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Heartworm Symptoms: What Dog Owners Need to Know
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Heartworm Symptoms: What Dog Owners Need to Know

Charlie, Critter Culture Staff
Updated Sep 10, 2020

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Heartworm is a preventable disease that does not spread from dog to dog. Yet, millions of dogs around the world have the disease. The cycle starts when a mosquito bites an infected animal. Through the bite, the mosquito picks up microfilariae, or baby heartworms, which develop into infective larvae. When the mosquito bites another animal, such as a dog, it deposits these infective larvae through the bite wound. The larvae travel to the pet’s heart, lungs, and blood vessels where they mature and produce offspring. While heartworm disease is treatable, owners may not notice symptoms until their dog is seriously ill.

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1

Sometimes there are no symptoms

infection abnormalities asymptomatic heartworm Ed Reschke / Getty Images

Veterinarians say that in milder or early cases of heartworm disease, the pet is sometimes asymptomatic, meaning it shows no clinical signs of infection. Lab testing may reveal no abnormalities. If the dog has underlying health issues, is older, or is highly active, the clinical signs may be more pronounced even in the early stages. Once the symptoms begin to appear, it means the larvae have been living inside your pet for at least six months.

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Fatigue is an early symptom

feels tired symptom fatigue dog gollykim / Getty Images

A dog that always feels tired means there’s something wrong. Owners know their dog’s energy levels and can recognize when their pets aren’t feeling well. While fatigue and listlessness may be a sign of many other types of common dog ailments, they can also be early indications of heartworm disease. Infected dogs may lose interest in activities they’re usually excited about, such as walks. If your pet is older than 7 months and is not on preventative medication for heartworm, keep an eye out for additional symptoms.

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3

Infected dogs often develop a cough

Puppy Dachshund looks at the camera katerinasergeevna / Getty Images

In the early stages of heartworm disease, you may notice that your dog has developed a cough. This cough is not strong or sporadic like kennel cough, but instead, it’s persistent and dry. You may notice that your pet starts to cough or breathe rapidly following even a small amount of exercise or active play. That’s because the heartworm parasites have traveled to the lungs. The microfilariae primarily target the lungs and liver. As lung tissue damage increases, the cough worsens.

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4

Weight loss is another early sign

infected eating weight listless canine ChristopherBernard / Getty Images

For heartworm-infected dogs, even eating becomes a chore that requires too much energy. Their appetite wanes, and even their favorite snacks don’t inspire the usual level of excitement. As a result, the dog starts to lose weight—often at a rapid rate. If your pet is coughing, listless, and losing weight, your pet could be showing early symptoms of heartworm disease. It’s essential to take them for a veterinarian exam and blood test to rule it out or diagnose any other issues.

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Dogs may cough up blood or faint

heartworm progresses breathing faint exam razyph / Getty Images

As heartworm disease progresses, breathing difficulties increase and the dog’s cough becomes worse. They may cough up blood. Fluid builds up not only in the lungs but also in the blood vessels that surround them. It becomes increasingly difficult for them to provide the necessary oxygen and nutrients to the blood. At this point, owners also notice their pet’s ribs and chest begin to protrude. Some dogs experience syncope, a loss of consciousness due to a drop in blood pressure.

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6

The dog’s belly swells

fluid buildup ascites belly GeorgePeters / Getty Images

A buildup of fluid in the animal’s peritoneal cavity, or belly, causes a condition called ascites. This is a sign that the heartworm disease is progressing and symptoms are becoming more severe. The heart is no longer functioning properly and has lost the ability to pump the blood at a rate that the dog’s body requires. A blood test at this point will reveal moderate to severe anemia. Urinalysis shows protein loss. Breathing difficulties continue to worsen.

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The liver gets larger

heartworm progresses enlarged liver exam filadendron / Getty Images

Once the heartworm disease progresses into later stages, other symptoms develop, including an enlarged liver. The infected larvae block the blood vessels that nourish and carry oxygen to the cells. This hinders the blood flow and leads to infection and liver damage in addition to lung damage. Liver injury results in cirrhosis, which leads to jaundice, anemia, and weakness. Chances are, if the vet detects liver damage, they will also discover kidney damage as well, which leads to additional toxin buildup in the dog’s body.

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Abnormal heart and lung sounds develop

abnormal heart lung sounds veterinarian andresr / Getty Images

Veterinarians often discover abnormal heart and lung sounds at this stage of the disease in addition to the physical signs that the owner observes. Dogs with this level of infection experience chronic heart failure, especially on the right side. The vet will also detect a rapid heartbeat and an elevated blood pressure in the latter stages of the disease.

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9

Pale gums and dark-colored urine are late-stage symptoms

final stage pale gums canine jirousova / Getty Images

The final stage of heartworm disease is canine caval syndrome, but not all dogs develop it. If this often-fatal condition occurs, it means the dog’s cardiovascular system is collapsing. The owner may notice that the dog’s breathing has become even more labored. If the main arteries are blocked, the dog develops pale gums. Because heartworm disease can also damage the kidneys, the dog produces dark, coffee-colored urine. The other organs begin to fail.

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10

Collapse is a late symptom

collapse disease long heartworms emergency Morsa Images / Getty Images

Serious heartworm disease usually indicates a long-term infection. Heartworms can survive and grow inside a dog’s body for as long as seven years. The seriousness of the disease depends on the number of worms living inside the dog. Animal health experts call this the worm burden. Without treatment, the disease progresses to the point of organ failure and severe breathing difficulties. In many cases, the dog is unable to survive the infection and collapses.

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