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Heartworm disease is a serious and potentially fatal condition caused by parasitic worms (Dirofilaria immitis) living in the arteries of the lungs and in the right side of the heart. Heartworm disease occurs in dogs, cats and other species of mammals, including wolves, coyotes, foxes, ferrets, and sea lions. Dogs and cats of any age or breed are susceptible to infection. Heartworm larvae are transmitted to dogs and cats through the bite of infected mosquitos.


Heartworm disease has been reported in all 50 states. 


Since its first discovery in dogs more than 100 years ago, naturally acquired heartworm infection has been identified as a worldwide clinical problem. Despite improved diagnostic methods, effective preventives and increasing awareness among veterinary professionals and pet owners, cases of heartworm infection continue to appear in pets around the world.


First, adult female heartworms release their young, called microfilariae, into an animal's bloodstream. Then, mosquitoes become infected with microfilariae while taking blood meal from the infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. Then the mosquito bites another dog or cat and the infective larvae enter through the bite wound. It takes approximately 6-7 months for the infective larvae to mature into adult worms. In dogs, adult heartworms are thin, spaghetti-like worms that can reach up to 12 inches in length and can remain in the dog’s heart for several (5-7) years. Microfilariae cannot mature into adult heartworms without first passing through a mosquito.


For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages.

  • Early Infection - No abnormal clinical signs observed
  • Mild Disease - Cough only
  • Moderate Disease - Cough, exercise intolerance, abnormal lung sounds
  • Severe Disease - Cough, exercise intolerance, dyspnea (difficulty breathing), abnormal lung sounds, hepatomegaly (enlargement of the liver), syncope (fainting), ascites (fluid accumulation in the abdominal cavity), abnormal heart sounds, death

Cats may exhibit clinical signs that are very non-specific, mimicking many other feline diseases. Chronic clinical signs include vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss. Signs associated with the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often mistaken for feline asthma or allergic bronchitis, when in fact they are actually due to a syndrome newly defined as Heartworm Associated Respiratory Disease (HARD).


Heartworm infection in apparently healthy animals is usually detected with a simple blood test for antigen or by direct examination of the blood for circulating microfilariae. Heartworms are not detectable with the commonly used antigen blood test until they are sexually mature (about 6 months after entering the patient). Female worms must be present for accurate test results. Heartworm infection may also occasionally be detected through ultrasound and/or x-ray images of the heart and lungs, although these tests are usually used in animals already known to be infected.

The earlier a diagnosis is made and treatment is begun, the better the chances are that the patient will recover properly.


Heartworm infestation is dangerous; untreated dogs die and treated dogs go through weeks of discomfort while the worms are killed and expelled from their bodies. If a blood test or the onset of symptoms alert owner and veterinarian to the presence of this devastating parasite, treatment is possible and successful if the disease has not progressed too far. Usually, all but the most advanced cases of heartworm disease can be successfully treated in dogs. The first step is to evaluate the dog and treat any secondary problems of heart failure or liver or kidney insufficiency so that he can withstand the treatment. 
Adult heartworms are killed using a drug called an adulticide (an arsenic compound) that is injected into the muscle through a series of treatments. Treatment may be administered on an outpatient basis, but hospitalization is usually recommended. When the dog is sent home, exercise should be limited to leash walking for the duration of the recovery period, which can last from one to two months. This decreases the risk of partial or complete blockage of blood flow through the lungs by dead worms.

Currently, there are no products in the United States approved for the treatment of heartworm infection in cats. Cats have proven to be more resistant hosts to heartworm than dogs, and often appear to be able to rid themselves of infection spontaneously. Unfortunately, many cats react severely to the dead worms as they are being cleared by the body, and this can result in a life-threatening shock reaction. Veterinarians will often attempt to treat an infected cat with supportive therapy measures to minimize this reaction; however it is always best to prevent the disease.

Re-infection during treatment is prevented by administration of a heartworm preventive medication. These preventives may also eliminate microfilariae if they are present.


