How Do Dogs Get Heartworms? 15 Things to Look Out For

How Do Dogs Get Heartworms? 15 Things to Look Out For

A mosquito bite is annoying for a human. For a dog, it can be downright deadly.

That's because mosquitos can carry a parasitic worm called Dirofilaria immitis. Burying easily into your pup's heart, lungs and blood vessels, this worm can continue to mature, mate and produce for up to seven years. The result? An infestation of heartworms.

While it sounds disgusting, the condition is far worse than the initial impact. As the worms continue to spread throughout your dog's body, they can cause myriad forms of physical damage ranging from severe heart disease to heart failure.

How do dogs get heartworms? What causes them and how can you prevent them?

Today, we're sharing the answers behind those questions, so read on!

A Closer Look at Heartworms

Heartworm prevention is on the minds of most pet owners, and for good reason. 

The worms themselves live inside the mosquito for a short period of time, which they use to perfect their infective nature. This makes the mosquito the intermediate host. Because those worms continue to develop while inside your dog, Fido is thereby considered the definitive host.

While there have been recorded cases of dogs with heartworms in all 50 states, it's the most common along the Atlantic coast. You'll also find plentiful cases along the Gulf coast, traveling from the Gulf of Mexico to New Jersey.

The Heartworm Lifecycle

Once a mosquito bites your dog and he becomes infected, the heartworms go straight to work. The adult female heartworm will release her offspring, known as microfilariae, into your dog's bloodstream. 

When a mosquito bites your newly infected dog, those microfilariae take up residence inside of the mosquito. After around 10 to 14 days, they will become infective larvae. Then, when the infected mosquito bites another dog, those larvae enter the dog through the bite wound.

In all, it takes around six to seven months for the infective larvae to grow into adult heartworms. Once developed, those adult heartworms mate with one another, producing even more microfilariae and completing the lifecycle. The FDA provides an illustrated guide to this cycle here.

How Do Dogs Get Heartworms? The Four Classes of Infestation

It's important to keep in mind that the only way a dog can contract heartworms is through the bite of an infected mosquito. The disease is not contagious, which means your dog can't catch it if he's simply around a dog who is suffering from the condition. 

Want to protect your best friend as much as possible? Knowledge is key! If you know the signs and symptoms of heartworms in dogs, you can take quick action that could potentially save his life. 

In most cases, the severity of your dog's condition is directly related to how many heartworms he has living inside of him. This is known as his "worm burden".

In addition, other factors include the duration of the infestation and your dog's physical reaction to it. You're also more likely to notice more adverse reactions in dogs who are normally active, as heartworms can impede on their respiratory function and make it more difficult for them to play.

As such, if your dog has a low worm burden, hasn't been infected long, or isn't normally active, symptoms could take a while to show up, while the opposite also holds true. Once confirming a diagnosis of heartworms, your veterinarian can also reveal which class, or stage, his condition is in.

These range from Class 1 (mildest symptoms) to Class 4 (most severe symptoms). Let's briefly review each class.

Class 1: No Symptoms

Dogs in Class 1 might not show any signs of a heartworm infestation at all. Or, they might have mild symptoms that are virtually undetectable.

Class 2: Mild to Moderate Symptoms

Dogs in Class 2 do show some kind of symptoms, but they are not severe in nature. This might include a mild cough or fatigue after moderate activity.

Class 3: More Severe Symptoms

Dogs with Class 3 heartworm disease will normally undergo a chest x-ray to determine the extent of internal damage. Symptoms can affect their internal organs as well as their outward appearance.

Class 4: Most Severe Symptoms 

Class 4 heartworm disease is also called Caval Syndrome. When dogs reach this stage, they have such a massive worm burden that there's a physical blockage of worms that's preventing blood from flowing back to their heart. This syndrome is life-threatening and the only way to treat it is to quickly remove the heartworms via high-risk surgery.

Is Your Dog Infected? Watch For These Signs

Keeping these four classes in mind, let's take a look at 15 symptoms that could point to a heartworm infestation in your dog.

