Heartworm disease: Doesn’t sound pleasant, does it? Well, it’s not but, unfortunately, it’s a big risk for many dogs in the US, and as global weather conditions change, that risk is growing. Heartworms can be fatal if left untreated or if it goes undetected for a long time. Even with treatment, the risk of long-term damage can affect your dog’s health for the rest of her life.

heartworms killWhat causes heartworms?

Heartworm-carrying mosquitoes infect dogs, cats, ferrets and other animals. When a mosquito bites an infected animal, they pick up baby (microscopic) heartworms, called microfilaria. The baby worms reach an infective stage after 10 – 14 days and then when the mosquito bites a dog or cat, the larvae are transmitted to the skin and absorbed through the bite wound.

Heartworms aren’t transmitted through physical contact or via the air. Transmission is only possibly through mosquito bites.

Heartworm life-cycle

After heartworms enter a new host they continue to mature. They are more at home in dogs, and use the optimal environment to grow into mature adult heartworms (development takes about 6 months) with a lifespan of 5 – 7 years. During that time they can reproduce to the point where hundreds of heartworms live in dogs’ heart, lungs, and blood vessels.

Heartworms don’t thrive in cats and most don’t even reach adulthood, so they don’t tend to reproduce. The result is that cats usually only have 1 – 3 worms in their body. However, it only takes a few worms to do lasting damage to cats. The low numbers means that cats don’t usually show any symptoms of heartworms until the damage is severe.

How do heartworms affect pets?

Heartworms grow to about a foot long, which is pretty big. So it stands to reason that the smaller the dog or cat, the more damage heartworms cause.

Heartworms affect liver and heart functioning, as well as blood flow and tissue oxygenation. They can cause lung disease, heart failure and other organ damage.

In cats, immature heartworms can cause heartworm associated respiratory disease (HARD), which causes severe breathing problems.

Preventing heartworms in dogs and cats

Remember prevention is better than cure, as there are no FDA approved heartworm treatments for cats.

Heartgard plus

You have several choices when it comes to preventing heartworms, including spot-on topical medication, chewable tablets, pills and injections.  You need to give treatment monthly so it can kill immature larvae entering the body and kill the microscopic worms that result after mature adults reproduce in the host.

The important thing to remember about heartworm preventatives is that they need to be administered strictly on schedule. Even if you are only a day or two late with that month’s dose, you risk allowing immature worms to grow into adults capable of reproducing.

The earlier preventative treatment starts the better.  The American Heartworm Society recommends that you start puppies and kittens on heartworm preventatives from when they are at least 8 weeks old. It’s important to know that dosage depends on weight, so rapidly growing puppies will need different dosages from one month to the next.

One of the benefits of prevention treatments is that they can also protect your pets against other parasites, including roundworms, tapeworms, hookworms, ticks, fleas, and mites.

According to the American Heartworm Society, monthly prevention is essential no matter where in the country you live, as heartworm occurs in all 50 states in the US. Climate change allows heartworm-bearing mosquitoes to travel further and stronger storms carry the mosquitoes even further afield. Transported rescued pets across the States, increases ‘imported’ infections. The Heartworm Society has a ‘Think 12’ policy: Test pets every 12 months and administer preventative treatment 12 months of the year.

Preventative treatments include:

  • Heartgard Plus for Dogs
  • Heartgard for Cats (available as chewable tablets)
  • Tri-Heart Plus for Dogs
  • Iverhart Max for Dogs
  • Sentinel for Dogs
  • Revolution for Dogs & Cats
  • Advantage Multi for Dogs & Cats (topical solution)
  • Trifexis
  • Interceptor Flavor Tabs for Dogs & Cats
  • Paradyne

Symptoms of heartworms in dogs and cats

The signs of heartworms are more obvious in dogs than cats, and symptoms very seldom crop up in the early stages of the disease. It’s only during the later stages of the disease or when dogs or cats are particularly active (they put strain on their hearts and lungs) or old that obvious signs appear.

Heartworm_lifecycleSymptoms in dogs include:

  • Mild but persistent cough
  • Lethargy (not interested in exercise or games)
  • Easily fatigued
  • Nose bleeds
  • Loss of appetite
  • Weight loss
  • Swollen abdomen (fluid retention)

Signs of advanced stages of the disease include heart failure, blocked blood vessels (blood clots), aneurysms, pneumonia (due to lung inflammation), hypertension (high blood pressure) and caval syndrome (cardiovascular collapse). Sudden laboured breathing, pale gums, and dark urine are also signs.

