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This is Dr. Christian. Welcome to JustAnswer. Heartworms are spread by mosquitoes. A mosquito must bite a infected dog. The heartworm then develops in the mosquito for a while before it bites another dog and then gives that dog heartworms. As long as your other two dogs are on monthly heartworm preventive they will be safe. Treatment for heartworms involves a series of injection over the course of a month (usually 3 injections, two 24 hours apart then another a month later). If you have more questions please reply back. Here's a link with more about heartworms: