Spread by mosquitoes, heartworm is a condition that affects many different animals, including bears, ferrets, foxes, cats, dogs, and even humans. Dogs are the natural hosts of the parasite. Once infected, heartworms wreak havoc on the cardiovascular system and lungs and can even prove fatal.

Let's dive in and find out what you need to know about this dog disease.


  1. Signs of Heartworm Infection in Dogs
  2. Are Heartworms in Dogs Contagious
  3. Preventing Heartworm in Dogs
  4. Heartworm Treatment in Dogs
  5. How Can the Emergency Fund Help with Treatment
  6. FAQ
  7. Conclusion

Signs of Heartworm Infection in Dogs

The signs and symptoms of heartworm in dogs can take months to show themselves — six months or longer. This is because of the four stages of heartworms.

Stage One: Infection

Your pup was bitten by an infected mosquito. Blood and other tests during this stage can come up negative even though the parasite can be present in the heart and cause damage.

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Stage Two: First Symptoms

This is the point at which dogs usually start to show symptoms. They usually dissipate with treatment, but some symptoms and side effects can carry on for a long time after the dog has been successfully treated.

Antigen tests will come back positive for heartworm infection in stage two of the infection when females are multiplying.

Common dog heartworm symptoms include:

  • No interest in food or water;
  • Cough that doesn't go away;
  • No interest in walks, playtime, or usual hobbies and items;
  • More tired than usual;
  • Weight loss;
  • Unusually grumpy or snappy behavior.

Not all dogs will have all the symptoms. Many won't show symptoms at all for a few weeks or months.

Stage Three: Very Serious Symptoms

When a pup has reached stage three of a heartworm infection, the parasites will have multiplied to such a high point (known as worm burden) that really serious symptoms start to show. These can include:

  • Coughing up blood;
  • Excessive tiredness;
  • Significant weight loss.

You'd see literal worms growing inside the organs if you could look through your pet's body, into the lungs and heart. Some vets describe them as "spaghetti-like", and they feast on and destroy the organs they hang from.

Female heartworms can grow to 12 inches (30 cm) in length. Males are shorter than their female counterparts, reaching up to 6 inches (15 cm). They can live for up to seven years inside your pet, causing serious organ damage without treatment.

Stage Four: Serious Organ Damage

This is the final and most advanced stage of heartworm in dogs. It is medically called "caval syndrome". Most dogs will be very seriously ill.

The average number of heartworms in a dog is around 15 to 20, according to the U.S. Food and Drug Administration study. There have been cases of worm burdens of 250 individual heartworms. The more worms there are, the more they eat, destroy, and breed.

Stage four is where the parasite becomes deadly in terms of numbers and effects. This can cause massive and catastrophic damage to the heart, lungs, and liver. The bodies of the parasite can disrupt blood flow, eventually causing complete blockages of arteries and veins.

There is good news, though: heartworm in dogs is 100% preventable.

Are Heartworms in Dogs Contagious

No, heartworms in dogs aren't contagious. One dog won't give it to another dog or another animal. This parasite is caused by bites from mosquitoes that carry heartworm larvae.

Heartworm larvae can't "activate" without a mosquito host.

Preventing Heartworm in Dogs

Prevention is always better than cure for heartworm in dogs. Even with successful treatment, the side effects of being infected with heartworm can last forever and can even reduce the life expectancy of your pets by a considerable amount.

The FDA recommends that dogs be administered heartworm prevention treatments at around seven months of age, following a negative test. Preventative methods do not kill active heartworm infections.

Preventative treatments for canine heartworm come in various forms, including injections administered one or two times per year, monthly topical treatments, and chewy tablets. These should be prescribed or administered by your vet.

Consumer products for deworming dogs usually only treat tapeworms or roundworms. These are intestinal worms, not heartworms.

Heartworm Treatment in Dogs


The unfortunate thing about heartworm in dogs is that symptoms take a long time to materialize. The damage that the parasite causes, however, starts immediately following the moment of infection.

Most owners do not spot heartworm symptoms until at least six months after infection when larvae have matured into adult heartworms and females start breeding. This can be stage two or three, and a significant amount of organ damage can already be present.

Later stages of heartworm infection aren't easily treated without causing more injury to the pet. Damage to the vital organs means that any kind of medical intervention could do much more harm than good.

Treatment of dog heartworm comes in stages. The larvae will need to be eliminated first. Any potential secondary bacterial infections are also eliminated with antibiotics. Finally, the treatment for adult heartworm will be administered, which is often in injection form. Some pups will need more than one course of adult heartworm treatment.

How Can the Emergency Fund Help with Treatment

There are no restrictions for pre-existing conditions when you sign up for Petcube's Emergency Fund Service. Many pet insurance policies won't cover a pet with pre-existing conditions, such as heartworm infections, which means that you might be out of pocket when it comes to paying for emergency care treatment.

With a 24/7 online vet service to take advantage of along with your $3,000 of emergency care per year, you can ask questions about your pet's health without having to make an appointment with a local vet.

It pays to have an on-call vet for those heart-stopping moments with heartworm infections.


Can you get dog heartworm medicine without a vet's prescription?

No, you can't. The treatment for this parasitic problem involves antibiotics, anti-parasitic drugs, and other strong medications that are only available on prescription. Products purchased online or in-store are either only for intestinal worms or they're not the appropriate course of treatment to deal with all aspects of the condition.

What is a dog's life expectancy after heartworm treatment?

The success rate of heartworm treatments very much depends on the stage at which they are administered. Stages three and four are considerably more difficult to treat than stage two. Modern treatments are up to 98% effective when given at the appropriate stage. Ongoing care, if necessary, can give your pup the same life expectancy and quality as a non-infected one.


Preventative treatments for heartworm in dogs can offer up to a year's worth of coverage. This is considerably cheaper, easier, and less heartbreaking than watching your pet experience the side effects of a parasite worm infection and subsequent treatments.

When a condition is this serious and potentially fatal, is it worth risking it?

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