7 Heartworm Symptoms You Need to Know

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Athena Gaffud, DVM , is a board-certified veterinarian and writer based in the Cagayan Valley of the northern Philippines. She runs the website countryvetmom.com. Dr. Gaffud earned her Doctor of Veterinary Medicine degree from the University of the Philippines Los Baños in 2011, ranking in the top 10 and receiving the Best Undergraduate Thesis Award in Large Animals. With over a decade of experience, she has worked as a researcher, a practitioner for small and large animals, and in veterinary technical sales, marketing, and pet insurance. A published author, Dr. Gaffud promotes responsible pet ownership and combats misinformation on animal care through her platforms, including the DocAthena Facebook Page and DocAthena YouTube channel. She is a writer and editor for various pet-related websites such as Total Vet, Honest Paws, PangoVet, Dogster, Catster, My Best PH, Paw Origins, Bully Max, Not a Bully, Paws and Claws CBD, and many others. She was also cited in different pet-related media articles such as The Dog People, USA Today, Newsweek, New York Post, Reader’s Digest, Smithsonian Magazine, Woman’s World, Dog Time, Patch, Kinship, Martha Stewart, and many others. Moreover, she is also a published fiction author on Kindle.
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Heartworm disease is a serious condition affecting dogs and cats, caused by the parasitic worm Dirofilaria immitis. Transmitted through mosquito bites, these parasites lead to severe health issues if not detected early. Heartworm symptoms can range from mild to life-threatening and may take months to appear, making early detection crucial.

Knowing whether a dog or cat has heartworms is essential because early detection improves treatment outcomes. This article aims to help pet owners identify heartworm symptoms in dogs and cats, recognize early heartworm symptoms, understand advanced heartworm symptoms, and answer the common question, “What does heartworm look like?” By learning to spot the warning signs, pet owners can act quickly and get their pup or kitten the necessary veterinary care before the disease becomes life-threatening.

How heartworms affect a pet’s body

Heartworms are long, thread-like parasites that infect pets’ hearts, lungs, and blood vessels and cause severe damage over time. When a mosquito carrying heartworm larvae bites a dog or cat, the worms grow inside the body, eventually clogging the heart and restricting blood flow.

In dogs, these parasites multiply and reach large numbers, leading to coughing, fatigue, weight loss, and even heart failure. Dogs may not show symptoms until the infection progresses. Heartworm symptoms in dogs often start with a mild cough and reduced energy, but without treatment, advanced heartworm symptoms can include difficulty breathing and a swollen belly from fluid buildup.

Conversely, cats typically have fewer adult worms, but even a minor infection can trigger severe lung inflammation. Because of this, heartworm symptoms in cats often mimic asthma, causing wheezing, vomiting, or sudden collapse. Unlike dogs, there is no approved heartworm treatment for cats, making prevention the only way to keep them safe.

Pet parents wondering how to tell if a dog has heartworms should watch for early warning signs, such as occasional coughing or reluctance to exercise. These parasites can cause life-threatening complications if left unchecked, so recognizing early heartworm symptoms is crucial.

If you’re asking, “What does heartworm look like?” these worms can grow up to a foot long, lurking inside a pet’s bloodstream and putting extreme stress on vital organs.

Understanding the signs of heartworm in dogs and knowing when to seek veterinary care can save a pup’s life.

7 heartworm symptoms you need to know

Recognizing the progression of heartworm disease in pets is vital for early intervention and effective treatment. Below is a breakdown of key symptoms, from initial signs to more advanced manifestations.

1. Mild cough

A persistent, dry cough is one of dogs’ first signs of heartworm. This occurs as the worms begin to inhabit the lungs and surrounding blood vessels, irritating them. Pet owners might mistake this for kennel cough or allergies, but a lingering cough warrants veterinary attention.

2. Fatigue after light exercise

A pup that used to enjoy walks and playtime may start showing early heartworm symptoms like tiring quickly after mild activity. This fatigue results from reduced oxygen circulation due to worms in the heart and lungs. Owners may notice their once-active pup becoming unusually sluggish or reluctant to play.

3. Weight loss & loss of appetite

A dog may start eating less and losing weight as heartworm disease progresses. This happens because the infection weakens the body and complicates normal activities. If a pup seems uninterested in food and slowly loses weight, it could be a warning sign of heartworm symptoms in dogs.

4. Breathing difficulties

When heartworms grow and multiply, they block blood flow to the lungs, making breathing harder for a dog. Dogs with advanced heartworm symptoms may pant heavily even when resting, take rapid, shallow breaths, or show signs of distress when lying down.

