As I understand it, foot and mouth is analogous to flu in humans - which can be fatal on an emormous scale, or can be little worse than a cold, depending on what strain it is, and what condition you are in to start with. I would imagine that, with the way animals are treated these days in the agricultural business, it would probably tend to be fatal to a high proportion, even if it is a relatively mild strain.

And since horses can't get it, talk about culling horses is irelevant, except for those that are free to roam around like Dartmoor ponies. The reason horses get mentioned in this context is that they carry earth around on their hooves, and so could move infected material around - as could cars, boots, hedgehogs etc., and apparently wind.