Hello! Your first job for this little guy is to get him eating. Regardless of what’s causing the fever, and especially if he’s a big fatty (he could be if his name is Moose!), cat’s who don’t eat for about 3 to 5 days can start to get EXTRA ill, just because they went and stopped eating (i.e. - their liver gets overworked then it goes on strike).
Sometimes it’s the fever alone that makes them stop eating. Sometimes they are nauseous at the same time. Sometimes painful (sometimes they wont yowl, growl or do anything you can see when they are painful). Sometimes their sense of smell is off. They are usually dehydrated. All of this to say, I am commonly addressing anyone of those things, very often at the same time. Many cat's wont show you they have one of those issues until you treat them for it. And the benefit of the doubt is always given to the cat to be in one of those states, because if you miss it, you miss the window of opportunity. You miss pulling them back from a place in which things get very complicated for some cats. You gotta hit them with your best shot!
In terms of what’s causing the fever? Fever is a non-specific body reaction. Which is frustrating for both your and your vet. It's a hot needle in a burning haystack. Anything can do it, from inflammation to infection. To insidious, hard to find problems, cancer, or a simple kidney infection. So I start to quickly narrow down the list from my history (all the questions I asked you and your response in the exam room - i.e. yes he likes to fight with stray cats ALL the time and has a penchant for swallowing bad things), physical exam, bloodwork, xray, or urinalysis to begin. It also matters where you live. Any cryptococcus in your area? Soil born issues?
Because I put the priority on getting them to eat, using medications specifically designed to reduce fever is something I often talk to people about and advocate. I personally have never been too happy with Metacam in terms of taking down a fever. Could be your vets have a different experience with it, given what they face in your area. I agree that lyme disease in cats is rare, but I would probably test him for heart worm disease eventually which would tell me his lyme status anyways (4DX test).
Sometimes I will treat these guys one thing at a time during a single day to see what takes the temperature down. Sometimes just room temperature IV or SQ fluids does it. Did the antibiotic do it? Did the Anafen do it (this is an on-label anti-fever medication, always done in a hydrated cat with gastroprotectants on board)? I once had a patient that was so feverish and going downhill…and simply choosing the right anti-inflammatory brought him back from the grave within minutes! Did the pain relief do it? Any one of these responses can clue you into what the body is up to.
And sometimes they need lots of nursing care and end up getting better with the diagnosis of FUO. Fever of unknown origin. Which simply means your cat got better, but I cant tell you why…
Good luck with Moose!