You will want to use something like Revolution, which is expensive and necessary in this area.
http://www.revolution4dogs.com/content.asp?country=USā©=EN&drug=RV&species=CN&sec=000
Stops fleas. Prevents heartworms.
Even the tiniest of pets can be fiercely protective of their owners, but all dogs need our help to protect them from parasites. RevolutionĀ® (selamectin) provides that help. Just a spot once a month provides protection against fleas, heartworms and other parasites. Ask your veterinarian about Revolution.