Well, at this point, this is otherwise called a fever of unknown origin. Fever of Unknown Origin
This next link is very important to read in detail, probably a few times:Fever Link 2
STarting intravenous fluids. Very good
Using antibiotics as a precaution. Also, very good
But, got to get to the underlying cause.
Additional testing to absolutely ask the vet about should include:
1. An abdominal ultrasound. Yields great information if there are any changes in the liver, kidneys, abdominal lymph nodes, and gastrointestinal tract.
2. If there is any possibility...any....to being exposed to ticks, ask about a blood test for tick borne diseases. Very important and may change the antibiotic the vet is using.
If any continued nausea and vomiting, ask about starting some Cerenia
immediately. An excellent once a day anti-vomiting/anti-nausea drug.
Lastly, ask if they could call a internal medicine specialist for a little guidance as well. Persistent fever is very serious and some of the causes like cancer, certain infections, and autoimmunes diseases can be very tricky to diagnose. Very sorry to hear about everything that is going on.
Just saw you second response:
With heart related concerns, that ultrasound would be great. So, it would be a bicavity ultrasound. A specialist can easily evaluate the heart and abdomen at the same time.
I hope that information has been helpful.
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