It's impossible to be 100% certain, but there's no animal reservoirs for polio that have ever been known to exist and it hasn't been seen in humans for a couple

It's impossible to be 100% certain, but there's no animal reservoirs for polio that have ever been known to exist and it hasn't been seen in humans for a couple of decades now - if it was still in existence we'd probably know about it by know, given how infectious polio is. All of the recent outbreaks are either type 1 poliovirus (mostly in Pakistan and Afghanistan) or have been linked to the vaccine via genetic testing.