Veterinarian with dog with heartworm illustration on dog's chest
vet with dog

What Is Heartworm Disease?

Heartworm disease is an invisible, but potentially fatal threat that occurs in dogs and other animals. Heartworms (Dirofilaria immitis) are parasitic worms that are transmitted to dogs by mosquitoes. These microscopic larvae develop under the skin, then migrate to the blood vessels of the heart and lungs of the infected animal where they rapidly grow, becoming adults that are 5-12 inches in length.1 


Heartworms are an invisible threat because they cause severe damage to the vessels of the dog’s lungs, typically long before any symptoms appear. While treatment is available, heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life, long after the parasites are gone. 

Number five in circle

5 Signs Your Dog May Have Heartworm Disease

Not all dogs develop noticeable symptoms. While blood tests performed by your veterinarian are the only way to confirm a diagnosis of heartworm disease in dogs, here are five warning signs that are reported in dogs with heartworm disease:

1. Mild Persistent Cough

A persistent, dry cough is a common sign seen in dogs with heartworm disease. The cough caused by heartworm disease can be one of the first signs you notice in an otherwise healthy-appearing dog.

2. Lethargy

Lethargy and reluctance to exercise are also common signs described in dogs with heartworm disease. If your pet loses interest in going for walks or is fatigued after activity, it may be a sign of heartworm disease.

3. Weight Loss

Some dogs have a decreased appetite and, as a result, lose weight.

4. Swollen Belly

As heartworm disease progresses, it can lead to heart failure. You may notice that your dog's belly appears swollen from fluid in the abdomen.

5. Difficulty Breathing

In the most advanced cases, dogs can develop more severe respiratory issues like rapid breathing in addition to coughing.

Heartworms in dogs
Heartworms in dogs

Keep in mind, these symptoms may be consistent with signs of other conditions, and a blood test performed by your vet is the only way to confirm a diagnosis of heartworm disease. If you have concerns, please contact your vet immediately.


If your vet determines your dog has heartworms, killing the adult heartworms is the next step. But the treatment can be long, difficult, and is not always 100% effective. That's why vets recommend keeping your dog on a heartworm disease preventive year-round and visiting your clinic yearly for regular testing.

Heartworm Disease Treatment For Dogs

If you suspect your dog has heartworm disease, don't wait - talk to your vet immediately. If heartworm disease is confirmed, your dog will undergo treatment at the veterinary clinic based on how advanced the disease is. Unfortunately, most treatments can be a long, painful, and expensive process.2

vet in office with puppy
vet in office with puppy

How Do Dogs Get Heartworm Disease?

Heartworms are transmitted from an infected animal to your healthy dog through mosquitoes. When an infected mosquito bites your dog, the mosquito can also pass on infective larvae. Over 6 to 7 months, these larvae develop into adult heartworms, causing severe health issues and potentially death.

Dog with red rope in its mouth
Dog with red rope in its mouth

How Does Heartworm Disease Prevention Work?

Heartworm disease prevention works by eliminating heartworm larvae before they grow into adults and migrate to the arteries of the lungs and heart. Before getting a prescription for a heartworm disease preventative, your dog must be tested for heartworms. Testing can be done by your local vet.

dog getting a treat
dog getting a treat

See why heartworm disease prevention is critical.

Get Prevention From Your Vet

HEARTGARD Plus is #1 vet-recommended for a reason.3 Once your dog has a negative heartworm test and prescription from your vet, you can purchase HEARTGARD Plus in clinic or through their online store. Use the search tool below to get started

Search to see if your vet carries HEARTGARD Plus

    Prevent Heartworm Disease Before It Starts

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