Skip to main content

Canine Heartworms

The heartworm looks like a long white worm and lives in a dog's heart. It can also live in the blood vessels near the heart and lungs.

Baby Heartworms:

  • First affect the lungs
  • Then enter the heart and slow down the oxygen flow to other organs

Adult Heartworms:

  • First affect the heart--enlarging it, causing long-term damage
  • The liver and kidneys may also get damaged

Heartworms are easily preventable but fatal if left untreated. The treatment can cause serious side effects.


Mosquitoes spread heartworms.

When a mosquito bites an infected dog, it ingests immature heartworms, which mature inside the mosquito. If that same mosquito bites your dog, your dog will get infected.


A lot of damage can occur before you notice any signs. These signs include:

  • Cough, sometimes bloody
  • Loss of appetite
  • Low energy

If the disease has reached a critical stage, signs include:

  • Difficulty breathing
  • Weakness
  • Fainting

At this point, damage to major organs is severe, treatment is complex, and the chance of full recovery is low.


To diagnose your dog with heartworms, your veterinarian will perform the following:

  • Blood tests: The number of heartworms that are in the blood show how advanced the disease is
  • Ultrasounds of the heart: to check the heart's condition
  • Chest x-rays: to review the lung's condition

Based on this information and your dog's medical history, your veterinarian will diagnose your dog.


If you catch heartworms early enough, there is an excellent chance for treatment to be successful.

Baby Heartworms: Heartworm preventives will kill baby heartworms. In most cases, your veterinarian will admit your dog to the hospital for observation and then do another blood test to ensure the heartworms are gone.

Adult Heartworms: If your dog has adult worms, the baby worms must be treated first. Your veterinarian will recommend 1 to 3 months of preventive treatment before treating the adult worms.

Once the baby worms are gone, your veterinarian will most likely prescribe three doses of ImmiticideTM.

  • First dose: painful injection into your dog's lower back muscles with a 30% chance of reaction at the injection site - treatment and pain medication may be prescribed, as well as an overnight hospital stay for observation
  • Home rest: your dog must rest for 30 days to allow the adult worms to die, disintegrate, and absorb into the dog's body. Any excitement will disrupt this and could lead to sudden heart failure and death
  • Last two doses: once the adult heartworms are gone, your dog will be admitted into the hospital for two more injections, given 24 hours apart. These get rid of the rest of the adult heartworms still in your dog.

After treatment, you must ensure your dog has complete rest to prevent lung damage from the decomposing worms. Avoid exercise for at least a month, and gradually return to regular activity.

Severe heartworm cases may require surgical removal of the worms.


If your dog was treated for heartworms, they could get the disease again if you are not careful to follow a monthly preventive plan.

Most veterinarians recommend:

  • Monthly heartworm-prevention medication beginning at 6 weeks of age
  • Heartworm tests to regularly start at 6 months of age


If you catch the heartworms early, there is an excellent prognosis. Depending on the number of worms and stage of infection, life-threatening complications may arise from both the disease and treatment.  

Medically Reviewed by Sara Ochoa, DVM

Get coverage today with Mint Wellness

Get a free quote