CAC Blog

A Heart-to-Heart About Heartworms

By May 10, 2022 No Comments
By Dr. Brittany Hassell

Heartworms are a significant and potentially fatal disease in pets in the United States. These worms live in the heart, lungs, and associated blood vessels of affected pets. Since we live in the Southeast AND right along the Tennessee river, we have a heavy burden here. And because infected mosquitoes can come inside, both outdoor and indoor pets are at risk.

How does this affect my pet?

Heartworms can cause severe lung disease and heart failure, along with lasting damage to the heart, lungs and arteries, and can affect your pet’s health and quality of life long after the parasites are gone.

What is the life cycle of heartworms?

The mosquito plays an essential role in the heartworm life cycle. When a mosquito bites and takes a blood meal from an infected animal, it picks up baby worms (microfilaria) that circulate in the blood, which develop and mature into “infective” larvae. When the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to develop into sexually mature adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats.

What are the signs of heartworm disease in dogs?

The dog is a natural host for heartworms, meaning heartworms will mature and produce more heartworms. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. For this reason, heartworm prevention for dogs is by far the best option, and treatment should be administered as soon as possible.

Signs may include:

  • a mild persistent cough
  • reluctance to exercise, fatigue after moderate activity
  • decreased appetite
  • weight loss

As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life-threatening cardiovascular collapse.

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What are the signs of heartworm disease in cats?

The cat isn’t a normal host for heartworms, and most worms in cats do not survive to the adult stage. Cats with adult heartworms typically have just one to three worms, and many cats affected by heartworms have no adult worms. While most cats will go without a diagnosis of heartworms, it’s important to understand that even immature worms cause damage in the form of a condition known as heartworm associated respiratory disease (HARD). Currently, there is no treatment for heartworms in cats, so prevention is VERY important.

Signs can be very subtle or very dramatic and may include:

  • coughing
  • asthma-like attacks
  • periodic vomiting
  • lack of appetite or weight loss.

Occasionally an affected cat may have difficulty walking, experience fainting or seizures, or suffer from fluid accumulation in the abdomen. Unfortunately, the first sign in some cases is sudden collapse of the cat, or sudden death.

How do I know if my pet has heartworms?

DOGS: Getting a heartworm test administered by a veterinarian each year is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. If your pet tests positive, further tests may be ordered.

Annual testing is necessary, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working. Heartworm medications are highly effective, but dogs can still become infected.

If you miss just one dose of a monthly medication, or give it late, it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit the pill or rub off a topical medication. Heartworm preventives are highly effective, but not 100 percent effective.

CATS: Heartworm infection in cats is harder to detect than in dogs, because cats are much less likely than dogs to have adult heartworms. The preferred method for screening cats includes the use of both an antigen and an antibody test (the “antibody” test detects exposure to heartworm larvae). Because there is no approved treatment for heartworm infection in cats, prevention is critical.

What happens if my dog tests positive for heartworms?

  1. The goal is to first stabilize your dog if they are showing signs of disease.
  2. A round of doxycycline (antibiotic) will be started for 30 days to help weaken the heartworms.
  3. Depending on the case, your dog will either be given 2 or 3 injections of melarsomine dihydrochloride 30 days apart. This will kill off adult heartworms. It is imperative that they stay on prevention to not allow a reinfection during treatment.
  4. Exercise restriction is important during treatment. When the worms are dying off, they are slowly destroyed by the body. Increased activity can cause an embolus, or clump of worms, to form, blocking the circulation through the lungs. This can cause a stroke or even sudden death.
  5. Nine months after the last injection, your vet will test again for heartworms and microfilaria to ensure treatment was successful.

Take Home Point

Overall, PREVENTION is KEY! Today, there are many options against heartworms, including chewable tablets, liquid topicals, and even injections. The important thing is to pick what prevention is best for you and your furry friend and stick with it! Your vet and their staff can help you make that decision and answer any questions you may have as well.

For TONS more information about heartworms, please visit