Kitty with unexplained high fever

Question
Hello, Doctor!

I'll try to make this as concise as possible.  

Right after Thanksgiving, my 4 year old kitty, Dylan, began acting EXTREMELY lethargic.  He was sleeping, literally, 23 hours a day and simply could not be bothered.  I would wake him up and he would eat and drink well, and I would take him to the litter box where he used the box fine.  Then he would go right back to his chair and go to sleep.  He also struck me as very stiff--especially in the hind end--and he was reluctant to go up and down the stairs or to jump up on furniture.   I was worried about him, but because he wasn't showing any other symptoms aside from lethargy and stiffness, I chalked it up to a side-effect of the intense stress he'd experienced while I was away over Thanksgiving.  What had happened was that some feral cats got into my basement (they tore through the insulation around the heating ducts) and into my house.  They terrorized my two (totally indoor) cats (Dylan & Chaplin) and urinated all over the basement floor.  The girl who was taking care of my cats told me about this, and the minute I got home, I checked both cats over very carefully for any wounds, because I was extremely worried that there had been a fight and the possible transmission of a disease such as FIV.  By the time I got home, Dylan was already acting lethargic.  Chaplin seemed completely fine.  After about a week, Dylan turned the corner and was his normal, active, cheerful self again.  I debated off and on as to whether or not to take him to the vet, but because he was still eating and didn't have an respiratory symptoms or anything, I decided to just wait and see.  Like I said, he eventually returned to his 100% normal self.

Fast forward a few weeks.  I went away for Christmas vacation and while my boyfriend was here most of the time, he left the cats in the care of a friend for five days.  The feral cats, by the way, were no longer able to get into the basement, because I secured the area around the duct work!  When I arrived back home on January 4th, I noticed my 3 year old kitty, Chaplin, seemed a little "off."  My boyfriend told me that he had just started acting lethargic, and we decided to keep an eye on him.  Like Dylan, he was sleeping ALL the time, but still had a fine appetite and litterbox habits (when woken up).  He also seemed stiff, but perhaps stiffer than Dylan.  Yesterday, I grew very worried because he was so lethargic he didn't even seem to want to sit up.  He also seemed a little "glazed."  I took him to the vet and he had a fever of 105!!  The vet ran a CBC and diagnostic panel, all of which came back completely fine.  Chaplin (who was very upset!) wouldn't walk for the vet, but the vet said he was fairly certain the stiffness/reluctance to move was from how terrible the high fever must have been making him feel.  He decided to start him on antibiotics and admit him overnight.  This morning, after one dose the antibiotics, the fever hadn't gone down.  The vet game him his morning dosage, and we checked his temp at noon.  His temperature had dropped to 104, and he seemed more alert.  Because it was the weekend, the vet told me I should take him home where I could be with him and monitor his temperature (I know how) and give him his medicine--essentially the same care he'd get at the vet's office.  He said it was especially good that Chaplin was eating and drinking so well (he ate all of his dry food as well as 1/2 a can of wet food in the vet's office!).

I took Chaplin's temp around 4:00 and it was down to 103.2.  I'm going to check it once more before bed.

I didn't sleep at all last night, because I keep hearing horror stories about cats with unexplained fevers dying of FIP.  I asked the vet about FIP, but he said Chaplin wasn't exhibiting any of the other symptoms, and that he had no reason to believe that that was what was going on.  I felt a little better, but I'm still worried.  I'm supposed to take Chaplin in on Monday for a followup exam, but in the meantime, I'm kind of driving myself crazy playing the "what-if" game...

I guess I have a couple of questions...

1). Is it bad that we haven't figured out what caused Chaplin's fever?  Do YOU think I should be worried about FIP?
2). Is it okay that, despite the fact his fever is coming down, Chaplin is still sleeping SO much.  He seems to be sleeping very deeply and peacefully, and he's never been that extraordinarily active, but I feel like he should start perking up soon (even though he's only been on antibiotics for about 24 hours).
3). Is it possible that Dylan contracted something and passed it on to Chaplin, who just seems to have it worse than Dylan did?  Their symptoms are SO similar, I can't help but retain that theory.
4). Is the fact that the fever is going down with antibiotic therapy a good sign that this probably ISN'T anything other than some odd illness, rather than something horrible like FIP?

Sorry for the novel...but I just feel so confused not having a NAME to put to this problem.  The vet (who is normally very doom and gloom) seemed surprisingly nonchalant, and just told me that sometimes cats get high fevers for no apparent reason.  I guess I just have a hard time believing that there's not something seriously wrong.  I think that if I knew what it was, I would feel much better.  

Thanks so much for any advice you can offer.

Answer
First of all, I am not a doctor, but a vet tech. So if that bothers you you might want to re-post to one of the vets here.

The only thing I can tell you Kelsie is that cats do get these weird fevers for no reason. We saw them so often here we coined it the Tuolumne County Cat Disease- which was a joke of course. We would see cats present exactly like you are describing, totally lethargic yet would eat or drink and use the litter box. Always watch for abscesses to form somewhere on a cat that could have been bit. Make sure your cats are vaccinated for rabies also.

The high fever is the reason they sleep so much. One thing I would do for both cats is get them tested for FELV, which is much more commonly spread around then FIP. Not that FIP isn't an issue, but the symptoms are not quite like that. There is a dry form of it, but testing is the only way to find out if they have it.

We used to give cats like this (when we knew they were FELV negative) is an injection of a drug that isn't made anymore but it was a combo of an antibiotic, a steroid and Chlorpheniramine, which is an antihistamine. It made them feel great overnight and their fever was gone by morning.

The best thing to do is wait until they are both feeling better then take them in and get them tested. There is a 4 way test now that tests for all the feline infectious and contagious diseases at once.
Let me know how they are doing in a few days and how the tests come out.