i going to go out on a limb here and guess that the number of jags that have had to be culled due to neurological impairments could be counted on one, maybe two hands.
it's not even a twitch, it's just a lack of motor control to varying degrees. they are not nearly as bad as all the spinning spider balls... bottom line, don't believe the hype.

as far as pure bloodlines... there are plenty of breeders who breed them. it's just more difficult because of australia's retarded laws.