Understanding Canine Heartworm Disease

Canine heartworm disease can be a potentially lethal disease of the heart and lungs. An infected mosquito bites and transmits the infection whereby worm larvae mature and eventually produce more worms. After traveling throughout the body for about 3 months, the worms eventually stop in the heart, residing permanently there and in the nearby arteries in another 3 months where they mature into adults. Adult worms can reach lengths of 6 to 14 inches. Depending upon the number of worms present, how long they have been there and the age and health of the dog, will determine the severity of the disease. It is possible for a dog to show no clinical signs of heartworm disease.

Any dog, no matter what age, gender or breed, is susceptible to heartworm disease. However, most infections (up to 45%) occur within a 150 miles radius of the Atlantic and Gulf coasts as far north as New Jersey and along the Mississippi River and its tributaries.

Heartworm Symptoms

Heartworm disease can cause several medical problems including dysfunction of the heart, lungs, kidneys and liver. Accumulation of adult worms impairs blood circulation causing damage to these vital organs. Sometimes there are no visible clinical signs at first infection. However, a cough may signal the first sign of infection.

Clinical signs include:

  • Cough
  • Exercise intolerance
  • Abnormal lung sounds
  • Difficulty breathing
  • Enlargement of the liver
  • Fluid accumulation in the abdominal cavity
  • Abnormal heart sounds
  • Listlessness
  • Weight loss

Temporary unconsciousness due to poor blood flow to the brain Enlargement of the pulmonary arteries of the lungs, main pulmonary artery of the heart and right side of the heart If the disease is not treated early enough, it can lead to death.

Diagnosing with a Heartworm Test

There is not just one heartworm test but also a combination of tests and clinical signs used to determine the present of heartworm disease.

Diagnostic tools include:

  • Patient's history
  • Physical examination
  • Observance of clinical signs
  • Blood tests
  • X-rays
  • Detection of microfilariae (juvenile worms that circulate in the bloodstream)
  • Angiography
  • Echocardiography (ultrasound of the heart)

Heartworm Treatment

Most infected dogs can be successfully treated. The goal of treatment is killing all adult worms and their larvae. The earlier the disease is treated, the better the success rate.

There are two types of treatments: a drug administered intravenously and another through shots administered by injections into the lumbar (lower back) muscles. Either method used requires the dog to crate rest. This is to prevent pulmonary thromboembolism, which may result from the obstruction of blood flow through the pulmonary arteries due to the accumulation of dead heartworms in the arteries and capillaries of the lungs. An increase heart rate due to exercise or excitement will only magnify this condition. Pulmonary thromboembolism can produce cough, fever, blood in the sputum and potentially sudden death. Often the dog is hospitalized during treatment to monitor any adverse reactions as the heartworms are dying off. The dead worms are carried by the bloodstream into the lungs where they can lodge in the small blood vessels. There they decompose and are absorbed by the body within a few months.


Prevention is less costly and safer than treating the disease. Daily and monthly tablets and chewables are available as well as monthly topicals and a six-month injectable. These products are extremely effective and will prevent heartworm infection when used correctly. These products also prevent intestinal parasites, which can infect the dog's owner, with an estimated 3 to 6 million people infected each year.