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Heartworm preventative isn’t really a preventative, it kills the larvae ALREADY in your dog, so you give it 4 – 6 weeks AFTER mosquitoes show up. Doesn’t matter if it’s Spring or Fall, wait 4 – 6 weeks after the first (or last in the case of Fall) mosquitoes. It does not stay in their system like the monthly flea treatments. It kills once when given, then leaves the body, usually within 24 hours, and they are not “protected” but they are “treated” again the next month.