Okay, well it's VERY IMPORTANT that you NEVER give him any type of medication for parasites that isn't made specifically for birds, and that is very carefully dosed for your Budgie based on how many grams he weighs...I'm actually quite surprised that your Budgie is still alive if you actually put a spot-medication on his skin that is dosed for dogs/cats. You got very, very lucky with that one.
****Here's the deal with ALL Anti-Parasitic medications...99% of them are literally Poisons (the only ones that are not are any of the "natural" medications for treating parasites, which are typiclly contain Eugenol (Clove Oil), and they are for treating only Fleas and Ticks, they won't do anything for any species of mites...
Birds are extremely sensitive to EVERYTHING, and when it comes ot Poisons they often die if they aren't given the EXACT AMOUNT that is carefully calculated based on the weight of the bird, and typically Avian Vets will NEVER give any bird/parrot any type of Anti-Parasitic Medications topically (on their skin) at all because ALL of those "Drops" you can buy to treat dogs and cats for Fleas, Ticks, Mites, etc. are Poisons that are meant to be absorbed through the skin OVER-TIME, which is extremly dangerous to give to a bird due to their small size and low-weights. The other issue (you don't remember the type of medication, but most of them fall into this category) is that most of those "Drops" that are meant to treat dogs and cats to treat and/or prevent Fleas, Ticks, and other Parasites including certain species of Mites will not treat ANY of the types of Mites that commonly live-on/effect birds...And if you did happen to give your Budgie a type of topical "Drop" medication/poison for dogs and cats that does treat any of the species of Mites that effect birds/parrots, then you are EXTREMELY LUCKY THAT HE DIDN'T DIE! The Anti-Parasitic medication that an Avian Vet would give your Budgie for most of the types of Mites that effect birds/parrots (the 2 most-common Mites that birds/parrots get are Feather-Mites and Scaly-Face Mites) is called "Ivermectin", and it's an extremely strong and toxic Poison that regularly kills many pet birds/parrots, rodents, reptiles/amphibians, etc. (this is a HUGE problem with Vets who are not Reptile Specialists but insist upon treating Reptiles, as their metabolisms are so, so slow that when given Ivermectin at all, 99% of the time they suffer in agony from the time they are given the Ivermectin up to as long as 2-3 months and then they die, usually from Kidney and/or Liver failure because their bodies just can't get rid of the Ivermectin)...
***When an Avian Vet properly and positively diagnoses a pet bird/parrot with either Scaly-Face Mites or Wing-Mites, the treatment is a single, one-time Injection of Ivermectin under the skin of the bird (a "Subcutaneous" Injection); the amount of Ivermectin they give the bird by injections is based solely on the weight of the bird...And here's where they have to be extremely careful and precise, because again, Ivermectin is an extremely strong and potent Poison, and if you give a bird, especially a little Budgie that typically weigh under 50grams, too much of a Poison, obviously they can easily and quickly die (if the Vet you saw found Mites on your Budgie's feathers then he/she probably has Feather-Mites; you can actually see them yourself without a Microscope if he/she still has them, just find a feather that has fallen-out in the bottom of her cage and then hold it up to a lightbulb and look right along the shaft of the feather, usually on the underside of the feather as opposed to the side of the feather that is facing outward when on the bird and that is seen, you'll be able to see the Feather-Mites sitting right up against the Feather Shaft).
