7 Dog Heartworm Symptoms You Shouldn’t Ignore

Heartworm in dogs is a serious and potentially fatal parasite-borne disease that can affect dogs of all breeds and ages, as well as other species including coyotes, foxes, cats, and even humans. Yet the condition often goes undiagnosed because some dog heartworm symptoms can be easy to miss.

Despite the major health risks posed by this disease and the rising number of cases throughout the US, a recent study found that only about 30 percent of US dog owners had purchased heartworm prevention within the last year (1). 

Keeping your dog on a monthly heartworm preventative medication such as Revolution, or a combination heartworm, flea, and parasite preventative such as Simparica TRIO, is one of the best ways to keep your dog safe. But what happens if you miss a dose or if your dog is unprotected?

While most dog parents are somewhat familiar with the risk of heartworms they are often unfamiliar with dog heartworm symptoms. In this article, we will describe common dog heartworm symptoms that all dog parents should know. 

How Does a Dog Get Heartworm? 

Heartworm disease is caused by a parasitic worm, Dirofilaria immitis, which is transmitted through mosquito bites. When a mosquito takes a blood meal from a dog that is infected with heartworms, the juvenile heartworms in the blood (called microfilariae) develop into infective larvae inside the mosquito. These can then be transmitted to the next dog the mosquito bites, causing them to become infected. 

Any dog can become infected with heartworms. However, dogs with a higher risk of mosquito exposure — such as outdoor dogs, stray dogs, hunting dogs, and dogs living in the southeastern United States — are more likely to develop heartworm disease. 

Once infected, these worms can cause significant damage to the dog’s heart, lungs, blood vessels, and even the liver and kidneys. 

7 Dog Heartworm Symptoms You Shouldn’t Ignore

Often, dogs that are diagnosed with heartworms don’t show any abnormal symptoms at the time of their diagnosis. In fact, one study reported that 32 percent of dogs diagnosed with heartworms were asymptomatic (2). This is likely due to routine testing for heartworms, which allows veterinarians to catch this disease early on. 

The longer a dog is infected with heartworms, the more the worms reproduce, and the more likely the dog is to show symptoms of heartworm infection. 

The tricky thing is that many of these symptoms aren’t specific to heartworm in dogs. They are also commonly observed in many other dog diseases and ailments. For this reason, an assessment by a veterinarian is crucial to help determine the cause. 

Still, in order to get that assessment, pet parents need to know what to look for. So here’s a helpful overview of heartworm symptoms in dogs to watch out for.  

What are the First Signs of Heartworms in Dogs?  

1. Coughing 

After migrating through the skin and muscles, juvenile heartworms enter the chest cavity and lungs. They fully mature within the pulmonary arteries, where they will live out their life cycle, which can span 5 to 7 years. 

Since heartworms are not a normal part of a dog’s body, the immune system reacts to the worms causing widespread inflammation. In the lungs, this inflammation causes a pesky dry cough that won’t go away. For this reason, any time a dog has a persistent cough, a visit to the veterinarian is in order for heartworm testing and evaluation.

2. Exercise Intolerance

Another early sign of heartworm disease is difficulty exercising or a sudden reluctance to exercise. 

This can happen for many reasons. But when a dog has heartworms, adult worms living in the blood vessels of the lungs cause those vessels to narrow and not work as well as they should. That can prevent the lungs from exchanging oxygen efficiently. And because moving the body requires oxygen-rich blood, exercise may be more difficult for dogs with heartworms

If you normally take your dog on an hour-long daily walk, and suddenly your pup can’t walk for 20 minutes without wanting to lay down, it’s time for a visit to the vet. 

3. Lethargy

As with infections of any kind, heartworms can make dogs more sleepy and lethargic because their body is busy trying to fight the infection. The direct impact heartworms have on a dog’s heart and lung performance can also contribute to lethargy and low energy. 

