Heartworm disease is a severe, life-threatening condition that is fatal if not treated. Heartworm disease involves a live parasitic worm infestation inside a dog’s heart and pulmonary arteries, preventing those organs from functioning properly. The result is heart failure, severe lung disease, other organ damage, and death.

Contracting Heartworms

A dog gets heartworms when it is bitten by a mosquito that had previously bitten another animal with heartworms. Microscopic baby heartworms get transferred into the new dog’s body, where they then grow, mature into adults, mate, and produce offspring. Virtually 100% of unprotected dogs exposed to infective heartworm larvae through a mosquito bite will become infected!

What Heartworms Do

Image: New Rattitude, Inc. Rat Terrier Rescue & Adoption

Mature heartworms living in a dog’s heart are about the size of a piece of spaghetti. One dog may have as many as 250 worms, which survive up to five years. During this time the females produce millions of young. Adult worms cause disease by clogging the dog’s heart and major blood vessels, interfering with the heart’s valve action and reducing the blood supply to other organs of the body, particularly lungs, liver and kidneys, which leads to malfunction of these organs. The baby worms circulate throughout the body, blocking blood flow in the small blood vessels and depriving the body cells of nutrients and oxygen. Destruction of the dog’s lung tissue leads to a dry, chronic cough and shortness of breath. Liver problems cause anemia and general weakness and loss of stamina. Affected kidneys may allow poisons to accumulate in the body. The dog may show nervousness, listlessness, and exhaustion that is especially noticeable after exercise, when some dogs may even faint. Advanced cases progress to severe weight loss, coughing up blood, swelling of the abdomen and legs, and, finally, congestive heart failure.

This is a horrific disease that no dog should have to endure. It is even more horrible because it is easily and inexpensively preventable.

Heartworm Disease Prevention

The only reliable way to protect your dog from heartworm disease is to give heartworm preventative each and every month. Dogs of any age are susceptible to infection. The disease is more commonly spread in spring through fall and in climates where mosquitoes are plentiful, but areas of the country where the disease used to be very rare are seeing an increasing number of new cases each year, and there is no part of the USA nor any month of the year that is considered completely safe if regular monthly preventative is not given. Dogs in all 50 of the United States are at risk.

There is no “natural” or holistic treatment that will prevent your dog from developing heartworm disease if he is bitten by an infected mosquito. Nursing babies do not get immunity from their mother’s milk. You can NOT protect your dog from getting heartworms by vaccinations, mosquito repellent, only letting your dog outside a few minutes per day, keeping your house or yard clean, or not letting your dog drink from streams or gutters.

A single heartworm preventative dose once a month will kill any heartworm larvae in the bloodstream before they can mature and invade the heart and lungs. The form of heartworm preventative is either a small, easy-to-swallow pill, a tasty chew, or (if a combo product with flea preventative) a topical liquid to apply between the dog’s shoulder blades. The monthly cost of heartworm preventative, depending on brand, size of dog, quantity purchased, and purchase source, is typically $1.50 – $20.00. The most common brands of heartworm preventative are:

  • Ivermectin (in the products Heartgard, Tri-Heart, and Iverhart)
  • Milbemycin (in the products Trifexis (heartworm/flea combo), Interceptor and Sentinel)
  • Selamectin (in the product Revolution, a heartworm/flea combo)
  • Moxidectin (in the product Advantage, a heartworm/flea combo)


  • Early infestation: In the beginning stages of heartworm infestation, no signs are usually detected.
  • Mild infestation: Typically the earliest heartworm sign is a mild, dry cough.
  • Moderate infestation: As the disease progresses, additional signs include abnormal lung sounds, lack of energy, loss of appetite, weight loss, general discomfort, and fatigue.
  • Severe infestation: More severe symptoms appear, including enlarged liver, pneumonia, swelling around ribcage and/or abdomen, fluid accumulation in the stomach, jaundice, anemia, fainting after physical activity, loss of consciousness, dark bloody urine, and eventual death.

Progression of Heartworm Disease

It takes about 6 months for the heartworm larvae deposited by a mosquito’s bite to mature into adult worms, and they then continue to grow. Heartworms may accumulate gradually over years, or quickly when conditions allow exposure to high numbers of larvae-carrying mosquitoes. A dog can have heartworm infestation for years without showing any outward symptoms; however, as the worms continue to grow, the symptoms will worsen and become more obvious.

