The 5 Signs Your Dog Has Heartworms, and How to Help Them Heal

A young female veterinary nurse smiling at a dog she's caring for

Written by Kirstin Harrington

Published: May 5, 2022

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Heartworms, a parasite that can cause serious health problems and even fatalities in canines, are naturally found in dogs. Most dog owners are aware, however, that their companion should be on heartworm preventative medicine. 

If your dog has heartworm disease, medical intervention is critical since it increases Fido’s chances of survival dramatically. Even if your pooch is on heartworm prevention medicine, it is still vital to identify the symptoms of heartworm illness so that you can get them medical care as quickly as possible if your pet contracts heartworms.

What is Heartworm?

A heartworm is a parasite that lives in the heart and pulmonary artery of a dog or cat. The worms may reach a length of one foot and mimic thin spaghetti noodles. They cause damage to the heart and vascular tissue by devouring the tissue. Heartworms can clog the pulmonary artery, limiting blood flow to the heart and lungs and leading to serious injury. 

Heartworm disease in dogs is very dangerous and can be fatal. Canines with this disease may need to be admitted to the hospital and treated right away. For a few months, limited mobility and rest are suggested while the heartworm illness is treated and the dog recovers.

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Heartworms in Pets

Heartworm illness is common in dogs, cats, and ferrets, but heartworms can also be found in wolves, coyotes, foxes, sea lions, and, in rare cases, people. Wild animals such as foxes and coyotes are regarded as key disease carriers since they reside in close proximity to many metropolitan areas.


Heartworms that survive within the dog develop into adults, mate, and create children since the canine is a natural host for them. If left untreated, their populations can grow to the point that dogs can have hundreds of worms in their body. 

Heartworm illness causes long-term damage to the heart, lungs, and arteries, and can have a negative impact on a dog’s health and wellbeing even after the parasites have died. As a result, preventive measures are by far the best choice, and treatment should be started as soon as possible after the onset of the condition.


Heartworm illness in cats differs significantly from that in dogs. A cat is an unusual carrier for heartworms, with most worms in cats never reaching adulthood. Adult heartworms in cats usually only have one to three worms, and many cats with heartworm disease have no adult worms. 

While this means that heartworm disease in cats is frequently undiagnosed, it’s important to remember that even immature worms can cause serious problems in the form of Heartworm Associated Respiratory Disease (HARD). Furthermore, because the medicine used to treat heartworm illnesses in dogs is ineffective in cats, prevention is the only way to protect cats against the disease’s symptoms.

How Do Dogs Get Heartworm?

It seems that mosquito bites are the main way dogs get heartworm disease. Microfilariae develop into infective larvae inside the mosquito over the following 10 to 14 days, depending on the environmental circumstances. When an infected mosquito bites a dog, the infective larvae pass via the puncture wound to the dog. 

The infective larvae grow into adult heartworms in a freshly infected dog in roughly 6 to 7 months. The lifecycle is completed when adult heartworms mate and the females discharge their young into the dog’s circulation.

Heartworm illness is not communicable, which means that a dog cannot receive it from another dog who is affected. The only way to get heartworm disease is to be bitten by a mosquito. 

A heartworm may live for 5 to 7 years inside a pooch. Adult heartworms resemble cooked spaghetti strands, with males reaching 4 to 6 inches long and females reaching 10 to 12 inches. The worm load is the number of other worms residing within an infected dog. In canines, the average worm load is 15 worms, however, this amount can vary from one to 250.

Signs of Heartworm in Dogs

If your dog hasn’t been medicated for heartworm disease on a regular basis, early discovery is your best bet. Sadly, treating your dog for heartworm illness may be complicated, expensive, and unpleasant. Here are five indicators that your pup may be infected with heartworms.

  • Weight Loss: When heartworm illness spreads, dogs struggle to find enough energy to do even the most basic duties. Routine acts such as eating or having a snack may be overly strenuous, leading to fast weight loss.
  • Fatigue: It might be an indication of heartworm if your pet suddenly loses the desire in taking a walk or being active. Any form of physical exercise may become too taxing for your dog as the disease advances. 
  • Persistent Cough: A heartworm-related cough is dry and chronic, unlike a typical kennel cough, which is powerful and irregular. Coughing may be triggered by even minor levels of exertion in the early stages when the heartworm parasites find their way into the lungs, causing obstruction and irritation. 
  • Swollen Ribs: Your dog’s chest may appear to protrude as fluid continues to fill the lungs. As a result of the weight reduction, the ribs will seem to bulge. Fluid accumulation in reaction to the parasite’s presence can also cause this. Your dog will develop extremely evident heartworm signs after the heartworms have fully matured in your pet’s heart and lungs, which regrettably come with long-term health consequences.
  • Difficulty Breathing: In addition to coughing, your dog may experience breathing issues that resemble an asthma attack. Fluid can also accumulate around the lungs’ blood arteries, making it harder for the lungs to carry oxygen to the blood. 

