All About Heartworm in Dogs


By Dr. Jess Trimble


Canine heartworm disease is a serious illness affecting dogs (and yes, it’s caused by a worm infecting the heart!).  It’s relatively common and highly preventable, but deadly if not detected and treated early.  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 mosquito carrying heartworm bites a dog, infective larvae enter the body and travel 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. The adults are about the size of a strand of spaghetti and grow up to 12 inches long. Scary, huh? 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 (e.g. occasional cough)

  • Class 2:  Mild to moderate symptoms (e.g. 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.

  • 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.

HOW IS HEARTWORM DIAGNOSED? Dogs should be tested for heartworm disease during their regular routine checkups. If you notice any of the symptoms listed above or other unusual behavior between preventive health visits, consult your vet immediately! Heartworm disease can be diagnosed via a quick blood test (antigen and microfiliarial concentration test) to check for the presence of either mature or immature worms in the dog’s body.  If your dog has a positive test, your doctor will confirm it with a different test and then do additional diagnostics to determine if there is additional organ damage. HOW IS HEARTWORM TREATED? Learning that your dog is suffering from heartworm can feel devastating – but the good news is that most infected pups can be successfully treated. 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 injections are administered deep within the back muscles and can be very painful, so most veterinarians will give a mild sedative. The post-injection recovery period can take up to eight weeks, sometimes longer. Dogs must be strictly confined during this time to prevent excess work for the heart and lungs while they are healing. During that time, dogs are also generally treated with a heartworm preventive to keep 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. Six 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, you need to 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? The best way to avoid canine heartworm is to be proactive in maintaining your dog’s health. Heartworm Prevention: There are many different choices it comes to heartworm medicine. Consult a veterinarian to understand which prevention plan is best for your dog, and to understand if you’re living in a highly endemic area. Regular Testing: Heartworm testing is an integral, routine procedure in preventive 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.