Because the American Heartworm Society support dosing every 30 days, and I can't risk a breakthrough infestation because I can't breed a heartworm positive girl, plus I just plain don't want a heartworm positive because it is preventable. I also test annually, just to be certain what I am doing is working. Thankfully *knock wood* I only have to give (flea and tick preventative) Frontline Plus a couple times spring through summer, because where I field train and off lead hike has ticks.
Check this PDF out, it is current (January 2014):
https://www.heartwormsociety.org/pdf/...Guidelines.pdf
"All orally and topically administered macrocyclic lactone chemoprophylactic products are labeled for a 30-day dosing interval. Beyond this interval efficacy against late fourth-stage larvae declines and is unpredictable. Juvenile worms, which can be found as early as 52 days post infection, are even less susceptible to chemoprophylaxis."