Just seen this Vix, and wanted to share our experience. We have a Golden Retriever who was bitten (we believe adder bite too) about 4 years ago. No where near as severe a reaction as youve had, possibly due to the size of dog, but his leg was huge, he was generally unwell and it took about 2 months for the wound to start healing properly. However, the fitting thing happened with us too and started about a month or so after the bite.
Our vet couldnt come to a clear conclusion as to what caused the fitting, but we do think it was a combination of drugs and possibly a conflicting reaction to his rabies vaccination which hed had only a few weeks prior to the bite. (Hes passported and has travelled extensively!)
Our Goldie had irregular fits for around 9 months. There were no specific triggers (stress, exercise, diet etc) and we do wonder if we missed a few during the night as he sleeps in the kitchen. But we would get a 3 minute warning one was about to happen as hed come flying into the room to be near us and tuck himself up against our legs and then the fit would start. He was terrified poor thing and occasionally hed defacate during the seizure.
The vet didnt want to start him on anti seizure medication until he was having several fits a week on a regular basis. They then stopped as randomly as they started, and he hasnt had one for 3 years. I think it will always remain a mystery. It went on too long to be a direct result of the bite, but they can be easily overloaded with drugs and we do tend to over vaccinate our animals compared to other countries. So we do think it was a drug interaction of one sort or another.
Hes absolutely fine now. No long term damage/effects whatsoever and I m just carefull to ensure his rabies vaccination is done at the max distance, 6 months, from his normal annual vaccinations. If we didnt travel so much, I may have cut down on vaccines, but he has been fine since his last one.
Do hope your boy is doing well, hes really been through it hasnt he, so am sending loads of good wishes.
PS We're experts on ticks! We get loads of them every year in France and on the whole theres never been any adverse reaction, just a small swelling where the bite is. The first time we experienced them, we took the dog to a local french vet to remove it as we didnt have a clue as to what we should do, and he was great. Showed us a technique using olive oil, (you cover the whole tick and it then cant breathe through its body so releases its grip a tad to pop its head out) and gave us a tool to twist and remove. We ve now removed loads ourselves and honestly a normal harmless tick bite should only leave the tiniest of swellings. Any larger swelling or spreading of swelling, or whats called a bullseye reaction (thats a red circle surrounding the bite) should seek veterinary help as soon as possible (days, not neccesarily hours). The same with humans. Any adverse swelling, spreading or bullseye reaction, please see a GP asap. Lymes disease can be easily treated by antibiotics if picked up within the first week or so after the bite.