How can I Tell if my Dog Has Heartworms?

Ken Black

At home, after considering whether preventative care has been applied in the past, basic observation is the best way to tell if a dog has heartworms. An owner sometimes can see that his animal has symptoms such as labored breathing, fatigue and joint pain, but in many cases, pets exhibit no symptoms until it is too late. Veterinarians are key in providing a formal, final diagnosis when someone suspects an infection of these parasites, as they can perform a variety of tests to confirm the condition. Individuals should reject the common misconceptions that their dogs cannot be sick because they are “inside” animals or don’t live in an area known for the condition.

A dog might exhibit no signs of heartworms.
A dog might exhibit no signs of heartworms.

Understanding the Disease

Heartworms, or Dirofilaria immitis, are a parasite that mosquitos carry. The insects pass them on to animals, including dogs, when they try to bite to feed. Once this happens, the larvae begin traveling through the host’s body tissues, eventually making their way to the heart, major blood vessels and the lungs. There, they continue to grow and, if left untreated, eventually result in heart failure and death.

Veterinarians recommend giving animals heartworm pills to prevent a heartworm infection, which can be expensive and difficult to treat.
Veterinarians recommend giving animals heartworm pills to prevent a heartworm infection, which can be expensive and difficult to treat.

To get a little more in depth, the passing of these parasites from one animal to another begins when a mosquito takes up blood already infected with microfilariae, or baby heartworms. These molt twice, after which they can move through the mosquito’s saliva into a new host. After the larvae are passed on, they keep growing and migrate, developing into adults in about six to seven months. Although a single, already-infected mosquito might bite more than one dog an owner has, passing the disease on to each animal, dogs cannot pass it to each other, because the microfilariae must initially develop inside the insect. For this same reason, it’s also extremely rare for an uninfected mosquito to bite an infected pet, live through the parasite’s incubation period and then bite and pass the illness on to a second pet.

Surgery may be required to rid a dog of a heartworm.
Surgery may be required to rid a dog of a heartworm.

Previous Preventative Care

Sometimes, owners and vets can draw an initial conclusion about whether a dog has heartworms by considering whether it has been on a veterinary-recommended treatment for prevention. Many vets prescribe medication in the form of monthly pills, but they are also available as topical creams and injections. If someone has applied one of these options conscientiously to the dog since birth, a heartworm infection is statistically unlikely: They are effective 98% of the time. The 2% failure rate, however, requires at least considering the possibility of infection if symptoms are consistent with the disease.

Canine liver damage is often attributable to heartworm disease.
Canine liver damage is often attributable to heartworm disease.

Warning Signs of Infection

Often, a dog with heartworms does not initially display any symptoms, because it takes months for the newly-present larvae to grow, move through the body and develop into fully reproducing adults. Early on, the animal might display some fatigue during exercise or develop a cough, becoming easily winded, because the parasites damage arteries, causing inflammation and blockages that eventually lead to fluid accumulation that makes it difficult to breathe. It’s sometimes possible to hear abnormal sounds coming from the lungs, as well.

Blood samples may help reveal certain infections.
Blood samples may help reveal certain infections.

As the disease progresses, the immune system tries hard to handle the problem, but the standard response of inflammation, which helps carry antibodies to injured areas, goes overboard, often damaging joints and causing problems with liver, eye and kidney function. A dog in this stage might also lose weight. In some cases, it will pass out unexpectedly, and death usually follows soon after, sometimes in as little as a day.

Veterinary Examination, Testing and Confirmation

Professionals usually can detect these infections by taking blood samples and standard X-rays. Technicians performing the first option look for a specific protein the female heartworm gets rid of during reproduction, and they often pair this investigation with the Modified Knotts test, which requires a professional to put a blood sample through a centrifuge and examine it for microfilariae. X-rays can detect much of the heart, artery and lung damage that initially occurs, enabling the vet and owner to see how bad the problem is and make some decisions about how to proceed with treatment.

Other tests also can confirm that a dog is sick. The animal might go through tests that measure how its organs are functioning, for example, or it might complete a cardiac stress test. Combined with blood testing, X-rays and a general, physical examination, these methods catch many cases of the illness, increasing the chances of survival.


If tests come back positive, a veterinarian typically treats a case of heartworms with melarsomine dihydrochloride, often sold under the name Immiticide®, which they give through injection. It can have severe side effects, including vomiting and fever. In very severe cases where a dog is collapsing, he might opt to try to remove as many worms as possible with jugular surgery. A guarantee of success is not possible, however, and often, even if the operation removes the parasites, the damage to the animal’s body from the condition is usually so severe that it dies. Owners frequently are willing to try everything they can to help their pet, but many cannot afford the added expense of a procedure that vets know might not help, and sometimes, they believe it is better to put it to sleep to spare it additional suffering.

Additional Considerations

Some owners believe they can assess their dog’s risk of having heartworms by whether it is an “inside” or “outside” pet, but in reality, where it spends most of its time makes little difference. Although dogs that live outside generally have more opportunities to become infected and can be considered to be at a higher risk as a result, nearly all dogs go outside at some point and, therefore, are susceptible to being bitten by an infected mosquito. Even animals that remain inside for extended periods of time can be bitten by mosquitoes that find their way indoors.

Another consideration is that, as Sheldon Rubin, who served from 2007 – 2010 as president of the American Heartworm Society, points out, the problem is spreading to areas where it was relatively uncommon in the past. Much of this is due to improvements in irrigation, which create areas of water where mosquitoes can breed even in arid climates. Rubin cautions owners against thinking that their pets are fine just because their region previously has had a reputation for being free of the problem.

Heartworms are typically transferred through mosquitos.
Heartworms are typically transferred through mosquitos.

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Discussion Comments


Give your pet apricot kernels. This will slowly kill the heartworms.


@Closerfan12: The symptoms are actually more obvious in dogs. The first signs are usually excessive coughing - then major depression / weakness - weight loss and ultimately, death. Congrats on the new puppy!


Although it can be hard to tell when a dog has heartworms, the signs of puppy heartworms are a little clearer.

Most puppies with heartworms will have trouble breathing, and cough a lot. They also have a hard time gaining weight, and often have very low energy.

Luckily, treating puppy heartworms is not terribly difficult. Your veterinarian will be able to recommend an appropriate medication, either in a chewable treat form, or as a pill or cream.

You can also go a long way towards preventing heartworms in puppies, so don't forget to give them their heartworm pills -- in this case, an ounce of prevention is definitely worth a pound of cure.


I had heard that it was possible to actually catch heartworms from your dog, but apparently that's not true. Humans can have heartworms, but they have to get them via a mosquito bite, just like dogs do. Besides, heartworms in humans is extremely rare.

However, just in case you're worried, the signs of heartworms in humans include coughing, fever, fatigue and chest pains. Again though, this is a very rare disease, and you cannot catch it from a pet.

That's no excuse not to protect your pets though -- you still need to take care of them even if they can't infect you.


Are the symptoms of canine heartworms the same for feline heartworms? I have never had a dog before, only cats, and while I'm all up on my cat heartworms symptoms, I am woefully unaware of the signs in dogs -- do the signs of heartworms in cats show up in dogs too? I've just gotten my first puppy, so I am really trying to educate myself-- thanks!

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