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Heartworm Disease in Dogs

What Is Heartworm Disease in Dogs?

Heartworm disease in dogs, cats, ferrets, and other mammals is caused by Dirofilaria immitis, a large worm that can grow to over a foot long. Its life cycle takes around six to seven months, during which it ultimately settles in the heart and pulmonary vessels, where it can survive for several additional years. As the worms accumulate in the heart, blood flow becomes restricted, leading to decreased circulation throughout the body and potentially resulting in heart failure.


The symptoms of heartworm disease in dogs vary in severity depending on the extent of the infection and which organs are affected, primarily the heart and lungs. Common symptoms include:

  • Coughing
  • Lethargy
  • Decreased tolerance for exercise
  • Episodes of collapsing
  • Sudden death

Additionally, some dogs may exhibit signs such as weight loss, breathing difficulties, and excessive panting. If left untreated, dogs may progress to develop right-sided heart failure and ascites, which is the accumulation of fluid in the abdomen.


Heartworm disease in dogs is primarily transmitted through mosquitoes, which act as the carriers of the disease. Transmission doesn’t occur directly from one dog to another. When mosquitoes feed on an infected host, they ingest young immature heartworms, known as microfilariae, circulating in the bloodstream.

Inside the mosquito, these microfilariae undergo three larval stages (referred to as L1, L2, and L3). When the same mosquito bites a dog, it deposits the L3 stage onto the dog’s skin. From there, it migrates into the dog’s body, progressing through stages of development until it reaches the L5 stage. The L5 larvae then move throughout the tissues and bloodstream, eventually settling in the heart, where they mature into adult worms. This entire process typically takes about four months to complete.

Once the adult female worms reach around seven months of age, they become sexually mature, mate, and begin producing microfilariae. Diagnostic tests used in veterinary hospitals detect antigens—unique proteins found on the surface of adult female heartworms. This is why testing for heartworm disease typically begins around the seven-month mark.


Veterinarians typically recommend that dogs aged seven months and older undergo annual testing for heartworm disease. Dogs who miss a dose of prevention should be tested more frequently. Testing is usually conducted in the hospital setting using a small blood sample.

The primary method for diagnosing heartworm disease is antigen-based testing, which detects specific proteins found on the surface of adult female heartworms. A positive test result indicates that the dog is infected. Additional tests, such as blood smears or modified Knott’s tests (often sent out for diagnosis), may be performed to check for the presence of circulating microfilariae.

Following diagnosis, veterinarians may recommend further testing to assess the severity of the infection and the risk involved in treatment. This can include chest radiographs, EKG, blood pressure monitoring, cardiac enzyme evaluation (NT-proBNP), echocardiography, blood work, and urine analysis.

Dogs are categorized into different classes based on the severity of their condition. Class I dogs have the lowest risk for treatment, while Class IV dogs, often diagnosed with caval syndrome, are at the highest risk. In caval syndrome, the worm burden is severe enough to block blood flow from the heart, necessitating surgical removal of the worms, typically performed by a specialist, to save the dog’s life.


Upon diagnosis of heartworm disease in your dog, your veterinarian will discuss the next steps, treatment options, and schedule follow-up visits.

Initially, it’s essential to restrict your dog’s activities to minimize the risk of dislodging heartworms and causing blood clots. Moreover, if your dog has circulating microfilariae, limiting outdoor exposure can prevent mosquitoes from transmitting the parasite to other animals.

Your veterinarian may prescribe specific medications, including:

  • Steroids to reduce inflammation caused by the heartworm.
  • Antibiotics like doxycycline to eliminate Wolbachia, a symbiotic organism within the heartworm. Removing the symbiote makes the host heartworm more susceptible to treatment and reduces secondary inflammation.
  • A particular type of heartworm preventive to halt the development of younger worms into adults and eliminate circulating microfilariae from the bloodstream.

Additionally, your dog will receive injections of melarsomine, an arsenic-based compound, 60 days, 90 days, and 91 days after diagnosis. These injections aim to kill adult heartworms and are typically administered deep into the muscle in the lower back. Due to the pain associated with the injections, your veterinarian may prescribe pain medications to manage discomfort during these visits.


Preventing heartworm disease in dogs is paramount, and the most effective approach is through year-round prevention measures. Fortunately, there are numerous affordable options available on the market.

Various forms of heartworm prevention exist, including tablets, topicals, and injectable versions, offering protection ranging from one month to a full year. Some products also combine heartworm prevention with flea and tick control for comprehensive preventive care.

All these preventive products are designed to eliminate L3 and/or L4 heartworm larvae, with some also capable of clearing circulating microfilariae from the bloodstream.

If your dog tests positive for heartworm disease, it’s crucial to discuss the appropriate preventive measures with your veterinarian during treatment. Certain preventatives should be administered to minimize potential secondary complications.

Consulting with your veterinarian is essential to determine the most suitable type of prevention based on your dog’s lifestyle and your budget. While limiting your dog’s exposure to mosquitoes is advisable, it may not always be feasible in certain locations. Remember, it only takes one infected mosquito to transmit heartworm disease.

Living and Management

Treating heartworm disease in dogs carries certain risks. Dogs undergoing treatment, as outlined earlier, may experience complications such as anaphylaxis (shock), emboli (clots), and even sudden death. Additionally, there’s a possibility of abscess formation at the site of melarsomine injection, along with the emotional toll of months of restricted activity.

Furthermore, dogs may face long-term health issues due to the damage inflicted by the heartworms on their heart and lungs. The scarring and inflammation caused by the worms can impede blood flow through these vital organs, potentially leading to right-sided heart failure, even after successful treatment.

The prognosis depends on the severity of the condition, with early detection and intervention offering the best chance of a positive outcome. Unfortunately, dogs affected by heartworm disease do not develop immunity and remain at risk of reinfection in the future. This underscores the importance of year-round prevention to safeguard your dog’s health.

Heartworm Disease in Dogs FAQs

Can humans be affected by heartworms?

Heartworm disease predominantly impacts dogs, cats, and ferrets. However, according to the CDC, humans can also be susceptible to the disease. It’s important to note that your dog is not contagious; human infection occurs through mosquito bites and is relatively uncommon.

Is heartworm disease in dogs treatable?

Yes, if detected early and managed effectively, dogs can often lead a good quality of life post-treatment. Unfortunately, some dogs may experience adverse effects either from the treatment itself or from the progression of the disease, leading to lifelong complications.

What are the initial symptoms of heartworm disease in dogs?

While some dogs may not exhibit any symptoms, particularly those with a more sedentary lifestyle, others may show signs such as exercise intolerance and coughing.

What is the survival rate for dogs diagnosed with heartworms?

Dogs diagnosed with Class I or II heartworm disease generally have a better prognosis and higher survival rates compared to those classified as Class III or IV. Class IV dogs typically require lifesaving surgery for treatment and would not survive without it.

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