Prey drive.

[MarcoPoloCX]

How do you reduce it?
There are lots of cats and foxes where I live.
Some in the park early in the morning.
I try to spot them before he does and then try and distract with squeaky toy and treats and running whilst making loads of noise. Less effective than effective.
If he spots it he's off.

You can't really reduce prey drive as such but there's lots you can do to make it much easier to manage.

Improving your dog's impulse control, working on building engagement and also providing appropriate outlets for their hunting behaviour will all help.

You're on the right track with the treats/toy/making yourself fun but you need to get him responding reliably to that at home first then work up to doing it in more distracting environments.

It would be a good idea to muzzle train him (though bear in mind being muzzled doesn't mean he's not capable of doing damage to something he considers prey) and use a long line with a harness in the meantime.

There was a similar thread a few weeks ago if you search OP. there may be some good advice on there.

Remember in particular a poster who's two Huskies had killed and eaten two cats and a few other things and were now calmer.

You can't really. As a previous owner of a sled breed, I'd worked intensively with them for years and their behaviour was impeccable. Until one of them got loose and attacked a sheep. Luckily the sheep was fine and had a thick fleece but it was like something clicked in her head and she was gone.

I've known other same breed owners to have dogs that have lived happily for years with a family cat, then killed it because it made a funny noise or ran. It's just in some breeds and sometimes, no amount of training can erase instinct.

However - have just read @ProfessorCat reply - maybe ignore my comment!

I've tried , to no avail , to desensitise my terrier in regards to small furries / cats / foxes / birds and have decided that the only really safe way to walk him off lead is to only let him off where I can see what is around and get him back on his lead before he sees anything . Fortunately we have a small Heath nearby which is ideal provided I don't do dawn and dusk walks when the rabbits are too lively .