Understanding Heartworm in Dogs: What Every Pet Owner Needs to Know

Heartworm disease is a serious and potentially fatal condition that affects dogs. It’s caused by a parasitic worm called Dirofilaria immitis, which is spread through the bite of an infected mosquito. Understanding what heartworm is, how it affects dogs, and how to prevent it can help you keep your furry friend safe and healthy.

What is Heartworm in Dogs?

Heartworm is a parasitic worm that can grow up to a foot long. These worms live in the heart, lungs, and associated blood vessels of infected dogs. The disease is called “heartworm” because the adult worms live in the heart and pulmonary arteries of the infected animal. You might wonder; well how do dogs get it?

How Do Dogs Get Heartworm?

Heartworm is transmitted through mosquito bites. When a mosquito bites an infected animal, it picks up tiny immature heartworms called microfilariae from the animal’s bloodstream. When the mosquito bites another dog, the larvae are transferred to the dog’s bloodstream, where they eventually grow into adult worms.

Life Cycle of Heartworm in Dogs

  1. Mosquito Bite: The life cycle begins when a mosquito bites an infected dog, picking up microfilariae.
  2. Development in Mosquito: Inside the mosquito, the microfilariae develop into infective larvae over 10-14 days.
  3. Transmission to Dog: The mosquito bites another dog, transferring the infective larvae into the dog’s bloodstream.
  4. Migration and Maturation: The larvae migrate through the dog’s tissues and bloodstream, reaching the heart and lungs. Over the next 6-7 months, they mature into adult worms.
  5. Reproduction: Once mature, the adult worms mate and produce microfilariae, which circulate in the dog’s bloodstream, ready to be picked up by another mosquito, continuing the cycle.

Symptoms of Heartworm Disease

In the early stages, many dogs show few or no symptoms. As the disease progresses, symptoms become more noticeable and severe, including:

  • Mild persistent cough
  • Fatigue and reluctance to exercise
  • Decreased appetite
  • Weight loss
  • Swollen belly due to fluid accumulation

In advanced cases, heartworm can cause heart failure, severe lung disease, and damage to other organs. Heartworm disease is diagnosed through blood tests that detect the presence of heartworm proteins or microfilariae in the blood. Your veterinarian may also use X-rays, ultrasound, and echocardiograms to assess the extent of the damage caused by the worms.


Treating heartworm disease is complex and can be risky, especially in advanced cases. The goal is to kill the adult worms and microfilariae while minimizing the side effects of treatment. Treatment typically includes:

  1. Stabilization: If the dog is severely affected, initial treatment may focus on stabilizing the condition.
  2. Medication: The dog will receive medications to kill the adult worms and microfilariae.
  3. Activity Restriction: Dogs must be kept calm and confined during treatment to prevent complications from dying worms, such as blockages in the blood vessels.


Prevention is crucial because treating heartworm disease is difficult and costly. Several preventive medications are available. These include monthly oral tablets, topical treatments, and injections administered by a veterinarian. These medications are highly effective at preventing heartworm infections when used consistently.

Heartworm disease is a serious threat to your dog’s health, but it is entirely preventable. Understanding how heartworm is transmitted, recognizing the symptoms, and knowing the importance of regular testing and preventive measures can help you protect your dog from this dangerous disease. Always consult your veterinarian for the best preventive plan for your pet and ensure your dog receives regular heartworm preventives to stay safe and healthy.