A vet inspects a dog for heartworms
A vet inspects a dog for heartworms

Signs of Heartworm Disease in Dogs

Heartworm disease is a serious and invisible threat to dogs. Unlike fleas and ticks, you can’t see heartworms on your dog. By the time heartworm symptoms become apparent, it’s often too late for the dog to make a full recovery. That’s why prevention is the only way to truly protect your dog against the dangers of heartworm disease. 

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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 advanced heartworm disease in dogs, here are 5 warning signs that are reported in dogs with advanced heartworm disease: 

1. Mild Persistent Cough

A persistent, dry cough is a common sign seen in dogs with advanced 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 common signs described in dogs with advanced 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.

A graphic depicting a heartworm infection
A graphic depicting a heartworm infection

In extreme cases, a dog may exhibit one of the most serious signs of heartworm disease: caval syndrome. This is caused by restricted blood flow to the heart. Dogs suffering from caval syndrome will have labored breathing, pale gums, and/or bloody urine. Unfortunately, dogs in this stage rarely survive without surgical intervention.1


Keep in mind that these symptoms may be consistent with signs of other conditions. 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.

What is Heartworm Disease?

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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 infected 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.2


Long-term effects of heartworms in dogs include 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, lung, and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. The best way to prevent heartworm disease is with a monthly heartworm disease preventive.  

A microscopic view of a single heartworm A microscopic view of a single heartworm
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An image of a microscope

How Do Vets Test for Heartworm Disease?

The only way to confirm whether your dog has heartworms is by visiting your vet. The most common way to diagnose heartworm disease is by performing an antigen test.2 This blood test detects specific heartworm proteins that are released by adult female heartworms into your dog’s bloodstream. A follow-up examination using a microscope could be done to detect microfilariae in a blood sample.

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A happy smiling dog

Heartworm Disease Treatment for Dogs

Treating heartworm disease in dogs is possible, but it is extremely costly and, depending on the severity of the disease, doesn’t always work.


If your dog is heartworm positive, the top priority will be to medically stabilize your dog. Next, your dog will undergo treatment based on how advanced the disease is. While your dog is under treatment, you’ll need to ensure your dog remains in a calm, relaxed environment with restricted exercise to minimize damage caused by the dying worms.

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A vet hands a client Heartgard Plus | HEARTGARD® Heartworm Protection

Continuing Your Dog’s Treatment

Once treatment is complete, your dog will have to be tested for heartworm disease again to evaluate the treatment’s effectiveness. You will have to continue giving your dog monthly, year-round preventive treatment to keep him healthy. 


With how easily heartworm disease spreads and how hard it can be to detect in dogs, prevention is the best course of action.

How does Heartworm Disease Prevention Work?

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

A vet pets a dog
A vet pets a dog

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.2

A mosquito on a dog's skin
A mosquito on a dog's skin

Are Heartworms Contagious?

Technically, heartworm disease is not contagious. You cannot contract heartworms by touching or standing near an infected dog.


However, since all it takes is one mosquito bite for infection to occur, heartworm disease can spread. If a mosquito bites an infected animal, it picks up heartworm microfilariae. Once they develop to infective larvae (10-14 days), the larvae can then be transmitted to other animals through future mosquito bites.1

A woman is about to throw a tennis ball for her dog
A woman is about to throw a tennis ball for her dog

See why heartworm disease prevention is critical.

Need a prescription? Find a vet near you using our Find A Vet tool.

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A woman gives her dog a Heartgard Plus chew | HEARTGARD® Heartworm Protection

Prevent Heartworm Disease Before It Starts

Just one bite from an infected mosquito can give your dog heartworm disease. Protect your dog with HEARTGARD Plus chews.


Save on Prevention
12 doses of Heartgard Plus | HEARTGARD® Heartworm Protection 12 doses of Heartgard Plus | HEARTGARD® Heartworm Protection
Get Your Rebate
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Buy 12 doses of HEARTGARD Plus chews and enjoy a $15 rebate.* Offer expires January 31, 2025.

Protect With Two
SAVE $60*
12 doses of Heartgard Plus and NexGard | HEARTGARD® Heartworm Protection HEARTGARD and NexGard offer
Save Big When You Buy Two
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Get a $60 rebate when you buy 12 doses of HEARTGARD Plus chews and 12 doses of NexGard® (afoxolaner) flea and tick protection. Offer expires January 31, 2025.

IMPORTANT SAFETY INFORMATION: NexGard® (afoxolaner) is for use in dogs only. Reported side effects include vomiting, itching, lethargy, diarrhea, and lack of appetite. Use with caution in dogs with a history of seizures or neurologic disorders. For full prescribing information, please click here.

*Available only on purchases from your veterinarian. One coupon required for every purchase. Terms and conditions apply. Must be of the same product and purchased at one time. May not be combined with any other offer. Rebate in the form of a Boehringer Ingelheim Mastercard® Prepaid Card. Please allow 6 to 8 weeks for delivery.