There are all sorts of tick diseases, some can be cured with aggressive antibiotic therapy, and some cannot be completely resolved, but can be kept in check. Being in South Africa, I am not as familiar with the type of tick diseases that you might have there.
Can you get copies of any bloodwork that has been done? Can you have your vet tell you exactly what parasite s/he has identified and is treating for?
I would also suggest that you go to the first link here: http://saluqi.home.netcom.com/ticklinks.htm
and join the tick list.