When Should You Test Your Dog for Heartworms?

All dogs should visit the vet for a heartworm infection check once a year. This can typically be done at a regular appointment for preventive care. Testing and timing guidelines are as follows:

Puppies under the age of seven months can begin heartworm preventative measures without the need for a heartworm test, but they should be given a test six months after your initial visit, again six months later, and then yearly thereafter to ensure they are heartworm-free. 

Adult dogs over the age of seven months who have never been on a heartworm preventative should be administered a test before commencing treatment. They, too, must have a test for six months, twelve months, and then yearly after that.

Even if dogs are on heartworm prevention all year, annual testing is a requirement to determine that the regimen is effective. Even though heartworm medicines are quite successful, dogs can still become infected. If you skip one dosage of a monthly prescription or administer it late, your dog may become vulnerable.

Your pooch may spit up or vomit a heartworm tablet or scratch off a topical medicine even if you give it to him as directed. Heartworm prevention is extremely successful, although it isn’t perfect. You won’t know whether man’s best friend requires treatment unless you bring your dog to the vet for a test.

Treatment for Dogs With Heartworms

Nobody wants to learn that their furry friend has heartworm, but the good news is that the majority of dogs that test positive can get effective treatment. If your dog is displaying signs of sickness, the objective is to first stabilize him, then kill off all adult and immature worms while minimizing treatment negative effects.

Get a Diagnosis

If a dog’s antigen test results are positive, the diagnosis should be verified by a separate test. Because heartworm treatment is both costly and complicated, your veterinarian will want to be certain that treatment is required.

Restrict Movement

This condition may be tough to follow, particularly if your dog is used to being active. However, as soon as the diagnosis is established, your dog’s typical physical activity must be limited, as physical exertion speeds up the rate at which the heartworms inflict damage to the heart and lungs. The less activity your dog does, the better the symptoms are.

Stabilize Your Dog 

You may need to control your dog’s health with suitable therapy before true heartworm treatment can begin. The treatment can take many months in extreme cases of heartworm illness or when a dog has another significant ailment.

Provide Treatment 

When your veterinarian determines that your dog is stable and suitable for heartworm treatment, he or she will propose a multi-step treatment program. The American Heartworm Society provides instructions for creating a strategy. Treatment effectiveness is excellent in dogs with no or minor indications of heartworm illness, such as cough or activity intolerance. 

The risk of consequences is higher in effectively treating more severe illnesses. Heartworm disease severity does not usually correspond with the degree of symptoms, and dogs with numerous worms may have little or no early signs in the disease’s progression.

Final Testing 

Your veterinarian will conduct a heartworm test six months after treatment is finished to ensure that all heartworms have been eradicated. To ensure that your dog does not get heartworm disease again, you should give him heartworm medication year-round for the remainder of his life.

How to Prevent Heartworm in Dogs

Thankfully, there are things you can do in addition to heartworm medication to ensure Fido doesn’t get worms. Here is what we suggest to keep heartworms at bay. 

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Feed Your Dog High-Quality Food

What your dog eats is essential to every aspect of his health. If you choose to use store-bought dog food, look for one high in protein. Make sure it uses real ingredients and whole foods instead of fillers with little nutrients. 

Regular Vet Visits

Just like humans, dogs should have regular checkups. This should happen at least once a year, depending on your dog’s health. You may need to go more often for puppies or senior dogs. During this visit, the vet will perform a physical and look for signs of parasites like heartworm.

Continue Heartworm Medication

Vets provide pet owners with a preventative medication that will kill heartworm larvae before it becomes an issue. There are several types of medication available. You’ll get to choose from topical, injectable, or oral tablets.

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About the Author

Kirstin is a writer at A-Z Animals primarily covering animals, news topics, fun places, and helpful tips. Kirstin has been writing on a variety of topics for over five years. She has her real estate license, along with an associates degree in another field. A resident of Minnesota, Kirstin treats her two cats (Spook and Finlay) like the children they are. She never misses an opportunity to explore a thrift store with a coffee in hand, especially if it’s a cold autumn day!

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