HomeResource CenterSymptoms Of Heartworm In Dogs

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10 Symptoms of Heartworm in Dogs + Treatment Tips

Content Marketing Manager

Jocelyn Stange, MA

Spending time outside with your dog is a great way to get some vitamin D and give them a healthy life. But, if you live in a warm, humid climate you'll likely encounter mosquitoes from time to time.

More than a pest that makes humans itch, mosquitoes can carry a dangerous and potentially fatal disease.

Dirofilaria immitis, also known as heartworm, is a parasitic worm that can affect cats, dogs, and humans. Transmitted by mosquitoes, heartworms can cause serious issues for your furry friends.

In this article, we'll explore:

  • Stages of heartworm disease
  • 10 symptoms of heartworm in dogs
  • Tips for achieving year-round prevention
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Stages of heartworm disease

Classes of Heartworm Symptoms in Dogs and Cats

According to the FDA, there are four classes or stages of heartworm disease. At each progressive stage, heartworms will continue to develop in your pet's heart, lungs, and associated blood vessels. As the heartworms grow, your pet’s symptoms worsen.

The progression and severity of heartworm disease in dogs depend on a couple of factors. This includes if you already have a healthy dog and the number of adult heartworms in your infected dog. As heartworms develop in your dog's body, a heartworm-associated respiratory disease can develop.

  • Class 1: In the early stages of heartworm, an infected dog may not show any noticeable signs except a mild cough.
  • Class 2: As heartworm disease progresses, coughing might become more prevalent. Your dog may lack the motivation to exercise.
  • Class 3: By this stage, heartworms have compromised your dog's heart and lungs. Coughing may increase and they may start to show more asthmatic signs.
  • Class 4: Caval syndrome is the final stage of heartworm disease. Adult heartworms multiply and create a blockage in your dog's heart which can lead to kidney, lung, or heart failure.

It's clear that the signs and symptoms from stage one to stage four vary. Make sure you're paying attention to the symptoms of heartworm disease in detail to avoid serious results.

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10 symptoms of heartworm in dogs

Symptoms of Heartworm Disease in Dogs

Heartworm disease starts with an infected mosquito that transmits parasitic worms to your dog. The most common symptoms of heartworms in dogs include fatigue, decreased appetite, persistent cough, difficulty breathing, and weight loss.

Heartworm disease symptoms may vary depending on the severity of the infection and the location of microfilaria (immature heartworms). These signs are pretty common in other infections, illnesses, and routine health issues. So, it's important to understand the details of each.

Here are the 10 most common heartworm symptoms in dogs:

1. Trouble breathing

When infective larvae mature into adult heartworms they will take up space in your dog's lungs. This makes it difficult for your pet to breathe and may bring on a dry cough. If your dog suffers from allergies, these symptoms could be asthmatic, so track them closely.

2. Coughing or wheezing

A dry cough is a very common symptom of heartworm disease. As the disease worsens, coughing can range from mild to severe. A light cough could also be a sign of allergies.

But, a mild, persistent cough could signal heartworm disease. If your pet has difficulty breathing or begins coughing up blood, contact your vet immediately.

3. Asthma symptoms

Irregular breathing and coughing are normal signs of heartworm in dogs. Dogs can also experience asthmatic symptoms such as:

  • Rapid heart rate or breathing
  • Excessive panting or drooling
  • Breathing with their mouth open

4. Loss of appetite

Due to heartworm disease, your dog may lose interest in their food, snacks, and even water. This is especially true if the worms have started to migrate through the lungs and into their throat. Dogs with severe heartworm disease may refuse food because it's too painful to swallow.

5. Weight loss

Low appetite and exercise can lead to unexpected anorexia. Rapid changes in your pet's weight are serious symptoms of which to take note. Inappetence beyond a few days is not normal for your pets and may signal a more severe health issue.

6. Distended chest

A bloated chest or swollen belly is a common symptom of heartworm infections. Large fluctuations in shape and size often lead to this symptom, so pay attention to any sudden changes in your dog's body and overall health.

7. Fatigue

With all that coughing and a low desire to eat, it's no surprise that lethargy and fatigue may follow. Fatigue isn't a concrete way to tell if your dog has heartworm disease. But, when paired with respiratory issues it's something worth exploring.

When diagnosing a health issue in your pet be sure to monitor symptoms and consult with your veterinarian for proper treatment.

8. Reduced activity

Another common signal of heartworm disease is a reduction in activity or motivation to exercise. Most dogs love walks and regular play unless you have older, adult dogs or ones with pre-existing conditions.

If your normally rambunctious pup is showing signs of fatigue, it's best to take precautions. But, before you rush out to get your dog (or cat) tested for heartworm disease, rule out other possible explanations first, like UTIs and ear infections.

9. Fainting

By stage 4 of heartworm disease, the blockage of adult heartworms can become so severe in your dog that they may faint. Blockages create a restriction of blood flow and shock to your pup's cardiovascular system. Collapsing and fainting are strong indicators of serious heartworm infection.

10. Fatality

Heartworm is an especially dangerous parasite because it can be fatal. For most dogs, heartworm disease can receive treatment to avoid serious complications. But, if you don't take immediate action heartworm disease can lead to heart failure, lung disease, and even death.

If you suspect that your pet has heartworms based on a combination of any one of these symptoms seek veterinary help immediately. Annual testing and preventative heartworm medication are your best strategies to avoid infection.

