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Heartworm Disease in Dogs

Prevention, Diagnosis, and Treatment of Heartworm Disease in Dogs


Mosquito by tanakawho on Flickr


by tanakawho on Flickr

Please see the Heartworm Answer Page for more heartworm topics.

About Heartworm Disease

heartworm disease is caused by a parasite that lives mainly in the blood vessels of the lung and in the heart.

Heartworm disease has been seen in several species, but dogs are very susceptible. It can be fatal and is difficult to treat, but fortunately heartworm disease is easy to prevent.

Cause of Heartworm Disease
Heartworm disease is caused by a roundworm called Dirofilaria immitis. Adult heartworms are long and skinny, and can be up to 12 inches long. They live mostly within blood vessels of the lungs and in the heart. Heartworms cause irritation and inflammation in the blood vessels, leading to damage. When there are large numbers of worms, or the worms die, serious complications can develop from blockages in blood vessels.

Heartworm Life Cycle
Adult heartworms reproduce and release their larvae, called microfilaria, into the bloodstream of infected dogs. When mosquitoes feed on a dog (or other animal) with microfilaria in the blood, the mosquitoes ingest the larvae. The larvae undergo changes within the mosquito, after which the infected mosquitoes can transmit the larvae to other dogs. The larvae migrate through the skin and tissues, and undergo further development, ultimately ending up in the blood stream where they grow into adult worms and settle in the blood vessels of the lungs and in the heart, a process that takes about 6 months. These worms can then reproduce, starting the cycle over again.

Risk Factors for Heartworm
Where heartworm is found, all dogs are susceptible to infection. Those that spend a lot of time outdoors may be at greater risk, but indoor dogs are at risk as well. Small dogs tend to be more severely affected by the worms than larger dogs.

Heartworm can only be transmitted where temperatures are warm enough to allow the larvae to complete their developmental stages within mosquitoes. It is found in widely throughout the world, mainly in areas with relatively warm climates. Heartworm has been found in all 50 US states, though spread between dogs has not been documented in Alaska.

Signs and Symptoms of Heartworm Disease in Dogs
The severity of symptoms depends on several factors including the number of worms present and how long the infection has been present. In mild cases, symptoms may be absent or take years to develop. Signs of infection can include:

  • Coughing
  • Low tolerance for exercise
  • Labored or rapid breathing
  • Bluish tinge to skin
  • Coughing up blood
  • Nose bleeds
  • Fainting
  • Weight loss
  • Fluid accumulation in abdomen

Diagnosis of Heartworm Disease
Diagnosis is made by a blood test to detect proteins produced by the worms ("antigen") in your dog's blood. The test may be falsely negative early in an infection or if only a small number of worms are present, so repeating the test after six months may be necessary to detect an infection. An older test method involves checking the blood for circulating larvae, but this test is not as accurate.

In addition to blood testing, radiographs and ultrasound can be used to assess changes in the heart and lungs associated with heartworm disease.

Treatment of Heartworm Disease
Medication is given to kill the adult worms, and in very severe cases, surgery may be required to retrieve large worms from the blood vessels.

Heartworm treatment is vital but risky because the dead worms in the bloodstream can cause some serious complications including blood clots in the lungs. Strict rest is prescribed for several weeks after treatment, since activity can increase the chance of complications. Complications often affect the lungs, causing breathing problems, and may require oxygen therapy and medications to combat inflammation and blood clots.

Because treatment is difficult, it is much safer and easier to prevent infection in the first place.

Prevention of Heartworm Disease
Fortunately, prevention of heartworm disease is relatively straightforward. There are several preventative medications available, most of which are given monthly (e.g., Heartguard®, Interceptor®, Revolution®, Advantage®, Sentinel®).

These medications work by killing any larvae that might have been transmitted by mosquitoes in the time between doses, thus preventing the development of adult heartworms and heartworm disease.

Dogs should be tested for heartworms prior to starting medication, unless they are started on medication as young puppies. Your vet will help you choose the right medication for your dog's situation, and also discuss a schedule for medication and regular testing.

Year-round dosing and yearly testing is recommended. Giving the medication at the proper intervals is the most important factor in the success of heartworm prevention.

If your dog misses a dose of heartworm preventative check with your vet, because sometimes a missed dose can leave dogs vulnerable to infection.

Please Note: this article is intended for informational purposes only. If your pet is showing any signs of illness, please consult a veterinarian as quickly as possible.

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