Heartworms in Dogs


Heartworms (Dirofilaria immitis) are large parasitic worms of dogs (and occasionally cats) that live in the heart and the blood vessels connecting the heart to the lungs. Dogs get heartworms after being bitten by an infected mosquito. The mosquito sucks microfilariae, which are baby heartworms, from the blood of an infected dog. The microfilariae then mature inside the mosquito. When it bites another dog (or the same dog), the microfilariae enter the bloodstream and grow into adult heartworms.

It takes six months for an immature microfilaria to develop into an adult heartworm. Dogs cannot pass heartworms directly to each other - a mosquito has to develop the microfilariae. Humans cannot become infected with heartworms. Heartworm infections are most common in the southern United States, but infections can occur anywhere mosquitos and dogs live and at any time of the year.

Symptoms and Identification

Because heartworms live in the heart and major blood vessels of the lungs, heart and lung disease can occur. Initially, dogs with heartworm infections may not show any symptoms. Unfortunately, heartworm infection is eventually fatal because the heartworms can get very big, clogging the heart and lung vessels, and cause a great deal of inflammation. Symptoms include:

  • Coughing

  • Trouble breathing

  • Inability to exercise

  • Swollen belly

  • Collapse

  • Pale gums

Many veterinary clinics have a rapid, in-house blood test that can be used to diagnose adult heartworms. If a pet is positive on the test, the blood can also be viewed under a microscope to see if microfilariae are present. A confirmation blood test can be sent to a laboratory to ensure that a positive test is truly positive.

If a pet has heartworms, lab work and X-rays may be performed to see how the internal organs, especially the heart and lungs, are performing. It helps to determine if the pet is healthy enough for heartworm treatment.

Affected Breeds

No age, sex, or breed of dog is especially predisposed to heartworm infections. Infections in dogs younger than six months of age are very uncommon because it takes about six months for a heartworm to develop from a microfilaria to an adult.


Killing adult heartworms is difficult. The American Heartworm Society recommends using an antibiotic called doxycycline to weaken the adults prior to treatment. If the pet is not currently on a heartworm prevention, that must be started as well.

After doxycycline, an injection of a medication called melarsomine is given by the veterinarian into the back muscles. The dog must be confined for 30 days, then two additional injections of melarsomine are given 24 hours apart. This is followed by another 30 days of confinement. These confinement periods are critical for the health and safety of the pet. Killing off such large worms can lead to significant inflammation within the heart and lungs and may result in death.

The pet needs to remain on heartworm prevention the entire time to avoid additional heartworms. Pain medication and anti-inflammatory medications are also administered during this time to help minimize the inflammation associated with killing off the worms.

Veterinary Cost

Heartworm treatment is very expensive. Costs range from $1,000-3,000 just for the treatment. If any issues arise during treatment, costs may increase. Preventing heartworms is much cheaper and healthier for the pet. A six-month supply of monthly prevention ranges from $25-150 depending on type of prevention and size of the pet.


Prevention is available by prescription from your veterinarian and is recommended year-round in high risk areas (Southern US and anywhere mosquitos live), even if your pet lives mostly indoors. Types of products include a monthly chewable pill or topical medication that goes on the skin between the shoulder blades.

Most preventive medications have the added bonus of also preventing intestinal parasites (GI worms) as well. Injections are also available that can be given every six to twelve months to prevent heartworms. It is important to know that skipping even one month of prevention can lead to heartworm infection, and a test will not be positive for at least six months. Heartworm prevention does not kill adult heartworms.


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