5. Swollen belly

In severe cases, dogs may develop a swollen abdomen due to fluid accumulation, known as ascites. This condition results from heart failure caused by the extensive presence of heartworms. A distended belly should prompt immediate veterinary consultation.

6. Collapse or fainting

Episodes of collapse or fainting can occur in dogs with heavy heartworm infestations. These incidents are often due to blockages in blood flow within the heart and lungs, leading to cardiovascular collapse. Such severe signs necessitate urgent medical attention. Understanding how to tell if a dog has heartworms can help pet owners seek emergency care before it’s too late.

7. Pale gums or dark-colored urine

Pale gums and dark-colored urine are indicators of caval syndrome, a life-threatening complication of heartworm disease. This syndrome arises when a large mass of worms obstructs blood flow, leading to cardiovascular collapse.

Immediate surgical intervention is often required to remove the blockage and save the dog’s life. Pet owners should know that these visible signs can indicate a severe infection while the worms hide inside the body.

Knowing the signs of heartworm in dogs and how to tell if a dog has heartworms can significantly improve your pet’s health.

While cats are less commonly affected, heartworm symptoms in cats can include coughing, asthma-like attacks, periodic vomiting, lack of appetite, or weight loss.

Grey and white striped cat lying upside down

Image by Orna from Pixabay

Suspect heartworm symptoms? When to see a veterinarian

Recognizing heartworm symptoms early is crucial for your dog’s health. Initial signs, such as a mild, persistent cough or fatigue after light exercise, should prompt a visit to the veterinarian for testing. These early indicators may be subtle but signify the onset of heartworm disease.

As the condition progresses, more severe symptoms like difficulty breathing, a swollen abdomen, or episodes of collapse may occur, necessitating immediate veterinary attention.

Without prompt treatment, heartworm disease can be fatal; however, early detection and intervention significantly improve the prognosis. If you observe any of these early heartworm symptoms or advanced heartworm symptoms, consult your veterinarian promptly.

How heartworm disease is diagnosed and treated

Diagnosis

Veterinarians diagnose heartworm disease with various tests.

Blood tests: Veterinarians use blood tests to detect heartworm proteins in dogs during antigen tests. In cats, antibody and antigen tests are employed to identify heartworm infections.

Microfilaria test: This test checks for microfilariae (immature heartworms) in the bloodstream, confirming an active infection in dogs.

Imaging: In advanced cases, X-rays or ultrasounds may be necessary to assess organ damage and visualize heartworms in the heart or lungs.

Treatment

Heartworm treatment depends on the severity of the case.

Medications: Approved drugs eliminate adult heartworms and microfilariae in the bloodstream. Melarsomine dihydrochloride kills adult heartworms, while products like Advantage Multi for Dogs target microfilariae.
Supportive care: Dogs with severe heartworm disease may require additional treatments, such as anti-inflammatories, antibiotics, and diuretics, to manage symptoms and improve heart function.
Exercise restriction: Limiting physical activity reduces the risk of complications during treatment.
Surgical intervention: In severe cases with heavy worm burdens, surgery may be necessary to remove heartworms causing blockages. Heartworm preventatives for cats require a veterinarian prescription.

German shepherd biting a stick beside a puppy

Image by Anja from Pixabay

Preventing heartworm disease

Preventing heartworm disease is far easier, safer, and more affordable than treating an infection. Monthly preventatives—available as oral chewable or topical treatments—kill heartworm larvae before they mature into adult worms, which can cause severe damage. Without protection, pets risk developing heartworm symptoms that worsen, leading to lung and heart problems.

Regular use of trusted preventatives like Heartgard Plus, Sentinel, and Advantage Multi helps ensure dogs and cats stay safe year-round. Annual vet checkups and heartworm testing are essential; even one missed dose can leave pets unprotected.

Here is a list of other heartworm preventatives:

For dogs

For cats

By consistently using these preventatives, pet owners effectively reduce the risk of heartworm infection in dogs and cats and avoid the severe health issues associated with heartworm symptoms.

Heartworm prevention: Final thoughts

Heartworm disease is a serious and potentially fatal condition caused by foot-long worms that live in affected pets’ hearts, lungs, and associated blood vessels. These parasites are transmitted through mosquito bites and can lead to severe lung disease, heart failure, and damage to other organs in the body.

Heartworm symptoms often start subtly but become life-threatening over time. Recognizing early signs like coughing or fatigue can save your pet’s life.

The best protection against heartworm disease is prevention. By staying proactive with heartworm preventatives and regular vet checkups, pet owners can help ensure their pups and kittens remain safe from this deadly parasite. If a dog shows any symptoms listed, prompt veterinary care is critical.