Where do heartworms come from?

Heartworm disease is one of the scariest complications you and your pet can face.


Where do heartworms come from? They’re spread from animal to animal by mosquitoes, usually unnoticed, and they can be fatal when left untreated. And no dog is safe: Dogs of all ages and all locations are susceptible to the disease.


How are heartworms transmitted?


Heartworm disease is caused by worms that live within the heart and the blood vessels that connect the heart to the lungs. The disease is transmitted by mosquitoes, which means one bite from an infected mosquito is all it takes for your dog to get infected.


How do heartworms get inside your dog?


When a mosquito bites an animal infected with heartworms, it picks up heartworm larvae, and when that same mosquito bites your dog, those heartworms are then transmitted onto their skin when the mosquito bites, then the larvae actively migrate into the bite wound when the mosquito leaves.


Once inside your dog, the heartworm larvae take about 70 days to reach your dog's heart and lungs, where they will continue to mature and start to cause harm.


Heartworms take about six months to fully mature, and they can live inside your dog for up to seven years. Female heartworms can grow up to 12 inches long.


What are the symptoms of heartworm disease?


Some dogs will show little to no signs at all of having heartworm disease until it is too late. Some dogs will show warning signs that may include:


  • A nagging, persistent cough
  • Extreme tiredness or fatigue
  • Unexplained fainting
  • Unusual loss of appetite
  • Weight loss
  • If infected, pets may not act like themselves, and owners can often tell that something is wrong. What they can't understand, though—without help from a veterinarian—is the cause of the sudden change in their dog.
  • Once the disease has progressed, some dogs may also have a swollen stomach due to excess fluid.
  • One of the most serious signs of heartworm disease is caval syndrome, which is caused by restricted blood flow to the heart. Dogs suffering from this will have:
    • Labored breathing
    • Pale gums
    • Bloody urine


    Unfortunately, dogs in this stage rarely survive without surgical intervention.


What treatment options are there?


Heartworm disease is serious, and it can go unnoticed for a very long time. That's why the absolute best treatment option is prevention.


Veterinarians will typically prescribe a heartworm prevention, which not only prevents heartworm disease, but also kills heartworm larvae before they can grow into adults.


According to the American Heartworm Society, heartworm antigen tests only detect infections that are over 6 months of age, so puppies under 7 months old do not need to be tested for existed heartworm disease prior to starting on heartworm prevention. If your dog is over 7 months old, it should be tested for existing heartworm disease and then immediately started on a preventive method.


Testing for heartworms


Regardless of your dog's age, you should continue to regularly test your dog for heartworm disease during its annual vet check up—even if they are taking a preventive treatment—to ensure the treatment plan is working and to make sure your dog has not become infected.


If your dog does test positive for heartworm disease, it’s important to seek treatment right away. Treatment can be expensive, however, so your veterinarian should conduct additional tests to confirm the diagnosis.


Heartworm treatment for dogs


Once heartworm disease is confirmed, the first step will be to medically stabilize your dog.

Next, your dog will undergo treatment based on how advanced the disease is. Unfortunately, treatment for heartworm disease can be a long and painful process, and there is ultimately no guarantee that the treatment will be successful. While your dog is under treatment, you'll need to ensure your dog remains in a calm, relaxed environment with restricted exercise to minimize damage caused by the dying worms.


Once treatment is complete, your dog will have to be tested for heartworm disease again to evaluate the effectiveness. You will have to continue giving your dog year- round preventive treatment to keep it healthy.


With how easily heartworm disease is spread and how hard it can be to detect in dogs, prevention is the best course of action. To learn more about heartworm disease and how you can protect your dog from infection, our heartworm disease FAQs provide answers to questions that many dog owners like you have.

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HEARTGARD® Plus (ivermectin/pyrantel) is well tolerated. All dogs should be tested for heartworm infection before starting a preventive program. Following the use of HEARTGARD Plus, digestive and neurological side effects have rarely been reported. For more information, contact your veterinarian or click here for full prescribing information.