What kind of approach do you see here? Yes you can create an infinite amount of samples to test on, but you still have to deal with the millions of copies in the wild.
Modulo any environmental or self-mutation factors, I suppose? Might be that a tick which sucks blood from deer with XYZ picks up a parasite or bacteria which protects it against your measure. Or one crawls through some chemical goop / radioactive slop / gets hit by a cosmic ray etc. and gets a mutation which helps.
(Both fairly unlikely, I'll grant, but when you're dealing with millions or billions of ticks, unlikely is likely to happen a lot.)