Dog: Heartworm

General information

Other common/scientific names: heartworm disease, heartworm infection, dirofilariasis, Dirofilaria immitus, microfilariasis, filariasis

The adult heartworm or Dirofilaria immitus lives in the pulmonary arteries and heart of an infected dog. The life cycle of the heartworm begins when adult male and female worms mate. Heartworms do not lay eggs but rather they give birth to live baby worms called microfilariae or larvae. There are five larval stages termed L1, L2, L3, L4 and L5. The microfilariae are the first larval stage or L1. These immature worms enter the bloodstream where they can live for up to two years waiting to be ingested by feeding mosquitoes. The immature heartworm develops into L2 andL3 inside the mosquito. The mosquito then bites a new dog and deposits L3 into the new dog’s skin where the immature worm develops into L4 and L5. The L5 is actually a young adult and enters the dog’s circulation, making its way into the heart and pulmonary artery. It takes approximately 5-7 months from the time the mosquito bites the dog for the immature worm to reach adulthood and the dog’s heart.

Adult heartworms have a bacterial like organism called Wolbachia, which lives inside their body. These organisms, thought to be beneficial to the heartworm, seem to be involved in complications which result from heartworm infections in dogs. The role of this organism is still being researched.

For heartworms to be prevalent in an area mosquitoes, capable of carrying heartworms must be present and the weather must be warm enough to allow the larval forms of heartworms to develop within the mosquito. Because of this, some areas in the US have a high occurrence of heartworm infection while other areas have virtually none.


The cause of heartworm disease in dogs is the intravascular endoparasite Dirofilaria immitus. These worms prefer to live in the pulmonary artery causing arterial inflammation and damage. If the dog has a heavy worm load, these worms can back up into the heart causing signs of heart failure.

Abb. GSR88MO9
Abb. GSR88MO9: Microfilaria or larva of a heartworm.


Dogs acquire heartworms through a mosquito bite. Unborn puppies can become infected when the microfilariae cross the placental barrier from their mother. However, these puppies will not develop heartworm disease because the immature larvae must pass through a mosquito in order to mature into adulthood.

Cardinal symptom



It is important to distinguish between heartworm infection and heartworm disease. Dogs with a heartworm infection may only have immature worms in their body and no adult worms in their lungs or heart. These dogs will not show any clinical signs of heartworm disease. Likewise, dogs with only one or two adult heartworms may be asymptomatic.

However, dogs with more than a few adult heartworms will show signs of heartworm disease depending on the total number of adult heartworms and their location. The more worms in the lungs and heart, the more severe the clinical signs. These signs include:

  • Coughing
  • Exercise intolerance
  • Weight loss
  • Abnormal lung sounds
  • Breathing difficulty
  • Nose bleeds
  • Pneumonia
  • Abdominal distention
  • Heart insufficiency and failure


Heartworm infection can be diagnosed with a blood test performed in the veterinarian’s clinic. This test is used as part of your dog’s annual preventative health care to diagnose a heartworm infection before clinical signs develop or it can be used to detect heartworm infection in the sick dog. All dogs should be tested before any heartworm prevention is administered. Your veterinarian will determine how often your dog should be tested for heartworms depending on your geographic location and administration of preventative medication.

Abb. GG3QUIBP: Negative Heartworm Antigen Test.
This is a photograph of a negative heartworm antigen test performed at a veterinarian’s clinic.

Because it takes 5-7 months after a dog is bitten by an infected mosquito for the larval stages to mature to the adult heartworm, it is useless to test dogs or puppies under 5 months of age for heartworms.

If a dog tests positive for heartworm infection and is showing clinical signs of heartworm disease, bloodwork, thoracic radiographs and ultrasonography should be performed to determine the extent and severity of the heartworm disease. These tests will also aid in choosing the appropriate treatment protocol.


Treatment of heartworm disease may vary depending on the severity of clinical signs. Typically, there are three stages:

  • The first stage of heartworm treatment is to clear the immature worms from the dog’s body. This is done by administering one of the monthly heartworm preventative medications for 1 to 3 months.
  • The second stage of heartworm treatment is to kill the adult worms in the lungs and heart. An arsenic based antiparasitic is used. This medication is an injection and is administered while the dog is hospitalized to observe for any possible side effects.
  • The last stage of treatment is to retest the dog. If the adult heartworms have been killed, antigen testing should be negative by six months post treatment.

Because heartworms live in the lungs and heart causing damage and inflammation to these organs, treatment can be difficult. Some dogs may be too ill to undergo treatment. Other dogs may need to be on antibiotics, anti-inflammatories and lung and heart medications prior to treatment. It is important to realize that a harsh arsenic-based drug is necessary to kill adult heartworms and that this treatment may pose health risks to the heartworm positive dog. There is a risk of pulmonary embolism and circulatory shock from the dying worms. Due to this fact, it is important that dogs undergoing heartworm treatment be prevented from exercise or overexertion.


If a heartworm infection is detected early before clinical signs develop, the prognosis is good for a complete cure. However, advanced heartworm disease with heart insufficiency and failure can be a life-threatening condition with a poor prognosis.


Heartworm infection can be easily prevented by regular administration of antiparasitic preventative medication. These medications prevent heartworm disease by killing microfilariae, L3 and L4 larval stages in the dog. Prevention is much safer and less expensive than treatment. There are several options for preventing heartworm infection, including daily and monthly tablets and chewables, monthly topical medications and a six-month injectable medication.

The American Heartworm Society and most veterinarians recommend year-round heartworm prevention. This better ensures owner compliance and most preventative heartworm medications also prevent intestinal parasites which is an added benefit.


If you live in a heartworm endemic area and your dog is not on regular preventative, you should see your veterinarian immediately. If your dog misses a dose or doses of its heartworm preventative, consult your veterinarian. If your dog develops a cough or seems less active, call your veterinarian for an appointment.

Update version: 4/24/2014, © Copyright by
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The information offered by enpevet Ltd. is intended solely for information purposes and and does under no circumstances replace a personal consultation, examination or diagnosis through a veterinarian. Thus, the information serves as an addition to the dialogue between pet owner and veterinarian, but can never replace the visit to the veterinarian. enpevet® would like to ask all users, whose animals have health concerns, to see a veterinarian as required. If you have any questions regarding the health of your animal, we recommend that you turn to your trusted veterinarian , instead of starting, changing or breaking off treatments on your own. The content of enpevet® cannot and should not be used for making your own diagnoses or for the selection and application of treatment methods.