Heartworm Disease

Jun 05, 2015

Heartworm Image


Warmer weather and the presence of mosquitoes are not only signs of spring but also the start of heartworm season. This deadly disease has been reported in all 50 states and our pets depend on us to keep them safe. The good news is that heartworm disease is highly preventable and the cost of medication is far less than the treatment.

What is Heartworm?

Heartworm, also known as Dirofilaria immitis, is a parasite that infects dogs and less commonly cats (as well as some other critters but not humans). These parasites are found just outside the heart in the pulmonary arteries, which carry blood to the lungs to get oxygen and deliver it to the rest of the body.

Initially, these parasites were discovered in the hearts of dogs after dogs had passed away from the disease. When blood stopped flowing through these vessels, the worms fell backwards into the heart giving them the name “heartworms.”

Signs of Heartworm Disease

Symptoms of heartworm infections in dogs and cats are very different.

Symptoms are caused by the adult worms and vary depending on the amount of worms present. Signs may include:
• No symptoms
• Coughing
• Exercise intolerance
• Difficulty breathing
• Death

Cats have a different response to infection. Symptoms are usually associated with a reaction to the presence of immature worms and/or the death of an adult worm.

Signs may include:
• Chronic vomiting
• Asthma-like symptoms
• Difficulty breathing
• Sudden death

Heartworm Life Cycle

Heartworms are transmitted to our pets by mosquitoes. The mosquito is necessary to complete the life cycle of the parasite. The process from the initial mosquito bite and injection of immature worms (microfilariae) into the blood of the pet, to the development of adult worms can take as long as 6-7 months.

Understanding the heartworm life cycle is important in testing for the disease. If your pet is bitten and infected in the summertime, a heartworm test may not become positive until the winter or even later.

When a heartworm test is performed using a blood sample, it looks for signs of adult female heartworms. Tests may come back negative, however, despite an active infection with adult worms. This occurs when the adult worms are male, or the females are still too immature, to show a positive result.

If your pet does test positive, your veterinarian may recommend further testing and consultation with a veterinary cardiologist. Additional tests can be extensive and include:
• Full bloodwork
• Evaluation of urine
• X-rays of the heart and lungs
• Echocardiogram or ultrasound of the heart to evaluate the chambers of the heart and vessels and to look for the worms themselves

Heartworm Treatment in Dogs

Heartworm infection in dogs is treatable and usually successful but also can be very dangerous.

Treatment in dogs involves:
• The use of an adulticide (a drug that kills the adult worms)
• Microfilariacide (a drug to kill the circulating offspring and immature worms circulating in the blood stream)
• Surgical retrieval of the worms under general anesthesia by a cardiologist may be necessary in some dogs with more severe infections involving large numbers of worms
Adulticide Injections
• Given in multiple stages into the muscles of your dog’s back
• Many dogs are hospitalized during this treatment
• Once at home, dogs need to be strictly rested in a cage for at least a month or two to avoid severe life-threatening complications

Cats & Heartworm Disease

Cat owners often assume that an indoor cat is safe from this disease. However, that is not 100% true. Mosquitoes can get inside your home and they are just as likely to bite your indoor kitty as they are you. One infected mosquito can easily put your cat at risk.

Currently, there is no licensed treatment for heartworm in cats. Cats without symptoms are usually given time for the infections to resolve; their immune systems are usually able to rid them of the parasite spontaneously. If they have symptoms, cats may be given antibiotics and anti-inflammatory medications, or if your cat’s signs are more severe, he or she may require hospitalization for oxygen and supportive care.


Prevention is by far the safest and most economical way to protect your pet from this infection. There are a variety of effective and inexpensive preventatives for both dogs and cats available through your veterinarian. This includes:
• Tablets (daily or monthly)
• Injections (given every 6 months)
• Topical medications

Don’t let cold weather fool you into stopping treatment. Warmer winters have enabled mosquitoes to survive for longer periods throughout the year. And if you travel with your pets to warmer climates during the winter months, your pet may be exposed while on vacation.

Before starting heartworm preventatives, it is important to have your pet examined and heartworm tested by your veterinarian, as well as annually during your pet’s regular veterinary visits. For more information on heartworm disease, visit the American Heartworm Society.

If you have any questions, contact your primary care veterinarian. To learn more about our Cardiology Department, visit, or call (732) 747-3636.