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Heartworm Disease

Heartworm Disease

Heartworm is a serious, life-threatening disease that can affect dogs, cats and ferrets. It is caused by the adult stage of the parasite Dirofilaria immitis. The infection may cause inflammation and thickening of the pulmonary arteries, damage to the heart, liver and kidneys, and, if untreated, can lead to heart disease and death.

Risk Factors

Cases of heartworm are regularly reported throughout the state of Oregon. Mosquito populations are capable of rising rapidly if conditions are warm and wet. Your pet's exposure to parasite-carrying mosquitoes increases with outdoor activities in areas that have high mosquito counts.


Mosquitoes carry the parasite that causes heartworm disease from animal to animal. Dogs, cats, ferrets and wild canids, including coyotes, are potential reservoirs of infection. The life cycle of a heartworm begins when a mosquito bites an infected animal carrying heartworm microfilariae in its blood. If that mosquito bites another cat or dog, it transmits the larvae to that animal. The larvae mature into adult worms in the heart and lungs of the host animal. The adult worms can reproduce, creating microfilariae about 6-9 months after the initial mosquito bite.


Your veterinarian may perform a blood test to determine whether your pet has the disease. A blood sample is tested for the antigens (proteins) produced by adult heartworms. The sample may also be examined under a microscope for the presence of the heartworm larvae. More laboratory tests may be required to make a diagnosis, especially in cats, as the disease can be harder to diagnose in felines. A negative test result for the larvae does not rule out feline heartworm infection, as the larvae often are found only temporarily in an infected cat. However, if circulating larvae are found, it becomes a confirmation that heartworm disease is present.

Heartworm in Dogs


Signs of infection in dogs include a chronic cough (which is the most common symptom and a sign of advanced illness), lack of energy or endurance, difficulty breathing, loss of appetite or weight loss.


If detected early enough, most dogs can be treated successfully. Melarsomine dihydrochloride is the only drug approved by the FDA for heartworm treatment. Dogs should be closely monitored by a veterinarian during treatment. Following treatment, dogs should have restricted exercise for up to six weeks because active dogs are at risk for blood clots in the lungs. Only when tests show a dog to be free from heartworms is a preventive medication prescribed.

Heartworm in Cats


Signs of infection in cats include: cough, difficulty breathing, vomiting, sluggishness or weight loss. Some cats never exhibit clinical signs, but even a small number of worms can be life-threatening. Recently, researchers discovered that respiratory signs in cats, which are often diagnosed as feline asthma or allergic bronchitis, may actually be caused by the presence of heartworms in either larval or adult stages. The acronym “HARD” is the term for this clinical presentation and stands for Heartworm Associated Respiratory Disease.


Currently, there is no approved product for the treatment of heartworm disease in cats. Most cats with heartworm infection that are not demonstrating clinical signs are allowed time for a spontaneous cure to occur. Treatment is aimed at helping cats tolerate the disease, rather than eliminating it. If there is evidence of disease in the lungs and their blood vessels consistent with feline heartworm infection, such cases can be monitored with chest X-rays every six to twelve months, as needed. Supportive therapy with small, gradually decreasing doses of prednisone (a cortisone-like drug) is recommended for cats with radiographic or clinical evidence of lung disease. Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat's body.

Heartworm in Ferrets

No drugs are FDA-approved to treat heartworm disease in ferrets, so prevention is critical. If you own a ferret, talk to your veterinarian about prevention options.


When it comes to detecting heartworm disease, observing your pet's health is not enough. There is no "off-season" for heartworm prevention and treatment. Clinical symptoms develop very slowly; in fact, there may be no visible warning signs that a dog or cat is sick until the disease has reached an advanced stage. Prevention is simple compared to the expense and risk of treatment.

American Heartworm Society Guidelines recommend annual testing to ensure continuity of care, retesting any time there is a change in preventive methods, and year-round heartworm prevention, even in areas which show only seasonal mosquito activity.

It is generally recommended that all dogs and cats be tested for heartworm disease prior to administration of a heartworm preventive. Several medications are available to prevent heartworm disease in dogs and cats, and some can also protect your cat or dog against fleas and other types of worms.

There is no vaccine for heartworm disease. Talk with your veterinarian about testing and the appropriate preventive treatment to help keep your best friend safe from heartworm disease.

Updated: April 5, 2021

Sources: American Heartworm Society, FDA, Dr. Gary Wood DACVIM, Dr. Kathryn Atkinson DACVIM