Symptoms of Heartworm and How to Prevent It

Heartworm season is upon us, so make sure to take special care of your furry friends. Take time to educate yourself on the various causes, symptoms, treatments, and prevention of a silent killer among dogs.


What Is The Source Of Heartworm Disease?


In short, heartworm stems from mosquitos; it’s when they transfer heartworm larvae from one dog to another. Once an infected mosquito bites your dog, they are instantly at risk, and the symptoms don’t always show immediately. There is no way to tell if a mosquito is infected, therefore using preventative medication is of importance.

On Average, it takes up to five months for the larvae to mature into developed (adult) heartworms. Once developed, the worms set up camp in the lungs, heart, and blood vessels where they will then reproduce. On average they can live for 5 – 7 years in your dog’s system, can grow up to 12 inches in length and can develop into over 200 worms. The treatment is costly and can be prevented with proper medication.


How They Infect:


An infected mosquito will bite your dog, allowing the transfer of larvae to enter the dog and develop within the tissues. Once the larvae have matured, they are present in the heart and pulmonary arteries. They further mature and reproduce, leading them into the blood vessels as well. Should other mosquitos bite your dog, they will become infected, and further spread heartworm to their next victim as the cycle continues and expands.




  • A Mild/Persistent Cough

Once the worms contaminate the dog’s lungs, they begin to multiply in the lungs and surrounding veins. Coughing (usually a soft/dry cough) may become more apparent after exercise.

  • Inactive / Exhaustion

Dogs infected with heartworm will find the simplest tasks difficult, even if they previously enjoyed doing them. Playing, walks, going outside and any physical activity will become a chore; you will find their energy levels minimal. They will be noticeably more tired than ever before.

  • Difficult or Rapid Breathing

When the worms infect the lungs and veins, along with coughing, breathing problems will start to occur. If fluid builds up around the veins, it will be difficult for the lungs to oxygenate blood causing rapid or difficulty breathing.

  • Collapse

When the heartworms reproduce and multiply, they can and will attack the heart causing a blockage of blood flow. This is referred to as ‘Caval Syndrome.’ Once this happens, the dog can suffer shock, red blood cell damage, and collapse. The condition becomes so severe that death may be imminent.

  • Weight-loss/Reduced Appetite

Due to the lethargy heartworms can cause, eating may become an exhausting task. Therefore the desire to eat will lesson, and weight loss will rapidly occur.

  • Protruding Chest

If the dog’s ribs become very visible and their chest seems to be protruding, it is possible that an adult heartworm infection is present. A fluid buildup can cause this due to the parasites existence, or weight loss/anorexia triggered by the heartworms.


Other Symptoms May Include:


  • Allergic Reaction
  • Heart Failure
  • Lung Disease
  • Sudden Death




If you suspect that your dog may be suffering from heartworm, immediately visit your veterinarian and do the proper testing.
Various blood tests will be able to diagnose heartworm disease. The blood testing will enable your veterinarian to view the blood sample under a microscope to check for antigens of the adult heartworms. Antigens are the proteins produced by the heartworms.
Along with blood testing, X-rays, an electrocardiogram, and ultrasound are also used to determine the overall health of your dog’s organs and the severity of the heartworms. These tests will also conclude if your dog can be treated for heartworm disease safely.


Treatment Process


Hearing that your dog has heartworm disease is gut-wrenching, however most infected dogs are entirely treatable. Heartworm disease is both challenging and expensive, so before proceeding with treatment, your veterinarian will run tests to make sure your dog is treatable, and this is the best option for them.

  • The first step will be to run additional testing to confirm the diagnosis. This will be done after an antigen test has been completed and confirmed.


  • Your dog will then be put on strict rest, being confined to a small area i.e. crate rest. Crate rest will be continued 6-8 weeks after the last treatment injection your dog receives. Your dog will even need to be leashed while going outdoors to the bathroom. Physical activity can increase the rate at which heartworms cause damage in the heart and lungs, so limiting any physical activity is necessary. This will be a hard task if your dog is usually very active.


  • Before heartworm treatment commences, your dog’s condition may need to be stabilized. This typically occurs with a suitable therapy. If your dog suffers from any pre-existing conditions, this process may take a bit longer. Your veterinarian will want to make sure your dog is stable before any proceedings.


  • Your dog will then be started on a monthly “preventative” medication to kill immature heartworms and stop the rise of new infection from emerging. This medication may be required throughout the entire treatment process, and sometimes for the remainder of your pet’s life.


  • The first injection of Melarsomine will be given to your dog after day 60 of the treatment process. Melarsomine is the drug that kills adult heartworms. Due to the severity of this drug, your dog will need to remain in the clinic until discharged as your veterinarian will need to monitor your dog for symptoms.


  • One month after the first Melarsomine injection, two more will be given 24 hours apart.


  • On day 120, your veterinarian will need to examine your dog thoroughly and run additional testing to make sure Microfilaria is no longer present. Additional medication will be prescribed if Microfilaria still exists.


  • Around day 270 your veterinarian will re-test your dog’s blood to determine the success of the treatment. By this point, the goal was to eliminate all stages of heartworm disease.


  • Once your dog is in recovery, it is crucial to watch for any reoccurring symptoms and to limit exercise until cleared from your veterinarian.


Know that when the worms die, they collapse and bury deeper into the vessels that supply blood to the lungs. So before and during treatment, it cannot be stated enough that ALL EXERCISE SHOULD BE COMPLETELY REDUCED. Exercise will increase blood flow to the lungs, and if the dead (or live) worms are blocking the blood flow, this can cause severe complications or death.

The American Heartworm Society is a great resource for any information pertaining to heartworm disease.




Your veterinarian is able to prescribe a monthly preventative medication that is most suitable for your pal.

Some of the more conventional treatments are listed below. They protect against heartworm as well as other common parasites. Browse through their websites to gain a better understanding of what their product protects against.

Please click below to read more information on the specified medications.



Annual blood testing can and should be done to check for heartworm disease. Your local veterinarian should be suggesting or reminding you to do your annual check before heartworm medication is prescribed. A quick blood sample can provide comfort in knowing that your dog is healthy and free of the disease.


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