Abbey,
 Yes, I have had a problem with Harvest mites for years with one of my cats.  Usually, by October he has no fur left on his paws with the constant irritation, which makes him chew his paws to bits.    My vet gives me Strong Hold flea treatment for the problem.  It does Harvest mites too and worms at the same time.   It is Systemic, so goes into the tissues and blood stream which kills the blighters much better.  It really has made a big difference.  Only slightly chewed paws!!  
No other pets in the house are affected by them, so she would prefer I use frontline for the others as their normal parasite treatments.  
My vet informs me that Frontline is less likely to go into the bloodstream as much. She feels it is better for their liver not to be given the systemic parasite treatments, if at all possible.  
However, needs must, for the poor wee cat that is badly affected, he is demented by them !! 
 Marian.