Mystery Sickness for over a year

[BluntSpoon]

Hi all, i've never used this site and admit to never really reading it either untill recently, I've made this account especially to ask about this because I'm running out of ideas.

I have three ball pythons who got sick at the same time. They are wheezing and not eating.

I know I know it 'sounds like a RI to me, take it to the vet' but I've been taking the snakes to the vet for a whole year with no improvement but also not getting any worse. When looking in the snake's mouth we saw no evidence of a RI and whether or not they are being treated in injections or not they stay the same level of sickness.

The ONLY two symptoms are wheezing and no appetite, would anyone know of a virus or condition that may be worth looking into. I had the snake sent in for multiple cultures and tests and everything came back negative. the conditions of the snakes were checked and the vets are happy with those too.

One of my snakes are from Germany so the vet and I suspect something could have come from there?