Great question - the prevalence of various serovars in a given area can change over time, but it's rare to see an odd Central American serovar become endemic in e.g. North America - they are fairly region-specific.

As far as species-specificity - each serovar has a presumed natural reservoir, a "maintenance host". That doesn't mean that it only affects certain species, it's just that some serovars infect a host without causing severe disease. Swine serve as maintenance hosts for serovars pomona and bratislava, for example, and dogs are maintenance hosts for canicola - one of the serovars in your dog's vaccine. The other in your vaccine is icterohemorrhagiae, for which rodents serve as a maintenance host.

How dangerous is it for a dog to work as a ratter? I think that vaccinating is a good place to start, but I'd go with the tetravalent vaccine, personally. It includes serovar grippotyphosa, which also has rodents as a maintenance host, as well as pomona (cattle/pigs). Clinical leptospirosis is not super common in dogs, but dogs exposed to rodents would be at the highest risk. I had a coonhound I used to hunt with, and I would always keep her up to date on her lepto vaccine. It was somewhat common where I lived at the time - upstate NY. Cases seem to be on the rise in NYC even, with an outbreak earlier this year linked to a dog park in Brooklyn.

I wish there were a good reference for serovar distribution in the US, sounds like maybe something someone should work on. There are maps with case distribution online, but not serovar distribution.