If I give my dog heartworm preventive medicine year-round do I still have to test him?
This is a great question. If heart worm disease was easily treated and caused only mild disease the answer would be no. However, within 6 weeks of the arrival of adult heartworms in the heart and nearby blood vessels, irreversible damage has already occurred. The signs of advanced disease include coughing, difficulty breathing, and reluctance to exercise, but there are no obvious signs of the disease during the early stages.
Although newer treatments have much lower rates of complications, treatment is not risk free. For treatment to be most successful the disease must be detected well before the onset of clinical signs. Over the long run, giving heartworm preventive medication and testing annually, is much less expensive, both emotionally and financially, and is far better for your dog. Skipping the annual blood test would assume the medicine was 100% effective, it was given on time and properly, and that your dog actually absorbed the medicine. Although these assumptions are generally valid, a mistake leads to permanent heart and lung damage and could cause the death of your dog.
When it comes to heartworm disease "an ounce of prevention is worth a pound of cure".