Symptoms of Late-Stage Heartworm Disease in Dogs

4. Weight Loss

Advanced heartworm infections can cause dogs to lose weight. As heartworms take up residence in a dog’s pulmonary arteries for longer periods of time, this forces their body to have to work harder to perform basic bodily functions, such as breathing and pumping blood.

This increased exertion requires more calories to sustain it. However, dogs with heartworms typically don’t make up for this need by eating more food. Instead, the inflammation caused by the worms makes infected dogs feel less hungry, which contributes to the weight loss. 

5. Labored Breathing

Another late-stage sign of heartworm disease is labored breathing, which is often a symptom of right-sided heart failure. 

Over time, the right side of the heart has to pump harder and harder to push blood through the narrowed, heartworm-damaged pulmonary arteries. Eventually, this can lead to heart failure and often labored or rapid breathing. Labored breathing should always be treated as an emergency and you should not delay seeking veterinary care, should this occur. 

6. Swollen Belly

The right-sided heart failure seen in late-stage heartworm disease can also lead to a buildup of fluid in a dog’s belly. This condition, known as ascites, happens when blood pools in the lungs because the heart isn’t pumping properly, and fluid leaches from the pooled blood into the abdomen. This causes a dog’s belly to appear more rounded or swollen. 

7. Collapse

Sudden collapse or fainting episodes, called syncope, can be seen in dogs with advanced heartworm disease. This can be a result of heart failure or caval syndrome. 

Caval syndrome is a severe complication of heartworm disease in which heartworms become lodged in the right side of the heart. The worms become tangled and prevent the heart chambers from properly closing. This clump of worms also causes red blood cells to burst, leading to severe anemia and weakness. 

Caval syndrome requires emergency surgery in order to prevent death and even then the chances of survival aren’t great. For this reason, should your dog suddenly collapse, it is important to seek emergency veterinary care right away. 

Heartworm Symptoms in Dogs: Next Steps

If your dog is not on year-round heartworm prevention medicine, such as Revolution or Simparica TRIO, and you notice any of the symptoms described above, it is important to consult with your veterinarian about the possibility of heartworm disease. 

A veterinarian can usually perform a quick heartworm test during your visit to provide an answer right away. And even if your dog tests negative for heartworms, your veterinarian may be able to diagnose another health concern with similar symptoms before it becomes more serious.

However, ignoring symptoms of heartworm disease or skipping annual heartworm tests could prove deadly, because it reduces your chances of catching the disease when it is still treatable. 

Heartworm treatments for dogs are most effective when the infection is detected before symptoms develop. A dog with heartworm symptoms can still benefit greatly from treatment. However, the cost of treatment is typically higher. And some things, like lung damage, are often permanent.

Do Dogs Need Heartworm Prevention? 

Heartworm in dogs is a serious disease that poses a significant threat to your pet’s health even if it is treated. So preventing heartworm disease from happening is much preferable to treating the disease, whether your dog is diagnosed when asymptomatic or after symptoms develop. 

The good news is that this disease is almost 100 percent preventable with proper, consistent use of preventatives, according to the American Veterinary Medical Association. And whether your dog prefers a topical treatment or oral medication, there are plenty of heartworm prevention options to choose from.

Revolution is a prescription topical solution that’s perfect for dogs who don’t like to take oral medications. Just apply monthly to your dog’s coat, just between the shoulder blades, to prevent heartworm disease, fleas, American dog ticks, ear mites, and sarcoptic mange. If a tasty chewable tablet is more to your dog’s liking, giving your dog Simparica TRIO once a month protects against heartworm disease, roundworms, hookworms, fleas, and five types of ticks.

Just note that your vet will require a negative heartworm test before prescribing any heartworm preventative, and annual heartworm tests are recommended.

The American Heartworm Society recommends dog parents in all 50 states administer year-round, FDA-approved heartworm prevention medication for dogs, as well as limit dog exposure to mosquitos and their breeding environments. 

Talk to your veterinarian about which heartworm prevention is best suited for your dog, based on your pet’s risk factors and lifestyle.