How Can I Tell if my Dog Has Heartworms?

dog in snow

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.

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.

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.

napping dog

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.

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.

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.

two playing dogs


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.

Similar Posts