I am not going to argue with you....just merely say that you are incorrect and terribly misinformed about heartworms. A healthy dog will DIE if it contracts heartworms...just a matter of time and suffering. I can only pray that you will perhaps read the following link and think again about your choice to expose your pups to something so deadly.
American Heartworm Society
I have used heartworm preventatives for years. One of my pups died at 17...I had her for 15 years and she took Heartgard every month that I had her. She suffered NO ill effects from the Heartgard. None of my pups have ever had a problem with heartworm preventatives.