I also strongly agree with your opinion that your cats, especially your female, had a bad reaction to the vaccines. Like you, I find it very hard to believe that a food that your cats have been on their entire lives will "all of a sudden" be allergic. It just makes no sense, at all. You sound like me-things have got to make sense!!
What many people do not realize is what these vaccines really do to the body!! They put the immune system in major over-drive. The vaccine introduces a foreign bacteria/virus into the blood stream for your white blood cells to recognize later so that it can be fought off BEFORE your cat becomes infected. SOOOOO, if someone gives a 3 in 1 vaccine it puts too much strain on the body. If an animal's white blood cells are ALREADY fighting something and this vaccine introduces MORE foreign organisms, then a cat will become very sick. You're introducing a virus/bacteria to a body that's already stressed by fighting something. A vaccine can also bring another disease to the surface by the stress of it.
Many people do not realize this, vets and human doctors do not tell you this. Ask your vet about this, or look up some of this on the web! There is some info on it. Personally, I don't even believe in vaccinating after the age of 3 yrs old. If it must be done, avoid the 3-way vaccines at ALL costs. I hate to say this, but the vaccines might have brought another illness to the surface in your cats. I pray to God, this is not the case with your babies!
Best wishes to you and your babies!!
Blood work has been done. Thyroid t4 came back okay in December but we may redo that in a couple weeks. We have not done any force feeding because her eating is not an issue (unless we can't get enough of this Hill's she does not care for in her). She eats like she is starving to death and will drink water if I put the water bowel on the toilet bowel lid (no kidding). My second opinion vet gave us a three day course of something to kill internal parasites, which we finished this morning. She is eating the Hill's z/d but it is being mixed with chicken or turkey or Hill's z/d canned or sprinkled with Parmesan cheese or anything to get her to eat it. I just have a hard time believing that she is having a food allergy issue after 11 years on the same foods. Thank you for your interest.
There is a possibility the reaction was caused by the shots. There's an adverse reaction that can happen where the vaccination can cause the formation of antibodies that attack feline kidney tissue. I'm skeptical that this would have been the case in all your cats, though. Likely the others just experienced a stress reaction which they then recovered from.
But the ill cat? That's difficult to say. Keep in mind this is still a disputed claim in the veterinary industry.
It is also possible that the shot set off a previous undiagnosed condition. The Hypereosiniphile Syndrome treatment you mention is usually done after extensive blooodwork (was this done?).
Generally, they do this treatment when they have no idea what the actual cause of the illness is. It seems a bit radical to jump right to that treatment, but that just may be my cautious nature.
Have you been using force feeding techniques, and if so, what food? The biggest danger your underweight cat faces is a systematic failing due to lack of nutrition and water. It's important you get some food into her regularly!
"Food allergy" seems like a vague attempt at treatment to me. But it could work. Your vet may be guessing at a pancreatic issue which is one of those difficult to diagnose situations (original cause to effect, that is). If the hills isn't working for kitty, try one of the single protein source foods. I believe instinct makes a good one for cats. Preferably low protein.