Hi Lovemychoclab,
I have had a case of rat lungworm in one of my Rottweiler puppies about 20 years ago. The only way to know for sure is by, as you say the spinal test. There is no cure for this and presently vets will treat it initially with cortisone. My boy was 3 months old when it manifested. It is transmitted by the dog ingesting or sniffing the infected snails or snail slime ....... not all snails carry the disease, only those that have been consuming rat feces that carry the disease. To be clear, not all rats carry the disease and not all snails carry the disease. The disease starts in the rat (hence the name rat lungworm) and is transmitted to the dog by the snail. The worm can burrow into the spinal cord and causes paralysis.
If you live in an area where rat lungworm is then you must always be diligent in keeping your dogs surroundings safe.
How it manifested in my pup was:- he was 3 months, he was fine, had all his shots. At 3 months we wormed him. This worming cause the worm to die and calcify in his spinal cord which in turn cause paralysis of the back and front legs. The vet told us he would never be able to walk and that we should have him euthanized. We decided to try intensive physio with swimming every day, massage etc etc and, over the course of 2 to 3 months he was walking, not walking great but walking none the less. Eventually over a period of around 6 months he managed to regain most of his mobility. He walked like a train, left back and front legs went in unison and right front back legs in unison, if you know what I mean, but he regained mobility and lived into his old age, lovely boy and glad we persevered.
I know a lot of people don't like using rat poisons as native animals can be effected by it and I do feel that way about it somewhat myself however, I have taken the view that to keep my dogs safe from this disease I must. I use rat baits to keep the mice population down and use snail bait (beer traps too) to keep the snail population down. I also never ever leave my dog bowls out after feeding, they go straight into the dishwasher for the next meal. I live in a rat lungworm area and I have not had a repeat incident of it and have had dogs for all that time.
Hope all is well with your pup now and that you have a good outcome.