Why Pet Owners Should Be Concerned About Heartworm

What is Heartworm and How is it Transmitted?

Heartworm, a form of pulmonary dirofilariasis, is a potentially fatal parasitic infection caused by an organism called Dirofilaria immitis, which relies on mosquitoes as their intermediate host for development and transmission. Mosquitos acquire microfilariae (immature larvae) when feeding on an infected animal, such as a coyote or fox. In the mosquito, the microfilariae go through three stages of developmental changes called molts. The microfilariae become infectious in larval stage 3, which can take anywhere between 1 and 4 weeks depending on environmental temperatures. These mosquitoes can now transmit these infectious larvae to dogs when the mosquitoes feed. The larvae migrate from the subcutaneous tissue at site of feeding to the tissue and muscles in the chest and abdomen where they remain for around 2 months. After the final molt occurs, the worms are transported via circulation to the pulmonary arteries and continue to grow. As the worms grow and take up more space in the arteries, they enter the heart’s right atrium and ventricle. Females can grow up to 14 inches and begin to produce microfilariae anywhere from 6-8 months after infection. In dogs, the number of adult worms living inside and around the heart can range anywhere from 1 to 250 and have a lifespan of 5-7 years. Heartworms can cause blood flow obstruction, allergic reactions, infection, severe tissue damage and death. 

While heartworm disease is diagnosed much more frequently in dogs than cats, cats can still be affected. Typically, heartworm infection in cats only produces a few adult worms. Heartworm associated respiratory disease (HARD) is a syndrome seen in cats, which is caused from the body’s immune response to the dying and dead juvenile worms in the pulmonary vasculature. HARD is frequently misdiagnosed in cats as allergic bronchitis or asthma. 

Clinical Symptoms Include...

  • coughing
  • difficulty breathing
  • exercise intolerance
  • poor perfusion resulting in pale/blue mucous membranes
  •  syncope (collapse)
  • epistaxis (nose bleeds)
  • hemoptysis (coughing up blood)
  • weight loss
  • lethargy
  • right-sided congestive heart failure
  • ascites (pooling of fluid in the abdominal cavity)

How is Heartworm Diagnosed?

Unfortunately, the prepatent period (period between infection and detection inside the host) of heartworm is typically around 5-7 months from time of infection, meaning it won’t be detectable with diagnostic testing during that time period. Heartworm is diagnosed through a serology test which is capable of detecting the body’s production of antigens specific to heartworm disease. Since the heartworm antigen will not be detected in the blood until 7 months after infection, dogs 7 months and older should be tested at the time of starting preventative therapy and again 7-12 months later. Dogs under 7 months should start preventative therapy and receive a heartworm test in 6-7 months to be sure they did not acquire heartworm disease sometime prior to taking preventative measures. Echocardiography (ultrasound of the heart)  is not effective in diagnosing heartworm disease in dogs as detection is not possible unless there is a high worm burden (the number of worms a host carries). 

Serologic antigen testing for heartworm disease in cats is unreliable and ineffective. Serologic antibody testing is performed in cats as the majority of cats produce antibodies to heartworm 5 months after infection. Due to heart structure and size, heartworm can often be visualized using echocardiography in cats. A standard CBC and chemistry blood panel should be performed as well as thoracic (chest) radiographs. Evaluation of thoracic radiographs provide important information regarding disease progression and severity. 

Annual heartworm testing is often required before refilling prescription preventatives.

How is Heartworm Treated?

Treatment protocol depends on disease progression and severity. Dogs with a high worm burden are at a higher risk of experiencing complications from treatment. The only FDA approved medication used to kill adult heartworms is melarsomine dihydrochloride, an injectable drug administered into muscle. Low risk dogs will receive two injections 24 hours apart. Dogs with a high worm burden are at a higher risk for post-adulticide thromboembolism, a possible serious complication from worm death. High risk dogs will receive an initial injection. Around a month after the initial injection, the series of two injections will be given 24 hours apart. Melarsomine dihydrochloride injections may cause local swelling and soreness at the injection site. It is recommended that Doxycycline, a broad-spectrum antibiotic be given alongside the monthly heartworm preventative. Until heartworm disease has been resolved, exercise restriction is crucial. 

Unfortunately, there is no effective treatment for heartworm positive cats. There have been cases where the immune system has been able to spontaneously clear the infection, however in most cases the infection is fatal. Supportive care can provide comfort by reducing the undesirable side effects of heartworm disease. For example, corticosteroids can improve respiratory side effects, such as difficulty breathing.

How Can I Prevent Heartworm in My Pet?


Luckily, there are several different preventative options available to greatly reduce your pet’s chance of being infected by this potentially fatal disease. Topical and oral preventative medications must be given once a month to remain effective. Prescription from a veterinarian is required. 

  1. Advantage Multi (imidacloprid and moxidectin) – topical medication applied monthly with coverage against heartworm, hookworms, roundworms, whipworms, fleas and mites. Available for dogs and cats.
  2.  Revolution (selamectin) – topical medication applied monthly with coverage against heartworm, fleas, mites and ticks. Available for dogs and cats.
  3. Revolution Plus (selamectin and sarolaner) – topical medication applied monthly with coverage against heartworm, roundworms, hookworms, fleas, mites and ticks. Available for cats.
  4. Heartgard Plus (ivermectin and pyrantel) – oral medication given monthly with coverage against heartworm, hookworms and roundworms. Available for dogs. 
  5. Heartgard (ivermectin) – oral medication given monthly with coverage against heartworm and hookworms. Available for cats.
  6. Interceptor Plus (milbemycin oxime/praziquantel) – oral medication with coverage against heartworm, hookworm, roundworm, tapeworm and whipworm. Available for dogs.
  7. Simparica TRIO (sarolander, moxidectin and pyrantel) – oral medication given monthly with coverage against heartworm, hookworms, roundworms, ticks and fleas. Available for dogs. 
  8. Sentinel (milbemycin oxime and lufenuron) – oral medication with coverage against heartworm, roundworms, hookworms, whipworms and fleas. Available for dogs. 
  9. Trifexis (spinosad and milbemycin oxime) – oral medication given monthly with coverage against heartworm, roundworms, hookworms, whipworms and fleas.  Available for dogs. 
  10. ProHeart12 (moxidectin)- injectable medication that provides 12 month coverage against heartworm in dogs. 
  11. ProHeart6 (moxidectin) -injectable medication that provides 6 month coverage against heartworm in dogs.

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