There are medications available from your vet that will keep heartworms from growing in your pet. These medications can only be started when your pet has been proven to not have heartworms, because it will be fatal for any animal that is already infected. This is the most commonly used method of prevention in the US today.
Offering a solid, healthy diet to your pet is a great base on which you can start preventative treatment of heartworm disease. A healthy diet is proven to boost the immune system, so load up your pets with raw, species appropriate food and let their bodies work like they were designed.
Certain herbs and essential oils can be used on your pet to discourage mosquitos, which are responsible for heartworm infections, from biting your pet in the first place. Things like mugwort, spearmint, clove and turmeric can all be used as a preventative in this manner. However, care should be taken when essential oils are used, as pets can have reactions to them if not used properly. Visit this website for more information on scheduling a heartworm test in San Jose.