Signs Of Heartworm In Dogs And How To Spot Them

what are signs of heartworm in dogs?

What Are The Signs Of Heartworm In Dogs, And How To Spot Them.

If you are anything like me, you may have forgotten to about heartworm medicine for dogs once or twice.

Now, the little red heart stickers on my calendar remind me each month to give Tilly her pill.

Though many of us have done it, this lapse could prove dangerous to our pets.

Before we discuss heartworm medication – both as a preventative and treatment after infection – let’s begin with an explanation of the disease itself.

Heartworms in Dogs

How do dogs get heartworm? Or what causes heartworm in dogs?

Heartworm, or dirofilaria immitis, is a parasite transmitted by mosquitoes.

The mosquitoes pick up the larvae, or microfilaria, from the bloodstream of infected animals.

It is difficult to imagine a worm, immature though it is, entering a dog’s bloodstream through the pinprick of a mosquito bite.

However, the microfilaria are only 1/100th of an inch long.

By the time they travel through the dog’s bloodstream and settle in the pulmonary arteries or on the right side of the heart, they can grow up to a foot in length.

The heartworms mature after 6 months and can live in a dog’s body for 7 years, constantly producing offspring. After a year a dog can shelter hundreds of worms, though the average is fifteen.

Over 70 species of mosquitoes carry and spread heartworms, and transmission can occur in cool as well as warm months in large indoor spaces such as kennels.

Dogs of all ages and breeds are at risk.

More than a million pets in the United States are infected each year.

Signs of Heartworm in Dogs

Heartworm symptoms in dogs are more obvious the further the disease progresses.

This disease progresses in four stages or classes.

Dogs who are in class one are either asymptomatic or have a mild cough.

In class two, the cough becomes stronger and more consistent. Dogs also may show an intolerance to exercise.

By class three, the signs of heartworm in dogs become exaggerated. Exercise intolerance becomes greater.

The dog may also exhibit abnormal lung sounds, a weak pulse, fainting, a decreased appetite, weight loss and a swollen belly.

Class four is sometimes referred to as caval syndrome. Here the signs of heartworm in dogs are worst of all.

It includes labored breathing, pale gums, and dark-colored urine.

These symptoms can lead to organ failure and death.

How Long Can a Dog Live With Heartworms

Heartworm disease starts 3 months after larvae are deposited in the dog’s blood. At 70-90 days, young adult worms are present in smaller pulmonary arteries.

The heartworms mature and begin reproducing a mere six months after the mosquito bite.

The disease can affect a dog’s vital organs, including the heart, lungs, kidneys and liver.

However, because it is such a complex disease, the outcome varies from dog to dog.

Some dogs develop severe cardiovascular issues while others show no signs of heartworms.

Treatment has a high success rate for dogs that are diagnosed early enough.

Heartworm Prevention for Dogs

Rather than watching for signs of heartworm in dogs to appear, veterinarians advocate prevention.

The greatest prevention comes in the form of an oral or topical medicine given to dogs once per month. This medicine repels mosquitoes.

The American Heartworm Society recommends starting puppies on preventative medication no later than eight weeks of age and giving the medication year round regardless of location. (In the U.S., heartworm has been diagnosed in dogs in all 50 states.)

Keeping mosquitoes away from your home and yard is a second line of defense.

One way to do this is by eliminating any source of free-standing water such as flower pots, buckets and children’s toys.

Blocking any points of entry into your home such as a broken screen is another precautionary action.

Do you see signs of heartworm in dogs in your own pet?

Heartworm Test for Dogs

The American Heartworm Society recommends testing for heartworm in dogs every year.

Why does your pet need to be tested if on heartworm medication for dogs?

For those of us who slip and forget to give our dogs a pill, missing just one dose can leave a dog unprotected.

Some dogs can also be resistant to the medicine; the medicine could be rejected through spitting, vomiting or rubbing off the ointment.

Heartworms are detected through blood tests.

These tests should begin as early as seven months of age, as the tests will make a positive detection six months after transmission.

Heartworm Positive Dogs

In order to confirm a blood test and establish the stage of the disease, several evaluation tools are available.

