Preventing Heart worms using Ivermectin

I can't possibly be the ONLY person disgusted with the price of heart worm medication. With our five dogs to treat, plus my father in law's dogs, we were spending rite at $400.00 every 3 months on heart worm preventative. So........ I did some digging, and began concocting my own heart worm preventative at home using the same active ingredient listed in heart guard and others (ivermectin). I purchase Ivermec 1% injectible for cattle (with nothing else added). I also bought some propylene glycol to help mix the ivermectin because you use so little. The little bottle of ivermectin was around 40 or 50 dollars and will last us YEARS. The propylene glycol was less than 20.00 and will also last years. I measure the amount of ivermec needed, mix it with 9 drops of the propylene glycol, squeeze a fish oil capsule to the mix (to help bland the taste), and squirt the solution in the dogs mouth (back of their mouth). I found a helpful website and will share it. I have my dogs tested for heartworms annually. They all come up negative:).
 http://www.dimestorebudget.com/money-matters/affordable-diy-heartworm-treatment/