Interesting, although not what we've been taught in school. Adult heartworms in an infected dog produce microfilaria. Mosquito bites infected dog, therefore ingesting microfilaria, which continue to develop in the mosquito. Infected mosquito then bites uninfected dog, introducing the infective stage of the larva to the dog.... THIS is the stage that HW preventative is supposed to kill. Which is also why when treating a HW+ dog, after Immiticide has been given to kill the adults, often the next step is to give something like ivermectin or milbemycin to kill the microfilaria (if there are any). It's the same concept as giving Heartgard monthly as a preventative--it kills the baby microfilaria so that they don't develop into adults.
Unless I'm just misunderstanding your post, which is entirely possible since it's 1am...
