What exactly is heartworm disease?
Heartworm disease occurs when a dog becomes infested with a roundworm called Dirofilaria immitis. These worms actually live in the pulmonary arteries (the arteries carrying blood from the heart to the lungs to be oxygenated). When a lot of worms are present, worms may also be found in the right ventricle.

How do dogs contract these parasites?
Many people hear "worm" and think the dog must eat something to become infested. This is not the case for heartworm, which is actually transmitted by the Culex mosquito. The mosquito must feed off an animal with active heartworm disease (microfilaria circulating in the blood). These microfilaria are a stage of heartworm larvae (L1), which, once in the mosquito, develop into the infective stage (called L3) over the course of a month. The L3 migrate to the mosquito's mouthparts, allowing the mosquito to deposit one or two microfilaria into the bloodstream of another animal (i.e., a dog). These L3 larvae develop in the subcutaneous tissue, and later enter the venous system over a period of 50-70 days. At this point, the larvae molt into a final larval stage, L4. These travel to the right side of the dog's heart, where they become adult heartworms.

How will I know if my dog has heartworms?
Many people associate worms with feces, which is not the case for heartworms! This disease can only truly be diagnosed by a blood test that your veterinarian can perform. When adult heartworms are living in the dog's heart, the females secrete a uterine antigen that can be detected by a special test, called an ELISA (Enzyme Linked Immunosorbant Assay). This can be done in the veterinary office and takes only about 10 minutes. The test is very specific. The only drawback is that a certain amount of antigen (and therefore, adult worms) must be in the blood for the test to detect it.

If my dog has a negative heartworm antigen test, does that guarantee he/she does not have heartworms?
Unfortunately, no. It takes about 6 months from the time the mosquito bites the dog to the time the adult heartworms are secreting detectable antigen, which is what makes the test positive. For example, if a dog was infected three months ago, and is not kept on heartworm preventative, he/she can be infected with the disease, but will have a negative test result. Even if a dog has a negative heartworm test and is immediately started on heartworm preventative, he/she can still have a positive test up to 6 months later! The L3 larvae which are injected into the dog's blood by the mosquito will "swim around" for about 6 weeks. Then they develop into L4 larvae, which migrate to the heart and become adult heartworms, which secrete antigen that the ELISA heartworm test can detect. The preventatives used today are very effective at killing the L3, so if given once a month, your dog is protected (you are within that 6 week window of maturation to L4). However, after that 6 week length of time, the larvae are now L4, and the preventative is not very effective in killing that stage. You may not know this until your dog has a positive heartworm test a few months later!

Does heartworm have any signs I should watch for?
Only severe infestations will affect a dog so much as to cause clinical signs. When the worm load gets so high that the blood cannot pass through the heart and pulmonary arteries, dogs will most frequently present with a cough. They will not be able to exercise as much as they used to, and some will lose weight and act lethargic. If your veterinarian takes radiographs (X-rays), the dog may have an enlarged heart. The worms may be seen on ultrasound as well. Heartworm disease can cause a variety of other syndromes as well, which your veterinarian can check for as appropriate. The goal of the heartworm antigen test is to detect the disease early (before clinical signs appear), so treatment will have greater chances of success.

Is there a way to vaccinate against heartworm?
There is no vaccine like that for rabies or distemper per se, but there is a safe and effective means to prevent heartworm in dogs. Many drugs have come on the market in the last decade with very similar mechanisms of action. These drugs, often tasty pills, are given to the dog once a month. They will effectively kill all migrating L3 larvae in the dog, which will prevent the disease from progressing. Some examples of monthly heartworm preventative include Ivermectin (Heartgard), Selamectin (Revolution), and Milbemycin (Interceptor and Sentinel). A new shot has been approved recently, called Pro-heart-6, which is injectable Moxidectin that is gradually released over the course of 6 months. The monthly heartworm preventatives have the added benefit of other drugs to kill intestinal parasites, such as roundworms, hookworms, and whipworms.

Can heartworm be transmitted to other people or pets in the household?
No! Heartworm is not contagious, so animals can be housed with a heartworm positive dog with no risk. Heartworm disease essentially does not happen in humans. Heartworm can only be transmitted via a mosquito bite. (Additionally, pregnant females have been shown in some studies to transmit the disease to their puppies.) Along with dogs, the other two species highly susceptible to heartworm is the ferret and the California Sea Lion.

Is there a cure for heartworm disease?
Fortunately, many newer, safer cures have emerged in the last decade. Treatment must kill the worms slowly and gradually, since they are living in the heart, and, as they die, they progress to the lung. These worms can measure up to four inches long, and that's a lot for the body to naturally reabsorb through the immune system's macrophages! Treating heartworm disease is therefore a risky endeavor, since the dead worms may lodge in the lung, causing a thromboembolism (a blood clot). The most commonly used treatment is Immiticide, an injection into the lumbar (lower back) muscles of the dog. There are many different schedules for administration, depending upon the health of the dog, the stage of the disease, and your veterinarian's personal preference. Immiticide is designed to slowly kill the adult worms. It is important for the dog to have complete rest for no less than 6 weeks during the treatment, since increasing the heart rate will increase the risk of complications, such as pulmonary thromboembolism, which can be harmful and sometimes fatal. If the dog has an exceptionally high load of adult worms, one treatment option is physical removal of the worms. This can only be done by a specialist, and is a very long, invasive procedure. However, with the worms removed, the risk of thromboembolism is greatly reduced. Either form of treatment is expensive, making prevention the ideal choice.