One Cat- Constant Parasites

Hi everyone. I'm so frustrated. I take care of 5 cats outside (I bring 2 bring them in when they want, and when it's cold). They are the only ones that want to come inside. The others have heated everything outside... Anyway, to my point. I also have two that are only indoors. One is about a year and a half old and he is the son of two of the outside cats (Yes, they are now all fixed).

He loves it when his mom gets to come inside as she's one of the two that want to be inside frequently. Everything is fine except Hulk (the one that's about a year and a half) keeps getting parasites - cocchidia or giardia. When I first took him in, he also had a tape worm. If that wasn't traumatic enough...

Throughout his 'kittenhood', he would get sick if the cats came inside. I was told once he was older, his immune system would be stronger, etc. Well, that has been sort of true... He's gotten sick a lot less than he did, but he just recently got sick again after they came inside for a short stint during some winter weather. Other times, he's been sick afterward and I jumped the gun, took in a stool sample, only for it to come back negative and he was fine after a few days. I've easily spent $2,000+ at the vet since I brought him into my home in September 2024 for these recurring issues.

I'm willing to treat all the cats but it's not realistic to try to get them all to a vet. Aside from that, most of them would never be willing to be forced to drink a liquid medication, which is what I have to do to hulk. There has to be a better way.

He gets sick. Stool sample. If negative, nothing. If yes, the vet says "We can help with that, here's the medicine. He's then fine for a while, then we repeat again. I know this probably seems silly, but why does he keep getting sick if none of the others are? What is the easiest solution? I've had some tell me I should go to Tractor Supply and buy what they give for horses, but obviously give a tiny bit. I just don't know what to do.

"Bringing in all the cats", though I'd be willing, is not going to work. I feed them all but not all want to come inside and would not be comfortable with that arrangement. Added to that, one of the inside cats (not Hulk, but the other) isn't a fan of the others coming inside. Adding more would make that situation worse. Hulk (and the ones who do enjoy coming in) would be heartbroken if I just abruptly stopped letting them come inside.