Inflammation, due to high levels of immune system activity, can cause tissue damage and pain in joints and organs.

Symptoms in cats include:

  • Mild but persistent cough
  • Asthma-like attacks
  • Occasional vomiting
  • Loss of appetite
  • Weight loss

In advanced cases cats may not be able to walk well, they may have fainting spells or seizures, and fluid may accumulate in their bellies.

Testing for heartworm

Veterinarians can test for heartworm in different ways. One the easiest ways is simply to test blood for heartworm proteins or antigens. Other heartworm tests include x-rays and ultrasound and an antibody test.

Heartworm testing in dogs

While you can start your puppy on prevention treatment when they are 8 weeks old, you have to wait until your pup is 7 months old before the first test for heartworm can be done. This is because it takes 6 months after infection for a positive test to result. Puppies should be tested at 7 months and then 13 months and then annually thereafter.

If your puppy has never received any preventative treatment, or if you adopt an older dog – older than 7 months – then they need to be tested for heartworm before you can administer prevention medication. They should be tested again after six months and then go on an annual testing schedule.

Blood tests for heartworm proteins are most commonly used to detect the disease in dogs.

Heartworm testing in cats

Antigen and antibody tests are most commonly used to detect heartworm in cats, as they can pick up on the presence of larvae instead of adult heartworms. Veterinarians also use urine analysis, x-rays and ultrasounds. In some cases an electrocardiograph (ECG) may be necessary to detect worms in the heart or pulmonary artery. Cats use a similar testing schedule as dogs. Some veterinarians may devise a specific schedule for cats in their care based on the level of risk. It’s very important that the schedule is strictly adhered to as heartworm can’t be treated in cats. Prevention is your only bet.

Treating heartworms in dogs and cats


Once a dog or cat has tested positive for heartworm, most veterinarians will do a second test, using a different method, to confirm the diagnosis. A CBC (complete blood count) is also carried out to determine how the organs are coping with the disease and to determine the severity of the infection.

The first step is to ensure that pets are stable. This means managing any complications, treating any pain, and restoring appetite and energy levels. It can take weeks of stabilisation before heartworm treatment is administered in severe cases.

There are 2 aspects to treating heartworm: Killing adult worms and then killing the larvae.

Bear in mind that some symptoms can get worse during treatment, especially when eliminating adult worms. This is because the worms die and start to decompose while still in your pet’s body. Not only is the decomposition toxic but the immune system also kicks into high gear to get rid of the invaders. This can increase inflammation and temporarily increase pain and swelling.

Some of the prescription medications used to treat heartworm also have unpleasant side-effects; for example, adulticides are derived from arsenic and can affect the liver and kidneys if not administered properly.

Treatment-related inflammation and pain decreases after 5 – 10 days, and anti-inflammatory medication is often prescribed to help manage pain and discomfort.

Larvae elimination usually follows one month after the adults have been successfully killed and removed from the system.


It’s important to restrict your pet’s exercise while they’re receiving to prevent strain on lungs and heart, and ensure they eat a healthy diet.

In severe cases, for instance, where there are so many heartworms that they cause blockages in the heart, liver and blood vessels, surgery to remove heartworms may be necessary.

Testing and retesting is necessary to determine if treatment has been successful.


Remember that heartworm in cats isn’t curable, but it’s manageable, provided the disease isn’t so severe that her condition remains unstable. Managing the disease is complicated, especially if heartworms have migrated to your cat’s brain, eyes or spinal cord. Complications also occur if the heartworms die and cause blockages (heartworms have a shorter lifespan in cats than dogs).

If your cat tests positive for heartworm, she’ll need another test to confirm the diagnosis. If the diagnosis is confirmed, you’ll need to monitor her for the rest of her life. It’s important to treat symptoms as soon as they appear, to prevent a setback. So if your cat has tested positive for heartworm and starts to cough, you need to get treatment immediately so that it doesn’t snowball and the disease remains manageable.

Cats with severe cases are hospitalised, and may need emergency surgery to remove heartworms via the jugular. Other emergency treatment includes oxygen therapy, parenteral corticosteroid therapy, ventilation therapy, and dilation of the bronchioles to aid respiration.

It’s important to keep up with prevention treatment even if your cat has heartworm, so she isn’t reinfected.

Whether your dog or your cat has heartworm, treatment is expensive, painful and success isn’t guaranteed, so once again, we can’t overemphasise the importance of prevention.