Testing & Diagnosis

Image: New Rattitude, Inc. Rat Terrier Rescue & Adoption

Heartworm infection is diagnosed through a simple blood test, and results are available immediately.
Since symptoms do not show up until heartworm infection is advanced, testing is the only way to know that adog is heartworm positive until the dog becomes seriously sick ‐ sometimes too sick for treatment. Even owners who believe that they gave the dogs preventative faithfully each and every month may not be aware that their dog spit out a tablet or vomited it, leaving the dog unprotected. Monthly heartworm preventative works against baby and juvenile heartworms but does not kill the adults, so if a dose or two of preventative are skipped, that can give the young worms a chance to mature into adults and start doing their damage. Resuming preventative after the skipped month(s) won’t keep the disease from progressing.

Adopters of new dogs must be particularly careful because heartworm infections are not detectable until about six months after a dog has been bitten by a heartworm-infected mosquito, making it possible for an infected dog to have tested negative. For this reason, it’s reasonable and recommended to run a heartworm test on an adopted dog with an unknown medical history six months after adoption. The same would apply if there is ever a lapse in heartworm preventative for more than two months. Thereafter, have the dog tested annually for the rest of his life. Puppies under 6 months old, however, are too young to test positive under any circumstances.


Fortunately, there IS a treatment for heartworm infection. While heartworm treatment is highly effective and most treated dogs do survive, the treatment is not something to take lightly. It is painful and dangerous for the dog, rough on the caretakers, and usually very expensive.

The main part of heartworm treatment consists of a regimen of three very painful injections of an arsenic-based product (melarsomine) into the muscle close to the dog’s spine in the lower back. The adult worms start to die immediately. As their bodies decompose over the next several weeks, pieces are “shed” into the dog’s bloodstream and filtered out through the dog’s lungs. This can cause the dog to cough and gag, or lead (like a blood clot) to a fatal pulmonary embolism or stroke.

The dog must be kept confined for a 4-6 week period following each injection to keep physical exertion to an absolute minimum in order to prevent a rapid heart rate and/or increased blood pressure from forcing pieces of the dead worms into the tiny blood vessels in his lungs and clogging them up. To enforce the activity restrictions, the dog must be kept crated or penned and only allowed out to potty briefly on a leash. 


Recovery is often rough. Pain from injection site often spreads throughout the dog’s lower back muscles and makes the dog feel nauseated. These symptoms will usually ease in a couple of days, but then others may appear. As the worms begin dying off, it is common for a dog to cough or gag. Symptom are generally at their peak at 7-15 days after the injections, and this is when the dog is in most critical danger of pulmonary embolism from the dead and decomposing worms. If the coughing/gagging is very heavy, seems uncontrollable, or causes the dog distress, or if the dog has vomiting or any bloody discharge combined with lethargy, fever and/or pale gums, this should be considered an emergency, and the dog should be taken to the veterinary or emergency clinic immediately. Corticosteroids, fluids, and oxygen may be needed at this time to help the dog survive.

Rescuing HW+ Dogs

Because heartworm treatment is expensive, painful, and burdensome, with no guarantee of success, many dog owners choose to abandon their dogs or have them “put down” when they get heartworm disease, and heartworm positive dogs in shelters have little or no realistic chance for adoption. New Rattitude believes that dogs with heartworms should not have to face a death sentence, and should be treated to give them another chance for life. But it can cost $200- $1000 to treat one heartworm positive dog, depending on its size, age, stage of disease, health, and area of the country.

Can you contribute to New Rattitude’s Heartworm Fund to help save a precious life? We appreciate donations of any amount! But perhaps you’d like to contribute in honor of a dog you have loved, or a friend/family member who has always had a heart for dogs. With your tax-deductible contribution of $50 or more, we will add your Honoree’s name with our clinic icon to our Donor Recognition Board with our undying “gRATtitude.”

Image: PayPal link to support New Rattitude's Rat Terrier rescue efforts through a donation

If you prefer, you can also mail a check or money order to:
      New Rattitude, Inc.
      P.O. Box 91
      DeWitt, MI 48820 
Please be sure to write “Heartworm Heroes” in the note field of your check.

For More Information about heartworms, go to the American Heartworm Society website.

Image: Courtesy of New Rattitude, Inc. Rat Terrier Rescue