing to the head of VCA
after 6 months of wating to get only $200.00.  I never went back to that vet,
in fact I won't go to any vet that is affiliated with VCA.

This is a known possible reaction to the rabies vaccine.  Don't believe your
vet if they tell you it is impossible to have a reaction that isn't immediate.
I now have a test for any new vet that sees my cats.  I tell them the
symptoms and that she had a vaccine the week prior and ask for their opinion
about what the problem might have been.  My current vet answered - a vaccine
reaction of course.