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Crucial Signs of Heartworms in Dogs Every Pet Parent Should Know


Heartworm disease in dogs is very serious, and potentially fatal. Unfortunately, it has been reported in all 50 US states; however, it is most prominent along the Atlantic and Gulf coasts, and along the Mississippi River. If allowed to progress, heartworms can cause lung disease, heart failure, and additional organ damage that may be irreversible, even with treatment. 

Here are the signs of heartworms in dogs plus the things every pet parent needs to know to keep their fur baby safe from this deadly parasite.

The Signs of Heartworms in Dogs Every Pet Parent Needs to Know About

Heartworm disease is progressive, so without treatment, the symptoms become more and more severe over time. According to experts at Bond Vet in Brooklyn, NY, “One of the many problems with heartworm disease is its ability to fly under the radar. In its earliest stages, your pet may not show any detectable signs of an infection.” This pre-symptomatic period is known as the Class 1 stage of the disease.

Class 2: Early-stage Symptoms of Heartworm Disease

Early-stage symptoms of heartworm disease become visible as the heartworms begin to make their way into the dog’s lungs, causing discomfort and blockage. A dog who has progressed to the Class 2 stage of heartworm disease will experience the following symptoms:

  • Dry, persistent cough: A heartworm related cough in dogs is dry and persistent. It is often brought on by even mild activity.
  • Lethargy and fatigue: Dogs with early-stage heartworm disease may display a lack of interest in activities they normally love as physical activity becomes too strenuous for them.
  • Weight loss: Weight loss occurs when the dog becomes too tired to perform even the simplest tasks, such as eating, which leads to weight loss. 

Class 3: Middle-stage Symptoms of Heartworm Disease

Middle-stage symptoms of heartworm disease become evident as the heartworms mature and begin to inhabit the dog’s lungs and arteries. This leads to more severe blockage and additional symptoms:

  • Difficulty breathing: Dogs who progress to this stage may experience coughing and shortness of breath similar to an asthma attack. They could also have fluid build up around the blood vessels in their lungs, which makes it difficult for the lungs to oxygenate the blood, causing rapid and shallow breathing.
  • Bulging chest and ribs: As the fluid builds up in the lungs, the dog’s chest may protrude, and the ribs may begin to bulge. The ribs will also become more visible due to more severe weight loss.
  • These signs of heartworms in dogs will continue to become more and more severe and visible as the disease progresses.
Heartworms in dogs can cause serious and life threatening harm

Class 4: Late-Stage Symptoms of Heartworm Disease

When a dog progresses into late-stage heart disease, his symptoms will become even more severe and they may be life-threatening. 

  • The lack of appetite, dry cough, and lethargy the dog has been experiencing will become even more heightened.
  • Severe complications will begin to occur, such as organ failure, abnormal lung sounds, enlarged liver, and a heart murmur.
  • Caval Syndrome- Caval Syndrome is characterized by collapsing or fainting due to blockage of blood flow to the right side of the heart. Not all dogs progress to this stage, but it is extremely serious, life-threatening, and requires immediate critical care.

Some dogs may also experience rare symptoms like nosebleeds, seizures, blindness, high blood pressure, or pneumonia.

How to Prevent Heartworms in Dogs

As you can see, the symptoms of heartworm disease in dogs are quite severe. Prevention is key because the disease can cause lasting damage to the heart, lungs, arteries, and other organs that can affect your dog’s quality of life even after successful treatment kills the parasites.

The best way to prevent heartworms in dogs is to provide year-round, FDA-approved, heartworm prevention medication, prescribed by your veterinarian. This medication kills the heartworm larvae before they have a chance to mature and reproduce.

Many of these heartworm prevention medications also protect against other worms and intestinal parasites that can be dangerous for your dog or even be transmitted to humans, such as tapeworms.

When to Have Your Dog Tested for Heartworms

Don’t wait until your dog begins to show signs of heartworms to have him tested. By the time a dog is displaying symptoms of heartworm disease, his condition is already serious. It’s also important to note that the test for heartworms only detects adult heartworms, not larvae, so your dog won’t test positive if he has just recently become infected.

If you have a puppy that’s less than six months old, he can start taking heartworm prevention without being tested since it takes at least six months for heartworms to show up in the test. However, he should be tested in six months to ensure he wasn’t already a carrier.

Dogs over six months should be tested before they start taking heartworm prevention. Then they should be tested again six months later. After that, they should be tested once each year. Your dog should also have a heartworm test anytime he misses a dose of his preventative, and then again in six months.

Annual testing is essential, even for dogs on year-round prevention. Although they are very effective, preventatives may not work 100% of the time, you may miss a dose or give it late, or your dog could vomit or spit out a pill without your knowledge.

Treatment of an Existing Heartworm Infection

The only way to confirm an existing heartworm infection is with a positive heartworm blood test. If your dog tests positive for heartworms, begin treatment immediately. Remember, early treatment dramatically improves your dog’s prognosis.

Thankfully, most dogs can be treated successfully. However, the treatment is expensive and comes with severe side effects for your dog.

Your vet’s immediate goal will be to stabilize any symptoms of heartworm disease that your dog is experiencing. Once he’s stable, the vet will use strong medications to kill the parasites completely while doing everything he can to minimize the side effects.

Dogs undergoing heartworm treatment will need lots of rest and may require several rounds of x-rays and lab work. If you notice weakness, red urine, pale gums, or vomiting at any time during your dog’s treatment for heartworms, seek emergency care immediately.

Read This Next: Why It’s Critical to Stop the Heartworm Life Cycle in Dogs

Frequently Asked Questions About Heartworms in Dogs

Here are answers to some of the most commonly asked questions pet parents have about heartworms in dogs.

  • What are heartworms?

Heartworms, or Dirofilaria immitis, are parasitic worms that live in the heart, lungs, and corresponding arteries of infected animals. These thin, threadlike worms can grow to a foot or more in length and can infect dogs, cats, ferrets, and several other mammals.

  • How do dogs get heartworms?

Heartworms are only transmitted by mosquitoes. When a mosquito bites an infected animal, it carries the heartworm larvae to other animals. When it bites another animal, that animal becomes a new host for the heartworms. 

According to the American Heartworm Society, it takes six months for the larvae to mature into adult heartworms in dogs. By the time they’re mature, the heartworms will have already migrated to the new host’s lungs and heart, where they can live for as long as five to seven years.

  • How do heartworms affect dogs?

Dogs are a “definitive host” for the heartworm parasite. That means the heartworms mature, mate, breed and live out their five to seven-year life cycle inside the dog if left untreated. They can grow to be a foot long or more and can multiply by the hundreds.

Heartworm disease is progressive. If there are large numbers of heartworms present, they can damage the dog’s heart, lungs, arteries, and other organs. When it comes to heartworms, prevention is the best defense.

  • Are heartworms contagious from one animal to another?

No, heartworms can only be transmitted by the bite of an infected mosquito. Your dog cannot get them from coming into contact with an infected animal.

The Bottom Line

When it comes to heartworms in dogs, prevention really is the best cure. If your dog isn’t already on prevention, have him tested immediately and talk to your vet right away about a prescription for year-round heartworm preventative. It’s much more affordable than treating an existing heartworm infection, and it could save your dog a lot of suffering… or possibly even save his life.

GUEST POST written by: Nicole McCray

Nicole is a die-hard animal lover, former vet technician, mom to her two rescue pups, and she grew up living and working at her family’s boarding facility. She loves using her writing talents to share her insight in the hopes that she can help other pet parents out there! Follow Nicole at:

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