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Heartworm Prevention

Heartworm Medication Tables

Heartworms, Dirofilaria immitis, belong to the same class of worms as roundworms and are similar to them in appearance. Heartworms spend their adult life in the right side of the heart and the large blood vessels connecting the heart to the lungs. They can be found in both dogs and cats.

How Your Pet Gets Infected

Stage 1

Heartworm disease, caused by Dirofilaria immitis, is transmitted by infected mosquitoes. When a mosquito takes blood from an infected animal, it ingests heartworm larvae as well. The larvae in this stage are also called microfilariae.

Stage 2

These larvae develop into their infective stage inside the mosquito.

Stage 3

When the mosquito bites your pet, the larvae are transferred over. There are no obvious signs in the early stages of infection, which makes heartworm prevention all the more important because advanced infection may result in death.

Stage 4

Without prevention, the larvae develop and reach the heart and lungs of your pet. Worms can live inside up to 5-7 years in dogs and 2 years in cats, with female heartworms that can grow up to 12 inches and male heartworms that can grow up to 6 inches.

Signs of Heartworm Disease in Dogs

  • Difficulty breathing
  • Coughing
  • Exercise intolerance
  • Reduced appetite and weight loss
  • Lethargy
  • Loss of consciousness
    In dogs, the adult worms can obstruct large blood vessels leading from the heart to the lungs. Worms may also enter smaller vessels in the lung and obstruct those vessels. In severe cases, called “caval syndrome” worms start to fill the right ventricle of the heart.

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    Blood testing is performed to identify pets infected with D. immitis. Because blood tests are not always accurate, we need to interpret test results in relation to the history and the symptoms the animal is showing. Radiographs (x-rays) and ultrasound (echocardiography) are often performed to look for typical changes in the heart and lungs caused by D. immitis, and determine the severity of the infection. Changes include enlargement of the pulmonary artery and the right ventricle. Certain types of cells (eosinophils) may increase in the blood or secretions from the lungs in heartworm infections. These additional findings can all help support the diagnosis.


    Considering the fact that the earliest heartworm antigen and microfilariae can be detected is 5 and 6.5 months respectively, prevention is key.
    Pets should be tested for heartworm infection before starting a preventive program. Preventives are not used to kill adult worms; drugs called adulticides must be used to kill them. Nevertheless, it is better to avoid heartworms through preventive measures.