All About Heartworm in Dogs

Canine heartworm disease is a serious illness affecting dogs.  It’s relatively common and highly preventable, but deadly if not detected and treated early on.  As a pet parent, it’s important to know what canine heartworm disease is and how to protect your dog from it.  Here are the facts you need to know:


What is canine heartworm disease?

Canine heartworm disease is a medical condition caused when the parasitic worm Dirofilaria immitis infects a dog.  Heartworms live in the blood vessels, heart, and lungs of an infected dog’s body and their presence can lead to serious, sometimes fatal complications such as lung disease and heart failure.


How do dogs contract heartworm disease?

Mosquitoes play an integral role in the transmission of larvae.

When a heartworm-carrying mosquito bites a dog, the larvae enters the body and travels through to the blood vessels and into the heart and lungs.  The larvae grow inside the body until they become adult heartworms.  The fully-matured worms then mate and produce pre-larval offspring called microfilaria.  The microfilaria circulate throughout the dog’s bloodstream, grow into adults and the reproduction cycle begins again.

An infected dog cannot directly spread the disease to another dog, but can indirectly do so through a carrier, in other words, the infected mosquito.  


What are the symptoms of heartworm in dogs?

Symptoms of canine heartworm depend on the number of worms living in the body and how long they have been there.  Some dogs are asymptomatic and show no clinical signs of the disease at all.  Active dogs and dogs with pre-existing medical complications tend to exhibit more severe symptoms.  Symptoms typically include a mild or persistent cough, decreased appetite, weight loss, difficulty breathing and decreased activity levels.

Heartworm disease and its symptoms are typically broken down into four classes.  The U.S. Food and Drug Administration defines them as follows:

  • Class 1:  No symptoms or mild symptoms such as an occasional cough.
  • Class 2:  Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
  • Class 3:  General loss of body condition, a persistent cough, and tiredness after mild activity.  Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
  • Class 4:  Also called caval syndrome.  There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms.  Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option.  The surgery is risky, and even with surgery, most dogs with caval syndrome die.  (Source: U.S. Food and Drug Administration)

What does heartworm look like inside the body?

They look like spaghetti.  Adults can grow up to 12 inches long.


How is heartworm diagnosed?

Dogs should be tested for heartworm disease at each routine checkup.  If you notice any of the symptoms listed above or other unusual behavior between preventative health visits, consult your vet immediately.

Heartworm disease can be diagnosed via an antigen and microfiliarial concentration test. Both of these procedures check for the presence of either mature or immature worms in the dog’s body.  Your vet will likely confirm the diagnosis through an additional test like a radiograph (an x-ray), which will allow him or her to fully understand the level of infestation.


How is heartworm treated?

It’s devastating news to learn your dog is suffering from canine heartworms, but the good news is that most disease-infected pups can be treated easily and recover fully.

Canine heartworm is typically treated through a series of drug injections designed to kill the adult worms.  Infected dogs are frequently hospitalized and monitored during early treatment.  The post-injection recovery period can take up to four to six weeks, sometimes longer.  During that time, dogs are also generally treated with a heartworm preventative to prevent the microfiliaria, or baby worms, from hatching and thriving in the body.  

Some dogs are prescribed antibiotics and additional medications in conjunction with heartworm treatment.

The American Heartworm Society recommends restricted exercise for infected dogs as physical activity exacerbates the internal damage caused by the heartworms.  

6 months after the initial treatment is completed, your vet will run a test to determine if your dog is heartworm-free.  If the results come back positive, get ready to gear up for round two of treatment.  In the most extreme cases, surgery may be necessary to physically extract the worms from the body.


How can I prevent heartworm in my dog?

Heartworm treatment takes an huge toll on a dog’s body and mind.  The best way to avoid canine heartworm is to be proactive in maintaining your dog’s health.

Regular Testing

Heartworm testing is an integral, routine procedure in pet preventative care.  According to the American Heartworm Society, all dogs should be tested for heartworm disease annually.  It doesn’t matter if your dog is on a prevention program and is taking anti-heartworm medication – regular testing is absolutely necessary as there is always a risk of infection.

Heartworm Preventative

There are many different choices when it comes to heartworm medicine.  Consult a licensed veterinarian to understand which prevention plan is best for your dog and to understand if you’re living in a highly-endemic area.


Additional Resources

For additional information about Lyme disease in dogs, refer to these resources:


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