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Heartworm Disease in Dogs: What it is, how it affects your dog, and how to treat it

Updated: Mar 28

Heartworm disease (dirofilariasis) is very common throughout the US and can be potentially fatal to your dog if left untreated. It is caused by a blood-borne parasite called Dirofilaria immitis which is transmitted through mosquito bites. Dogs are usually asymptomatic during the initial several-month infection process, showing very minor or even no signs of infection at all. Because of this many dogs have advanced heartworm disease at the time of diagnosis.

How they affect Dogs

Once lodged in the dog's heart, they can restrict blood flow through the dog's arteries, affecting other organs throughout the body, including the lungs, liver, and kidneys. There are several stages of infection ranging from mild symptoms to death:

  1. Class One: No symptoms, mild cough

  2. Class Two: Mild exercise intolerance. persistent cough

  3. Class Three: Greater exercise intolerance, strange lung sounds, weak pulse, fainting, decreased appetite, weight loss, and swollen belly,

  4. Class Four: Known as Caval Syndrome; A life-threatening cardiovascular collapse marked by labored breathing, pale gums, and dark coffee-colored urine.

Identifying Heartworm disease in your dog

Since Heartworm disease can affect other areas of the body and potentially result in death, it is recommended that adult dogs are tested annually, even if they aren't showing any potential symptoms. Early diagnosis is key to treatment. Blood tests can determine the presence of parasites within a dog's bloodstream, as they look for the presence of proteins released by worms while attached to the heart. If your dog's test is positive, your veterinarian will conduct further tests to determine the extent of the disease and develop a treatment plan.

These include:

  • Radiographs

  • Echocardiography

  • Complete blood cell count and serum chemistry

Once a heartworm disease diagnosis has been confirmed, your veterinarian will go over the treatment and potential risks of treatment. Because many dogs have advanced heartworm disease at the time of diagnosis with extensive damage to the heart and other organs, heartworm prevention is strongly recommended.

Preventing Heartworm Disease in Dogs

The American Heartworm Society recommends giving heartworm prevention monthly year-round and testing for heartworm every 12 months. Heartworm preventatives are highly effective, but dogs using these medications can still get heartworm hence the yearly recommendation for heartworm testing. Puppies under 6 months of age can begin taking heartworm prevention and then take a heartworm test at 6 months, while dogs over the age of 7 months should be tested prior to starting heartworm. Other recommended prevention methods include:

  • Use of FDA-approved mosquito repellant

  • Environmental control to reduce mosquito breeding grounds such as pools of standing water; set up mosquito traps around the house; limit your dog's access to the outdoors at dawn and dusk, which are prime hours for mosquito activity.

Medication should only be used if instructed and prescribed by your veterinarian. Below is a list of preventatives that you can discuss with your veterinarian to find which one is best suited for your dog.


Heartworm disease is very serious and can be fatal for your pet, which is why veterinarians are very adamant about preventative care to reduce the possibility of infection. If your pet is showing any of the clinical signs listed above, please call us today at 559-434-5470 for more information, or contact your regular veterinarian to see if testing is necessary. Heartworm can be scary and expensive, but you and your pet are not alone in the battle.



  1. American Heartworm Society. Heartworm in Dogs

  2. American Kennel Club. Heartworm in dogs; what you should know about the mosquito-born parasite

  3. FDA. Keep the worms out of your pets heart! The facts about heartworm disease.

  4. American Veterinary Medical Association. Heartworm Disease

  5. Pets & Parasites. Heartworms

  6. Veterinary Partner (VIN). Heartworm Disease in Dogs

  7. Lifeline Animal Project. Facts about Heartworms

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