Start with an annual test to see if your dog is already carrying these deadly worms. This test involves having a blood sample taken and analyzed.
If the test is negative, your veterinarian will recommend a preventive heartworm medication. This may be given either once a month or daily. These medications keep heartworms from maturing. Some veterinarians recommend giving your dog the medication year-round, but it should definitely be given during the months when mosquitoes are active.