Occasional Cough

While your pup might have food allergies or sneeze around dust, an occasional cough that persists even in clear conditions might be a red flag. In Class 1 infestations, it's often the first and only sign you'll notice. The cough will get more persistent as more worms crowd your dog's heart and lungs. 


Does your pup seem especially tired after moderate activity? Especially when this fatigue is combined with a cough, you could be dealing with a Class 2 infestation.


Another common allergic reaction, nosebleeds can occur in dogs with heartworms. Check your pup's nostrils for signs of any that you might have missed.

Reluctance to Exercise

In keeping with extreme tiredness, you might notice that your once-active dog now prefers to lie indoors rather than play fetch. If yours seems unwilling to exercise or even go for a walk, it's worth checking into.

Sickly Appearance

As pet parents, we just "know" when our furbabies don't look right. If your pup is looking sickly, heartworms might be to blame. If this appearance change is coupled with tiredness and a cough, you're likely in a Class 3 heartworm infestation.

Decreased Appetite

It's common for dogs with heartworms to refuse their daily kibble. If yours is suffering from the condition, quick medical attention can help sustain him until his appetite levels regulate again.

Weight Loss

You can't gain any weight if you don't eat! A dog with heartworms will often appear skinnier than normal, due to the fact that the parasites are overtaking them in a way that makes simple digestion a task.

Difficulty Breathing

Trouble breathing is another sign of a Class 3 infestation. You might also notice other signs of congestive heart failure, such as pacing before bedtime and an elevated respiratory rate.

Swollen Belly

As the disease progresses, many dogs will get a swollen belly. This is due to the fact that the heartworms cause excess fluid to build up in his abdomen.

Protruding Chest

You might also notice that your dog's chest protrudes or has a bulging appearance. While the worms themselves can create the problem, it's exacerbated by his extreme weight loss.  

Pale Gums

One of the trademark symptoms of Caval Syndrome, pale gums are an indicator of cardiovascular collapse.

Worried you'll miss this sign? Don't be. If your dog is suffering from Caval Syndrome, there will be other, more obvious signs that he's in distress.

Abnormal Lung Sounds

Does your dog make strange sounds when he's breathing? Abnormal lung sounds can be another sigh of heartworms, though they are also linked to other upper respiratory conditions. 

Bloody, Dark Brown Urine

We all know that clear urine is an indicator of good health and strong kidney function. That's why dark brown urine or urine with blood in it should be taken seriously. Call your veterinarian if you notice this symptom with your dog, as it's another telltale sign of Caval Syndrome. 


As worm burdens increase and blood flow is further prevented, this loss of blood to the brain can cause your dog to collapse and lose consciousness. Again, this symptom is most closely related to Caval Syndrome. 

Official Tests

Rather than monitor your dog at home, it's always best to bring him in for a visit at the veterinarian's office.

With a simple blood test, the veterinarian can screen your dog for heartworms, though it's best for your dog to undergo a second test to confirm the results. Veterinarians will perform these blood tests on both dogs currently suffering from heartworms, as well as those on preventative heartworm medication.

Other tests commonly administered to dogs with heartworm disease include:

  • Complete blood cell count 
  • Blood chemistry panel
  • Chest x-rays
  • Urinalysis

Seizures and Blindness

Sometimes, the heartworm parasites get lost and accumulate in other places besides your pup's heart and lungs. If they wind up in his brain, they can lead to seizures. In the same vein, those that wind up in his eyes can lead to blindness.

Healthier Dogs, From the Inside Out

You'd do anything if it meant securing the health and wellbeing of your beloved pal. Now that you know the answer to the question, "How do dogs get heartworms?", you're ready to take the next steps toward preventing the disease.

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Thank you for stopping by! We're Wild Earth, a dog nutrition company that makes dog food and treats that contain zero animal ingredients. We make a high protein, high fiber dog food that has everything your dog needs and nothing they don't. Dogs love the taste and you can rest easy knowing you are giving the healthiest, cleanest food on the market. We also have treats that come in peanut butter, banana & cinnamon, and strawberry & beet.