Heartworm Disease

From the AVMA brochure, revised March 2010

Heartworm disease is a preventable, but serious and potentially fatal, parasite that primarily infects dogs, cats and ferrets.

How is heartworm disease transmitted and what does it cause? Heartworms are only transmitted from animal to animal by mosquitoes. When an infected mosquito bites an animal, microscopic young heartworms enter into that animal's bloodstream. It takes six months for the infective larvae to mature into adult heartworms and reach the blood vessels of the lungs.  Adult females grow to lengths of up to 14 inches. The worms damage the blood vessels, and reduce the heart's pumping ability, resulting in severe lung and heart disease. When the animal shows signs of illness due to adult heartworm infection, it is called heartworm disease. Adult heartworms can survive for 5 to 7 years in dogs and several months to years in cats.  If adult worms of both sexes are present, they will mate and reproduce. Baby worms can cause immune reactions that damage other organs. The life cycle continues when a mosquito bites the infected animal and becomes infected by the baby worms.  Two weeks later, when bitten by that mosquito, another animal becomes infected.

Where are heartworms found?

Geographically, heartworms are a potential threat in every state as well as in many other countries around the world. All dogs, regardless of age, sex, or living environment, are susceptible to heartworm infection. Indoor, as well as outdoor, cats are also at risk for the disease.

What pets should be tested for heartworm? 

Because heartworms are spread by mosquitoes, any pet exposed to mosquitoes should be tested. This includes pets that only go outside occasionally.

What are the signs of heartworm disease in dogs?

 If your dog has been recently or mildly infected, it may show no signs of illness until the adult worms have developed in the lungs.  As the disease progresses, your dog may cough, become lethargic, lose its appetite or have difficulty breathing. You may notice that your dog seems to tire rapidly after only moderate exercise.

How is heartworm diagnosed in dogs?

The most common test, an antigen test, detects the presence of adult female heartworms.  It is very accurate in dogs. Further tests, such as chest radiographs (x-rays), a blood profile and an echocardiogram (an ultrasound of the heart), may be necessary to confirm the diagnosis, to evaluate the severity of the disease, and to determine the best treatment plan for your dog.

What are the signs of heartworm disease in cats?

Signs of possible heartworm disease in cats include coughing, respiratory distress, and vomiting. In rare cases, a cat may suddenly die from heartworms.

How is heartworm diagnosed in cats? 

The diagnosis of heartworm infection in cats is more difficult than it is with dogs. A series of different tests may be needed to help determine the likelihood of heartworm infection as the cause of your cat's illness and, even then, the results may not be conclusive.

How can my dog be treated?

As with most medical problems, it is much better to prevent heartworm infection than to treat it. However, if your dog does become infected with heartworms there is an FDA-approved treatment available. There is substantial risk involved in treating a dog for heartworms. However, serious complications are much less likely in dogs that are in good health and when you carefully follow your veterinarian's instructions.  The goal of heartworm treatment is to kill the adult and baby worms present in your dog, as safely as possible. However, when a dog is treated it is important to consider that heartworms are dying inside the dogs lungs. While your dog is treated, it will require complete rest throughout hospitalization and for some time following the last treatment. Additionally, other medications may be necessary to help control the body's inflammatory reaction as the worms die and are broken down in the dog's lungs.

How can my cat be treated?

There is currently no effective and safe medical treatment for heartworm infection or heartworm disease in cats. If your cat is diagnosed with heartworms, your veterinarian may recommend medications to reduce the inflammatory response and the resulting heartworm disease, or surgery to remove the heartworms.

Can heartworms be surgically removed? 

Surgical removal of heartworms from dogs and cats is a high-risk procedure and is typically reserved for severe cases. However, in many cases surgical removal of heartworms may be necessary to afford the best opportunity for the pet's survival.

Can heartworm disease be prevented? 

Heartworm infection is almost 100% preventable in dogs and cats. There are several FDA-approved heartworm preventives available in a variety of formulations. Of course, you have to remember to give your pet the preventive in order for it to work! 

Why does my dog need a heartworm test every year? 

The preventives do not kill adult heartworms, and will not eliminate heartworm infection or prevent signs of heartworm disease if adults are present in the pet's body. Therefore, a blood test for existing heartworm infection is necessary to assess a dog?s current heartworm status. Heartworm tests should be performed annually afterwords to ensure that your dog doesn't subsequently become infected with the disease and to ensure the appropriate amount of medication is being prescribed and administered. There have been reports of dogs developing heartworm infection despite year-round treatment with a heartworm preventive, so having your pet tested regularly is the best way to keep them protected.