So think about that, even the smallest breed of dog in the world weighs at least 1 pound, and assuming that the topical "Drops" that you bought and put on your Budgie's skin was for the smallest size/weight of dogs/cats, even if you only put a tiny little dab of the liquid-drops onto your Budgie's skin, it is an overdose. And that's assuming that it was even Ivermectin that you gave him, I'm assuming it is, as if it said that it was for treating "Mites" on dogs and cats, it's referring to either the species of Mites that causes dogs and cats to get Sarcoptic Mange (Scabies Mites), or it's the species of Mites that causes dogs and cats to get Demodex, and both of these are treated with Ivermectin...But once again, I'll say it again just to make sure you understand just how lucky you got, not only would a tiny little dab of the liquid Poison insiide a single-tube of those "drops" you put on dogs and cats skin overdose a little Budgie (or most species of parrots/birds for that matter, even large Macaws and Cockatoos), but the fact that you put it on your bird's skin topically like you do with other animals could have also killed him, but it would have been a situation where he would have suffered greatly for days to weeks to even months before his Kidneys shut-down, because those "drops" of Poison are meant to absorb slowly over-time into their skin and blood-stream in order to not only kill any active parasites the dog or cat has at the time it's applied, but to also prevent new parasites from infesting the dog or cat, and a little Budgie who weighs less than even 50grams will not be able to constantly be filtering the Poison out of their bloodstream, and will typically result in the bird dying from Kidney failure. This is why Avian Vets treat birds/parrots with a single-injection of Ivermectin that is the exact dose that will be safe for the bird based on it's weight, rather than applying any types of Poisons topically that will be absorbed continually over-time, which will cause their Kidneys to fail...
****All of that being said, the search-tool that Jen posted in her reply above is a World-Wide search tool for Avian Specialist Vets and CAV's (I'm assuming it's the same link that is often posted on this forum, if so then it has found Avian Specialists all over the world for people, from all over the Middle-East to the UK to Australia to Canada to Asia and everywhere else...So if there is an Avian Specialist Vet or CAV in Norway, it will find them for you!
****So just to be clear here, is your Budgie still currently scratching himself like he did when the Vet found the Feather-Mites on him? Or has he stopped? I couldn't tell from what you said about "it's been months now", so I wanted to make sure this is still an active problem...WHEN DID YOU PUT THOSE DOG/CAT DROPS ON HIM? DON'T DO IT AGAIN!!! You need to use the tool to see if you have an Avian Vet anywhere near you, if so then make an appointment ASAP for him...JUST DON'T PUT THOSE DROPS OR ANY OTHER MEDICATIONS ON HIM AGAIN!
I'm a bit confused about what's going on, because you first said that the Vet found no signs of Mites but that you medicated him anyway with the dog/cat Anti-Parasitic "drops" that you bought and did yourself, but then you said that the Vet found Mites on his feathers under his Microscope...If that's the case, then I don't know why the Vet wouldn't have treated him right then and there in the office with an injection of Ivermectin, which is the standard of care for a bird/parrot with Feather-Mites...If that Vet actually told you to just go out and buy an Anti-Parasitic drop for dogs and cats and to topically put it on your bird's skin, then that's a HUGE problem and that Vet needs to be stopped from giving out those directions, because he's going to kill a lot of pet birds; I'm still shocked that your Budgie did't die within a few weeks to a month of you putting that stuff on his skin, or at least become very, very sickly for a long period of time due to Kidney failure. So one more time, if your Budgie is acting the same way again then he probably does have Feather-Mites again (if the Vet did in-fact find Mites on his feathers), and he's going to need to be treated for them, but not with ANY medications you buy yourself, especially those meant for animals/birds other than parrots (poultry birds like chickens and ducks, quail, chukkars, Doves/Pigeons, etc. not only weigh more than a little Budgie, but they do have a lot of physiological differences as well)...
In the meantime, there is a reason, or rather a "source" of these Feather Mites that is causing your Budgie to keep getting them...Like I wrote above, if you can find a new/fresh feather in the bottom of his cage that has recently fallen-out of him, you can hold the feather right up in-front of a lightbulb and look for little, tiny dots along the shaft of the feather, as well as in-between each, individual little plume coming out of the shaft that all come together to form the feather as a whole...I would completely dismantle his cage, take every single toy, perch, ladder, swing, Cuttlebone, Mineral Block, food and water dishes/containers, the grate in the bottom, etc. out of the cage (just take everything out of his cage and take the cage apart) and then soak it in very hot water with some bleach, scrub everything very well, and then make sure you rinse everything until you cannot smell the bleach any longer...Then you need to vacuum all around where the cage is, the floor, etc. because there might be Feather-Mites all over the place, they're very small and you'd never see them.
