Over in The Field magazine, they have an article up on Ratting with terriers. Which terrier is top?
This about sums it up:
“Any self-respecting canine will catch rats, though terriers and small lurchers are probably the best,” says David Harcombe, editor of Earth Dog – Running Dog and several volumes of memoirs, entitled A Terrierman’s Life. “I have seen a miniature long-haired dachshund do a decent job on rats in yards and fields, and that was just a Kennel Club, toy-bred thing. They would not be good for killing rats in large numbers, however.”
For general information on ratting with terriers, see this link.