Pet Health Library

Heartworm Disease in Dogs

A dog stares at a mosquito

Background

Heartworm disease is a potentially deadly, but preventable, infection that develops when a dog is bitten by a mosquito carrying the heartworm larvae (juvenile worms) of a parasite. When an infected mosquito bites a dog, larvae are deposited on the dog and penetrate the skin through the bite wound. The larvae then migrate to the blood vessels that serve the dog’s heart and lungs. It takes about 6 months for the larvae to mature into adult heartworms, which can grow to lengths of up to 12 inches and live for more than 5 years. The presence of the adult worms stresses the dog’s heart and causes inflammation of the blood vessels and lungs. Severe complications are also possible when the worms multiply or when they die.

Risk Factors

Heartworm disease occurs most often in dogs who are housed outdoors. However, any dog can become infected since all it takes is a single bite from a mosquito carrying the infective larvae.

Signs

Common signs of heartworm infection include:

  • Coughing
  • Exercise intolerance
  • Failure to grow
  • Labored breathing
  • Blue or purplish discoloration of the skin and gums
  • Spitting up blood
  • Fainting
  • Nosebleeds
  • Accumulation of fluid in the abdominal cavity

The severity of the signs is often related to the amount of lung damage and the dog’s activity level. More active dogs tend to show more dramatic signs of infection than less active dogs.

Diagnosis

The antigen detection test is the preferred method for diagnosing heartworm infection. This test is easy to perform and is considered the most accurate test available for dogs. The one drawback is that it cannot detect early infection since it only identifies adult, female worms.  For this reason, annual testing is recommended for all dogs to ensure the preventive medication is working. Other diagnostic tools include echocardiography, blood tests, and chest x-rays.

Heartworm (Dirofilaria immitis) exposed in internal organs of a dog during necropsy.

Treatment

Untreated heartworm disease can be fatal. If the infection is detected early, the disease can be treated before permanent damage occurs. Standard treatment involves a series of injections with a medication that kills the heartworms called melarsomine dihydrochloride. As a side effect of this treatment, there is a risk that dead heartworms will cause severe respiratory problems, especially if activity isn’t restricted. Dead worms in the heart can also significantly block blood flow. These dogs may need surgery to manually remove the worms.

Prevention

Heartworm disease is preventable. Several preventive medications are available from your veterinarian. The American Heartworm Society recommends annual testing along with giving your dog heartworm preventive medication 12 months a year.

Preventive treatment in dogs is recommended beginning at 6 to 8 weeks of age. No pretesting is necessary at this age. When beginning preventive treatment in dogs 7 months of age or older, an antigen test is recommended to make sure the dog isn’t already infected. Due to the worm’s 6-month development period, a second test should be performed 6-7 months later to ensure that the dog is not infected.

The most important thing that pet owners can do to protect their dogs from heartworm infection is to remember to give the prescribed dose of medication at the correct time. If you miss a dose, you should contact your veterinarian.

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