Because heartworm disease is preventable, the American Heartworm Society recommends that pet owners take steps now to talk to their veterinarian about how to best protect their pets from this dangerous disease. Heartworm prevention is safe, easy and inexpensive. While treatment for heartworm disease in dogs is possible, it is a complicated and expensive process, taking weeks for infected animals to recover. There is no effective treatment for heartworm disease in cats, so it is imperative that disease prevention measures be taken for cats.

There are a variety of options for preventing heartworm infection in both dogs and cats, including daily and monthly tablets or chewables and monthly topical medications. All of these methods are extremely effective, and when administered properly on a timely schedule, heartworm infection can be completely prevented. These medications interrupt heartworm development before adult worms reach the lungs and cause disease. 

  • All dogs should be tested first, before starting ANY heartworm preventative unless they are less than 7 months old. Dog’s over 7 months of age that are started on preventative without first testing for heartworm are at an increased risk of developing severe reactions.
  • Puppies should be started on heartworm preventative at approximately 8 weeks of age  and then blood tested at 7 months of age.
  • Dogs should be tested yearly even if no doses were missed and preventative is given year around.
  • Dogs that miss more than 2 doses (2 months) of preventative should be tested prior to restarting the preventative medication.

Please be aware that ALL heartworm preventative medications must be purchased directly from a veterinarian, or from a veterinary representative with a valid prescription. If you are able to purchase heartworm preventative medications from any other source without a prescription DO NOT USE THESE MEDICATIONS and report the source to the Food and Drug Administration (FDA). These medications may be counterfit and will not provide proper protection for your pet against heartworm infection.


Cats are susceptible to heartworm disease too! Until the last few years, it was thought that feline heartworm disease was so rare that prevention was not needed in cats. This is simply no longer known to be true. Although there are similarities between dog and cat heartworm infection, there are several differences between dogs and cats regarding heartworm disease. Some of these differences were mentioned above, but because heartworm infection in cats is a developing area of veterinary medicine it is important to highlight these differences to bring them the attention they deserve.

With Feline Heartworm Disease, Prevention is Key.

The Dangerous Facts:

1) Any area where dogs can get heartworms, cats can get them as well.  The current rate of cat heartworm infection diagnosis ranges from 5 to 20% that of dogs in the same area. Heartworm disease is being recognized as an increasing problem in cats.

2) There is no age predilection to Dirofilaria immitis infection in cats and a wide age range of clinically infected cats is reported (6 months - 17 years).

3) Indoor cats are at risk, too! It's a common misconception that indoor cats aren't in danger for parasitic infections. This simply isn't true. Mosquitos can enter your house putting indoor cats at risk. 25% of heartworm infected cats are reported to be indoor only cats. 

4) A higher incidence in males compared to females in experimental and clinical cases may represent a sex susceptibility.

5) Many cat heartworm infections are overlooked since feline heartworm disease does not commonly present with the same signs as dogs. Clinical signs may be confused with other common feline diseases, such as asthma.

Common signs of feline heartworm include:
- Difficulty breathing
- Coughing
- Vomiting
- Sluggishness
- Collapse
- Convulsion
- Diarrhea
- Sudden death

6.) In cats, even one worm can cause sudden death or sudden (acute) respiratory signs that are indistinguishable from asthma without a medical work up.  

7.) The detection of adult heartworms in cats can be difficult and tests are not always reliable. There are both antibody and antigen blood tests that can be used to detect heartworm in cats.

8.) In the cat, the presence of heartworm is difficult to detect with a blood test alone, therefore preventative medication is often started without testing unless signs of a heartworm infection are noticed. This is NOT true in dogs. All dogs must be tested before starting preventative medications.

9) No approved heartworm treatment exists for affected cats.

10) Most cat owners are unaware that feline heartworm disease even exists.


If your dog or cat is not on heartworm preventative please call 516-379-6200 to schedule an appointment for an exam, a heartworm test (dogs), and to purchase heartworm preventative medication today.


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