Heartworms are one of the most serious diseases a dog can contract and are very common, especially in dogs that have not been receiving preventative therapies. Though heartworms are large enough to be seen with the naked eye, dogs to do not pass heartworms the way they may pass intestinal worms. Therefore, they are sometimes harder to detect.

Unless a dog is taken in for surgery or X-rays, the average down owner will never know that his or her dog has heartworms. Often, dogs may exhibit no outward signs of heartworms until it is too late and the worms have already done their damage, by which time it is usually fatal. Some warning signs of heartworms include: coughing, lethargic behavior, inability to exercise for extended periods of time and anemia. Dogs that exhibit one or more of these symptoms should be thoroughly examined by a licensed veterinarian.

The vet is the only person who can truly tell for sure if the dog has heartworms. If X-rays and other diagnostic tests come back positive, a dog will likely need to undergo extensive medicinal therapy. This therapy is often very drastic and comes with severe side effects, but may be the only option for saving the dog's life.

A popular misconception is to think the best way to tell if a dog has heartworms is to determine whether the dog is an "outside" dog or "inside" dog. This truly makes no difference. While dogs that spend a great deal of time outside may have more opportunities to become infected, nearly all dogs go outside at some point and therefore are susceptible to mosquito bites, which is the major way heartworms are transmitted. Even dogs that remain inside for extended periods of time can be bitten by mosquitoes which find their way inside.

The best way to tell if a dog has heartworms or not may be to determine if it has been on an effective, veterinarian-recommended treatment for prevention. If that program has been applied conscientiously to the dog since birth, a heartworm infection is very unlikely. The preventative treatment, often in the form of a pill, does not allow heartworms to develop and therefore, they can cause no damage to the heart and eventually die.

However, even animals who have received preventative treatments cannot totally be ruled out of having heartworms. The preventative treatments are effective in approximately 98 percent of the cases, but no treatment is 100 percent. Even with preventative measures, a veterinarian should always consider the possibility a heartworm infection has occurred.