Why is early detection and diagnosis so important for a dog?

Because dogs tend not to show any signs until they have a very mature, advanced infestation. By that point, they often have had damage to their heart and lungs, so this is a disease we want to catch early before they have signs. We want to screen for this disease every year. If a dog has had heartworm for years, they're more apt to not do well through treatment because they've had this infestation for so long. There's often a lot of permanent damage to their lungs and heart.

In some cases, the damage is reversible. Usually, the earlier that you catch it, the better, but the more prolonged the infestation, the more apt the dog is to have complications during treatment, which can be fatal, quite frankly. And also, they can develop some permanent lung changes that lead to heart problems later on.