1. Protection limitations: Heartworm preventives can't stop mosquitoes from carrying heartworm larvae to your dog. Instead, they kill heartworm larvae before they mature, reproduce and cause damage, but they only work against certain larval stages. Any larvae too young to be stopped by the preventive at dosing time will not be covered until the next month's dose. Any larvae too old will be able to grow into adults that can't be killed by preventives. So any lapse in dosing could allow a larva to mature. A heartworm test is the only way to know it's there before serious damage is done.
2. Testing limitations: No commercially available test can detect heartworm larvae until they become adults, so there is a limbo period when heartworms can't be detected by a test or stopped by a preventive. With a yearly test, you have a chance of catching any heartworms before they cause extensive damage to the vessels of the heart and lungs. If we skip testing, we won't know your dog has heartworms until it shows clinical signs of cardiac impairment and irreversible damage has been done.
3. Resistance: Resistance to heartworm preventives is growing. This is because veterinarians used to put infected dogs on heartworm preventives in hopes of keeping the disease from getting worse, allowing the worms to adapt and build resistance. In the Mississippi Delta region, for example, researchers have identified heartworms that can develop in dogs that are receiving heartworm preventives. Yearly testing can help prevent resistance development and can help veterinarians know when resistant worms move into the area.
4. Insurance: Heartworm preventive manufacturers offer product guarantees. If your dog gets heartworms and your veterinarian can show that you purchased preventives without lapsing and did yearly testing, the manufacturer will pay for your dog's heartworm disease treatment (as long as your dog has a negative test on record each year).