A heartworm is a parasitic worm that is very common both in dogs and in cats. As the name suggests, these parasites live in the dog's heart and are generally found free-floating in the blood vessels that surround the heart and in the right ventricle. Heartworms are transmitted from dog to dog by mosquitoes, which pass the larvae through their saliva into the dog. Heartworms can be very dangerous and even fatal to a dog. It is difficult to detect the presence of heartworms, as the dog will show no signs of infection until the disease has progressed considerably.

Once infected with heartworms, a dog will show no obvious signs of infection and the heartworms will not be detectable with a blood test. Once the larvae reach the maturity and infect the heart, a routine X-ray will confirm their presence almost immediately. Damage to the blood vessels surrounding the heart and lungs will be apparent and the heartworms will continue to grow and multiply until the flow of blood is blocked and the dog will exhibit other symptoms such as pain, hypertension, fainting, breathing difficulties, and lethargy. Advanced heartworm infections will eventually lead to heart failure and death, although by this time the dog's owner will have noticed that something is wrong and will have sought medical care for the dog.

Once a dog has been diagnosed with heartworms, treatment must begin immediately. There are four stages to heartworm infection:

Stage one is when the dog is at the lowest risk for permanent health issues. Heartworms will appear in X-rays but there will be no other obvious signals.

Stage two indicates a moderate infection and the dog may have trouble breathing and/or have severe coughing.

Stage three includes a severe infection of heartworms and the dog may lose weight, have an extremely hard time breathing, and blood tests will show liver and/or kidney damage.

Stage four generally includes Vena Cava Syndrome and the dog may be in shock, essentially dying slowly. During this stage surgery is the only method of removing the heartworms and there is no guarantee that this will save the dog's life.

Prevention is the best medicine to ensure that your dog does not become infected with heartworms. Preventative measures should be taken from the time your dog is a puppy, before he reaches seven months of age. Older dogs are at greater risk for negative reactions and resistance to preventative treatments.