Your veterinarian may recommend an echocardiography to determine if the right ventricle of the heart is enlarged.

Another test that can be done is a thoracic x-ray to ascertain if artery enlargement has occurred.

What To Do If My Dog Has Heartworms

What about how to treat heartworms in dogs?

After confirming the diagnosis, your veterinarian should follow a protocol which includes the administration of an immiticide.

At this time, melarsomine is the only heartworm medication approved by the US Food and Drug Administration. The drug is given through an injection in the lumbar region while the dog is in an animal hospital.

The protocol also includes administering a heartworm preventative medication two months prior to the melarsomine treatment.

During this time the dog should be on cage-rest with restricted exercise.

Heartworm Treatment for Dogs

In a disease that is not past class three, the immiticide should kill the adult worms.

This necessary treatment has risks of its own. Adult worms lodge in the pulmonary arteries and can cause an obstruction of the blood flow.

The dead and dying worms can fragment and be swept through the arteries, potentially causing a thromboembolism, or blood clot.

Because treatment itself is risky, prevention against heartworm becomes that much more essential.

In extreme cases, surgery is sometimes recommended.

During this surgery, the heartworms are removed through the use of a surgical tool such as forceps.

Tests that have been done on dogs with heartworms before and after surgery demonstrate a decrease of the burden on the heart.

During treatment your veterinarian may want to assess your dog through the evaluation of biomarkers.

Biomarkers are substances that are released in the blood when a heart is damaged or stressed.

Can Humans Get Heartworm From Dogs

Do you seem to see similar signs of heartworm in dogs occurring in a human?

Though much more common in canines, humans can contract heartworm disease. The microfilaria are transmitted via mosquito from the bloodstream of an infected animal. The effects on humans are not as severe as canines.

Human cases are most often reported in areas of high canine heartworm prevalence.

A blood test is not available for humans so in order to diagnose heartworm disease, humans must undergo invasive procedures to distinguish heartworm from other more serious conditions.

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Where Are Heartworms Most Commonly Found

Heartworms are most prevalent in areas with a high mosquito population. Four countries with the highest rate of heartworm disease include the United States, Japan, Italy and Australia.

Heartworm societies have been developed in the US and Japan in order to foster awareness and stimulate research. They also provide guidelines for treatment and prevention.

With environmental changes some areas have seen an increase in infection potential as the transmission season has also increased.

Best Heartworm Prevention for Dogs

Using precautions before signs of heartworm in dogs appear is the best way to avoid this serious canine disease. Routine vet visits are essential. Be sure that heartworm testing is included in these visits.

Being consistent with administering your dog’s heartworm medication each month is the most effective and least expensive way to keep this nasty parasite at bay.

As the old adage goes: an ounce of prevention is worth a pound of cure.

Signs Of Heartworm In Dogs And How To Spot Them


  • The American Heartworm Society
  • Carreton, E. et al “Utility of Cardiac Biomarkers During Adulticide Treatment of Heartworm Disease (Dirofilaria immitis) in Dogs” Veterinary Parasitology, 2013
  • Carreton, E. et al “Variation of d-dimer Values as Assessment of Pulmonary Thromboembolism During Adulticide Treatment of Heartworm Disease in Dogs” Veterinary Parasitology, 2013
  • Ishihara, K. “Changes in Cardiopulmonary Values after Heartworm Removal from Pulmonary Artery using Flexible Alligator Forceps” The Japanese Journal of Veterinary Science, 1988
  • Lee, A. et al “Public Health Issues Concerning the Widespread Distribution of Canine Heartworm Disease” Trends in Parasitology, 2010
  • Losonsky, J. “Thoracic Radiographic Abnormalities In 200 Dogs With Spontaneous Heartworm Infestation” Veterinary Radiology And Ultrasound, 1983
  • Nelson, C. “Heartworm in Dogs” Infectious Disease Management in Animal Shelters, 2009.
  • Rawlings, C. “Effect of Monthly Heartworm Preventatives on Dogs With Young Heartworm Infections” The Journal of American Animal Hospital Association, 2002