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Tips for achieving year-round prevention

Heartworm disease is a serious disease for pets. To give your pet the healthiest and safest life possible, follow these tips for preventing heartworms in dogs and cats.

Get educated about heartworm infection.

Learn About Heartworm

The initial development of heartworms takes about 6 months. But, from heartworm larvae to adult heartworms, this disease can progress quickly. That's why it's important for dog owners to know:

  • The signs of heartworm disease in dogs
  • How to get dogs tested for heartworms
  • When to contact your veterinarian
  • The best way to protect your pets from heartworm

Take your dog to regular vet check-ups.

Dog Check Up

If you are unsure of your dog's heartworm status consult with your veterinarian right away. During their annual check-up, your vet will administer a few blood tests, including a heartworm test.

Getting your dog tested for heartworm is the best way to avoid a heartworm infection. Your vet can also answer any questions you have about finding the right heartworm preventive for your dogs.

Protect your pet with monthly heartworm prevention.

Monthly Heartworm Treatment

Heartworm preventives are the best way for pet owners to ensure their pets don't get infected with heartworms. The American Heartworm Society recommends year-round heartworm prevention.

The most common treatment comes in the form of a chewable tablet that you give your pet once a month. Talk to your veterinarian about the best medication for your dog or cat.

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Frequently asked questions about heartworm disease

Frequently Asked Questions: Heartworms in Dogs and Cats

There are a lot of unknowns when it comes to the symptoms of heartworm, knowing how to test, and what medication your cats and dogs need.

Here are some frequently asked questions about heartworms:

How do dogs get heartworm disease?

The heartworm life cycle begins the moment an infected mosquito bites your dog:

  • The mosquito bites and passes infective larvae into your dog's bloodstream
  • The larvae mature into adult heartworms and begin to infect their body
  • Heartworm larvae mature into adult heartworms in about 6 months
  • Finally, adult heartworms mate with females and reproduce in your dog

Mature worms live inside your dog's heart and lungs in the bloodstream. As they develop and grow, they can cause a lot of damage. In extreme cases, this can lead to severe lung disease and heart failure.

What about another infected dog?

Heartworm disease is not contagious. The only way for the disease to make its way to your dog is from infected mosquitoes (not an infected animal). But, not all dogs take monthly heartworm prevention.

So, if a mosquito bites a heartworm-positive dog and then bites your dog, it could pass the heartworm parasite to your pet. This is also true if another infected animal is in your area when mosquitoes are present. Consider a topical treatment to help avoid a mosquito bite in addition to heartworm treatment.

Can cats get adult heartworms?

Cats, dogs, and humans are all at risk of heartworm infections. But, heartworms in cats don't:

  • Live as long as in dogs with heartworm disease
  • Grow as large as heartworms in dogs
  • Mature into adult worms as often

Because of their size, cats who have only a few worms in their body are heavily infected. Protecting all pets in your home, cats included, with a heartworm preventive is the best way to avoid disease from an infected mosquito.

How can I reduce the risk of heartworm disease in my dogs?

Avoiding the outdoors isn't realistic when giving your pet the best life possible. But, if you live in an area where mosquitoes are common, you're more susceptible to mosquito bites and heartworm associated with them.

Start by reducing your exposure as much as possible. Clear standing water where mosquitoes can breed. Avoid or limit outdoor exposure during peak mosquito feeding season and times. Most importantly, use a monthly preventative and take your dog (and cat) to the vet every year for testing.

Can dogs get tested for heartworms?

Yes, it's important to test your dogs for heartworm every year. Blood tests help your veterinarian determine if your dog has an active heartworm infection.

If the heartworm test is negative, your dog is clear to start heartworm prevention to stay protected. If your dog tests positive, they will need to undergo treatment to remove adult worms in the bloodstream.

What does the test look for?

Heartworm testing is available in two primary ways:

  • Microfilariae test: A microfilariae test checks your dog's bloodstream for heartworm microfilariae (infective larvae) in their body.
  • Antigen test: An antigen test is more straightforward and looks for specific heartworm proteins produced by adult female heartworms.

Heartworm tests look for evidence of worms in your dog's blood vessels. Both determine if your dogs test positive for heartworm in the bloodstream.

If I use heartworm prevention, does my dog need annual testing?

Unfortunately, heartworm prevention isn’t 100% effective in preventing heartworm disease in dogs. Difficult to see, an infected mosquito can easily bite your pet without you noticing. The earlier you detect heartworms in dogs (or cats) the sooner your veterinarian can start treatment.

But, because symptoms are pretty generic, they can be hard to detect before it's too late. Testing your dog each year should be a part of every dog’s check-up in addition to heartworm preventives. Tests can help you determine the right course of action for your dogs and cats.

Does my dog really need heartworm preventative?

Preventing heartworm in infected dogs and cats is much safer than treating it. As soon as immature worms (larvae) enter the bloodstream they can cause irreversible damage to your pup. Treatment for heartworm requires veterinary help which can be pricey and risky.

Starting heartworm prevention when your dog is at least 6 weeks old is the best way to prevent heartworm disease. No matter where you are in your heartworm journey, watch out for symptoms and visit your vet annually for testing.


Meet Jocelyn Stange

As a Content Marketing Manager, Jocelyn is committed to creating expert content to help pets stay healthy and keep their humans happy. She received her MA in Journalism and Mass Communications from the University of Nebraska-Lincoln. When she's not zeroed in on content strategy you can find her attempting to teach her goldendoodle Murray a new trick.