While mosquitoes may be abundant up in the northern regions...rats aren't. You find the situation in the Alps with a very small rat population.
I think the thing to take home from this article...is that there were diseases out there...hundreds, if not thousands of years ago...which might have killed you, but if you survived...it left a note in your DNA to halt AIDS infection. Perhaps our logic today might be...its not a good thing to wipe out all diease because eventually a major disease will erupt which we should have been immune to...but we aren't because we never had measles in our lifetime.
This sounds like one of those Star Trek episodes where you could change time, but recreate totally new problems. You never truely fix a problem without causing another.
The density of rats in the coastal plain that constitutes the Arctic or North coast of Norway, Sweden, Finland and Russia depends on the availability of food suitible for rats.
The density of rats in the Alps depends on the availability of food, and altitude. Same with the Arctic areas. You can estimate roden density by checking out the habits of the raptors that prey on them. An article on the net indicated that the Snowy Owl, primarily an Arctic bird, usually doesn't breed in areas above 600 ft. elevation EXCEPT IN NORWAY where they breed in mountains up to 3,000 ft! No doubt there are numerous voles, mice, rats, lemmings, rabbits, etc. available in those mountains, if not the Alps.