We have been using heart worm preventative for our dogs for a long time now. We use a once a month tablet called Interceptor which in addition to preventing heartworm disease, it is supposed to control aldult hookworm infection and remove and control adult roundworms and whipworms. We have to have the dogs tested each spring and then start on the tablets in April. The usage is determined by mosquito season in our section of the country. I think in other parts of the country, the tablets are given year round. If you have mosquitos in your area, I would say it is a good idea to use this.
I could have gotten the tablets cheaper from the Drs. Foster and Smith catalog but you have to have your vet fax them a prescription.