Heartworm Basics

What is heartworm disease?

Heartworm disease is a serious and potentially fatal disease in pets. It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body.

The dog is a natural host for heartworms, which means that heartworms that live inside the dog mature into adults, mate and produce offspring. If untreated, their numbers can increase to harboring several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, heartworm prevention for dogs is by far the best option, and treatment (when needed) should be administered as early in the course of the disease as possible.

How is heartworm disease transmitted?

The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms live in an infected dog and produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog, the larvae enters the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to develop into sexually mature adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs.

What are the signs of heartworm disease?

In the early stages of the disease, many dogs show few symptoms or no symptoms at all. The longer the infection persists, the more likely symptoms will develop. Active dogs, dogs heavily infected with heartworms, or those with other health problems often show pronounced clinical signs.

Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockages of blood flow within the heart leading to a life-threatening form of cardiovascular collapse. This is called caval syndrome, and is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.

What do I need to know about heartworm testing?

Heartworm disease is a serious, progressive disease. The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins.

When should my pet be tested?

All dogs should be tested annually for heartworm infection, and this can usually be done during a routine visit for preventive care. Annual testing is necessary, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working. Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication—or give it late—it can leave your dog unprotected.

What happens if my dog tests positive for heartworms?

No one wants to hear that their dog has heartworm, but the good news is that most infected dogs can be successfully treated. The goal is to first stabilize your dog if he is showing signs of disease, then kill all adult and immature worms while keeping the side effects of treatment to a minimum.

What to expect if your dog tests positive:

Confirm the diagnosis. Once a dog tests positive via an antigen test, the diagnosis should be confirmed with an additional and different test. Because the treatment regimen for heartworm is both expensive and complex, your veterinarian will want to be absolutely sure that treatment is necessary.

Restrict exercise - This requirement might be difficult to adhere to, especially if your dog is accustomed to being active. But your dog’s normal physical activities must be restricted as soon as the diagnosis is confirmed, because physical exertion increases the rate at which the heartworms cause damage in the heart and lungs. The more severe the symptoms, the less activity your dog should have.

Stabilize your dog's disease - Before actual heartworm treatment can begin, your dog’s condition may need to be stabilized with appropriate therapy. In severe cases of heartworm disease, or when a dog has another serious condition, the process can take several months.

Administer treatment - Once your veterinarian has determined your dog is stable and ready for heartworm treatment, he or she will recommend a treatment protocol involving several steps. The American Heartworm Society has guidelines for developing this plan of attack. Dogs with no signs or mild signs of heartworm disease, such as cough or exercise intolerance, have a high success rate with treatment. More severe disease can also be successfully treated, but the possibility of complications is greater. The severity of heartworm disease does not always correlate with the severity of symptoms, and dogs with many worms may have few or no symptoms early in the course of the disease.

Test (and prevent) for success - Approximately 6 months after treatment is completed, your veterinarian will perform a heartworm test to confirm that all heartworms have been eliminated. To avoid the possibility of your dog contracting heartworm disease again, you will want to administer heartworm prevention year-round for the rest of his life.

FAQ's

At what age should puppies be started on heartworm prevention?

The risk of puppies getting heartworm disease is equal to that of adult dogs. The American Heartworm Society recommends that puppies be started on a heartworm preventive as early as the product label allows, and no later than 8 weeks of age.

The dosage of a heartworm medication is based on body weight, not age. Puppies grow rapidly in their first months of life, and the rate of growth (especially in dogs) varies widely from one breed to another. That means a young animal can gain enough weight to bump it from one dosage range to the next within a matter of weeks. Ask your veterinarian for advice about anticipating when a dosage change will be needed. If your pet is on a monthly preventive, you may want to buy just one or two doses at a time if a dosage change is anticipated (note that there is a sustained-release injectable preventive available for dogs 6 months of age or older). Also make sure to bring your pet in for every scheduled puppy wellness exam, so that you stay on top of all health issues, including heartworm protection. Confirm that you are giving the right heartworm preventive dosage by having your pet weighed at every visit.

Does the age of a dog affect the success of heartworm treatment?

The age of the dog is just one factor affecting the success of heartworm treatment. When making a diagnosis and administering treatment, veterinarians consider the dog’s overall health and the severity of his symptoms, as well as the results of X-rays and laboratory test results. Older dogs with long-term heartworm infections may have damage to their lungs, hearts, livers, and kidneys that can complicate heartworm treatment. To ensure the best chance of success, it is vital to follow your veterinarian’s instructions carefully. This includes severely restricting the dog’s activity, as exercise during the treatment period is the leading cause of complications.

How is heartworm typically treated?

Only one drug (melarsomine) is approved by the U.S. Food and Drug Administration (FDA) for treatment of heartworm in dogs. This drug is administered by injection in a veterinary hospital. Although there are some risks associated with this medication’s use, most adult worms die quickly and can be eliminated within 1 to 3 months. Cage rest and drastically restricted exercise during this period can decrease the chances of complications from treatment.

Along with melarsomine, the heartworm treatment protocol recommended by the American Heartworm Society includes several other medications that help improve the chances of treatment success and reduce the incidence of side effects. This includes administering a heartworm preventive medication to an infected dog for 2 months prior to melarsomine treatment. Long-term, continuous use of heartworm preventives alone to treat heartworm infections, is not recommended as an alternative to melarsomine, because it is well documented that additional damage to the heart and lungs occurs the longer adult heartworms are present.