Heartworm Disease

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

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

Signs of heartworm disease

A recently infected dog may show no signs of illness until the adult worms have developed in the lungs and signs of heartworm disease are observed. 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.

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

The signs of heartworm disease in ferrets are similar to those in dogs, but they develop more rapidly because the ferret’s heart is small and even one worm can cause severe problems. The American Heartworm Society recommends year-round prevention for ferrets as well as regular checkups with a veterinarian to ensure they stay healthy and heartworm-free.

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Preventing heartworm disease

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 to eliminate adult heartworms, but serious complications are much less likely when you carefully follow your veterinarian’s instructions. 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. Surgical removal of heartworms from dogs and cats is a high-risk procedure and is typically reserved for severe cases.

There is currently no FDA-approved 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.

Heartworm infection is almost 100% preventable in dogs and cats. There are several FDA-approved heartworm preventives available in a variety of formulations. Your veterinarian can recommend the best method of prevention based upon your pet's risk factors and lifestyle. The American Heartworm Society recommends year-round prevention for dogs, cats and ferrets as well as regular checkups with a veterinarian to ensure they stay healthy and heartworm-free.

Of course, you have to remember to give your pet the preventive in order for it to work! 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 recommended before beginning a prevention program to assess the pet’s current heartworm status. Because it is more difficult to detect heartworms in cats, additional testing may be necessary to make sure the cat is not infected.

For more information about heartworm, visit the American Heartworm Society’s website.

The content on this page is a condensed version of our brochure, Heartworm Disease, available in English and Spanish.