Heartworm Disease

What Is Heartworm Disease?

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are filarids, one of many species of roundworms. Dogs and cats of any age or breed are susceptible to infection. Learn more…

Where is Heartworm Disease?

Heartworm disease has been reported in all 50 states. The map below shows particularly endemic areas based on the number of cases reported by clinics.

heartworm map


The first published description of heartworm in dogs in the United States appeared more than 100 years ago in an issue of “The Western Journal of Medicine and Surgery.”1 Heartworm in cats was first described in the early 1920’s.2, 3

Since then, naturally acquired heartworm infection in cats and dogs is 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.

1 Osborne, TC. Worms found in the Heart and Bloodvessels of a Dog; Symptoms of Hydrophobia. The Western Journal of Medicine and Surgery, 1847.
2 Riley, WA. Dirofilaria immitis in the heart of a cat. J Parasitol 1922;9:48
3 Travassos, LP. Notas Helminthologicas. Brazil-Med. An. 1921;35 2(6):67

How Heartworm Happens: The Life Cycle

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. After that, the mosquito bites another dog, cat or other susceptible animal, and the infective larvae enter through the bite wound. It then takes a little over 6 months for the infective larvae to mature into adult worms. In dogs, the worms may live for up to 7 years. Microfilariae cannot mature into adult heartworms without first passing through a mosquito.

Heartworm Lifecycle

What Are the Signs of Heartworm Disease?

For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages, as the number of heartworms in an animal tends to accumulate gradually over a period of months and sometimes years and after repeated mosquito bites.

Recently infected dogs may exhibit no signs of the disease, while heavily infected dogs may eventually show clinical signs, including a mild, persistent cough, reluctance to move or exercise, fatigue after only moderate exercise, reduced appetite and weight loss.

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).

How Do You Detect Heartworm Disease?

Heartworm infection in apparently healthy animals is usually detected with blood tests for a heartworm substance called an “antigen” or microfilariae, although neither test is consistently positive until about seven months after infection has occurred.

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.


Because heartworm disease is preventable, the AHS 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 and chewables, monthly topicals and a six-month injectable product available only for dogs. 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.

It is your responsibility to faithfully maintain the prevention program you have selected in consultation with your veterinarian.


heartwormsHeartHeartworms in the heart of a dog

Usually, all but the most advanced cases of heartworm disease can be successfully treated in dogs. 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 tend to react severely to the dead worms as they are being cleared by the body, and this can result in a shock reaction, a life-threatening situation. 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.

heartwormsPulmonaryHeartworms in the Pulmonary Artery of a dog

Adult heartworms in dogs are killed using a drug called an adulticide 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.

Re-infection during treatment is prevented by administration of a heartworm preventive. These preventives may also eliminate microfilariae if they are present. Dogs in heart failure and those with caval syndrome require special attention.

Cats versus Dogs

  Cats Dogs
Parasite Dirofilaria immitis Dirofilaria immitis
Transmission Mosquito Mosquito
Susceptibility to infection Lower than dogs – 61% to 90% of cats exposed to infective larvae become infected Very high – virtually 100% of dogs exposed to infective larvae become infected
Longevity of worms 2-3 years 5-7 years
Ectopic infections Not uncommon Occasionally
Number of worms Usually less than 6, 1-2 worms most common Not uncommon to find more than 30
Single-sex infections in meso- to high-endemic areas Common Unusual
  • Transient (Lasts about 1 month)
  • Seen in less than 20% of naturally infected cats
  • Persistent
  • Very common (80%-90%)
  • Can last years, even after death of adult worms
Organ with greatest pathology Lungs Heart and lungs
Clinical importance of small worm burdens Potentially fatal Clinical importance depends on the size of the dog, the size of the worm burden,
and exercise level
Diagnosis Complex Relatively simple
  • None approved
  • High risk of complications
  • 1 compound approved
  • Complications manageable
Compounds for prevention 4 approved in US Several approved in US


Dirofilariasis in Cats

Heartworm in cats is caused by infestation of the organism Dirofilaria immitis, a parasitic nematode (roundworm) commonly referred to as the heartworm. The severity of this disease is directly dependent upon the number of worms present in the body, the duration of the infestation, and the response of the host (the infested cat is the host).

The prevalence rate of heartworm disease in unprotected cats that have not received the proper preventative medication, or prophylaxis, is significantly lower than that of unprotected dogs — approximately one-tenth the rate of dogs. Additionally, most cats have only a few heartworms present, and the worms infecting cats are physically smaller and have a shorter lifespan than those infecting dogs. Outdoor cats are at increased risk, and are twice as likely to contract heartworm disease as indoor cats.


Symptoms and Types

Signs of heartworm infestation in cats include coughing, labored raspy breathing (known as dyspnea), and vomiting. Respiratory problems and vomiting are the predominant symptoms in cases of chronic infestation. A physical examination may also reveal a heart murmur or otherwise irregular heart rhythm.


Heartworms are spread through mosquito bites carrying infective heartworm larvae. Because the lifecycle of heartworms affecting cats is shorter than the cycle of those infecting dogs, a look at the (more prevalent) infestation process in dogs is useful. In dogs, the larvae migrate from the bite wound through the body until they reach the heart and blood vessels of the lungs. Here, the larvae mature and reproduce, releasing immature heartworms, known as microfilaria into the animal’s blood. It is important to note that the presence of microfilaria in the blood is in fact uncommon in cats, and has been seen in less than 20 percent of infected cats.

It is also important to note that because heartworms in cats have a much shorter lifespan than those that infest dogs, a spontaneous cure without treatment is more likely to occur.


There are no specific tests that are able to diagnose heartworm disease in cats. A variety of tests that may be done to aid diagnosis include a urine analysis, heartworm antigen and antibody tests, x-rays which may reveal the enlargement of certain veins or arteries associated with heartworm disease, and an electrocardiograph (ECG), which may allow for identification of worms in the heart or pulmonary artery. An ECG can also exclude or confirm other heart diseases that may exhibit similar symptoms.


There is currently no approved adulticide therapy (a treatment that kills the presence of adult heartworms in the body) for cats. Therefore, a surgical procedure to extract the adult worms may be the best option. However, because heartworms in cats have a much shorter lifespan than those that infest dogs, a spontaneous cure is more likely to occur, so that no such treatment is necessary. Various medications may be used in order to help treat symptoms as well.

Living and Management

After treatment, your veterinarian will schedule your cat for follow-up exams in order to test for progress, as well as to attend to any side effects of treatment.


Heartworms are a preventable disease, and there are a number of medicinal preventatives that are highly effective and commonly used. Your veterinarian can determine which medicine is best for your cat.