What happens if your dog gets heartworms?

Most dog owners know that we recommend giving heartworm preventive medication every month, all year to all dogs. But what really happens if your dog is infected with heartworms? It can be treated, right? Well, yes, but there is a lot of long-term and irreversible damage that can be prevented, and treatment is lengthy and expensive. Let’s look at what really happens inside your dog first.

Damaged blood vessels and more

We always think about heartworms as a parasite that lives in the heart, but that is rarely the case. They spend most of their lives and do their worst damage in pulmonary arteries, which are the blood vessels that bring blood from the heart into the lungs to receive oxygen. And the damage starts early: as soon as 120 days after a dog is bitten by an infected mosquito, the maturing heartworms start moving through the pulmonary arteries and causing inflammation. But a heartworm test cannot accurately diagnose heartworm disease until 180 days after infection, meaning that there is two months’ worth of damage before we can diagnose the disease!

The longer the worms are present in the pulmonary arteries, the more scarred and constricted the arteries become. There is a risk of clots forming around the heartworms, and these clots can break free and travel into the larger vessels of the lungs (a condition known as a pulmonary embolism). The worms themselves also cause blockage of the pulmonary arteries, which causes poor oxygen delivery from the lungs into the heart, severe inflammation of the vessels, and ultimately congestive heart failure. And even after treatment, the dead worms just get pushed farther down into the pulmonary arteries where they can sit for years.

Here is a photo of heartworms blocking the pulmonary artery of a dog.

Photo courtesy of Dr. Stephen Jones

Left untreated, heartworms can cause the heart to have to work harder due to increased resistance in the infected pulmonary arteries. Ultimately, this process can lead to congestive heart failure. There are rare cases in which the worms do travel into the heart causing valve damage or even blockage of the vena cava, the main vein bringing blood to the heart from the general circulation. The inflammation associated with heartworm disease can also cause damage to the liver and kidneys.


Okay, these worms are really gross! We just need to give the medicine that kills them, and we need to do it right away, right? Not so fast. Treating heartworm disease isn’t just about killing the adult worms, although that’s an important part of the treatment. These worms are not just hanging out in the pulmonary arteries; they’re also reproducing and producing microfilaria, the larval form that transmits the disease. So in addition to killing the adult worms, we need to sterilize the females and kill the microfilaria. And if we try to do all of these things really rapidly, we can cause the immune system to have a severe reaction to the dying worms and microfilaria, so treatment can take a long time. A typical heartworm treatment timeline goes like this:

  • Positive test
  • Additional blood testing to check organ function, confirm the positive test result and check for microfilaria. Also, chest x-rays are recommended.
  • Start heartworm preventive medication – this allows the microfilaria to be exposed to the immune system and killed. Continue preventive monthly throughout (and after) the treatment.
  • Start a 28-day course of doxycycline – this antibiotic kills an organism that lives within the heartworms and allows them to reproduce. Doxcycline also makes the microfilaria non-infective.
  • Begin cage rest.
  • Give the first dose of adulticide (the drug that kills the adult worms). This drug is given deep into the muscles of the back and can be very painful for several days after the dose. This dose starts the process of killing the adult worms, but doesn’t bombard the immune system with huge amounts of dead worms.
  • Wait a month with continued strict cage rest.
  • Give the second and third dose of adulticide, 24 hours apart. The dog is usually hospitalized for these doses. These two injections should kill the remaining adult worms.
  • Continue strict cage rest for another 6-8 weeks.
  • Test for the presence of adult worms 4-6 months after treatment.

When you add this up, the treatment process takes about 4 months from diagnosis to the completion of cage rest. And the dog isn’t immune to a new infection, so you still have to give preventive for the rest of his life or you’ll have to do it all again.


Some people tell us that it’s too expensive to give heartworm preventive to their dog. Here’s a comparison between the cost of a year’s worth of Sentinel for a 55-pound dog and the cost of treating the same dog for heartworm disease (prices approximate as of May 2019):

1 year of Sentinel:  ~$120 (about $10 per month)

Heartworm treatment:

  • Blood tests: $160
  • Chest x-rays: $175
  • Doxycycline: $55
  • Adulticide injections: $600 for the 3 injections
  • Hospitalization overnight (with overnight nursing care): $90
  • Additional pain medications: $30-40

Total cost of treatment: $1100-1200 (plus the Sentinel)

And now you know why we strongly recommend monthly heartworm preventive for ALL dogs!

For more information:

American Heartworm Society

April is National Heartworm Month. Is your pet protected?