Heartworm Symptoms

Heartworm symptoms do not manifest until adult worms invade your dogs heart. Annual visits to the veterinarian will detect an infection in the initial stage of development. This is the stage before the larvae become adult worms and lodge in the heart. If you do not provide annual care for your dog, you may not detect the symptoms of infection until the worms have already settled into the chamber of the heart.

Understanding the lifecycle of these parasites will provide a clear understanding of the difficulty of early detection of heartworm symptoms and beginning treatment.

The larvae of the heartworms reside in the saliva of mosquitoes. When a mosquito bites a dog, the mosquito’s saliva is injected into the wound. The amount of worms your dog may develop depends on the amount of larvae contained in the mosquito’s saliva. Your dog may contract several worms from several mosquito bites.

Once a larva is injected into the dog’s bloodstream it circulates in the bloodstream as it begins to grow and develop. As the larvae become adult worms, the worms will attach to the tissue in the heart chamber. Heartworm symptoms will become apparent at this stage in a dog that is normally active. The worms can impede blood flow and this results in the heartworm symptoms detailed below. In extreme circumstances, death of the dog could result.

Heartworm Disease In Dogs


Seek medical attention if your pet is experiencing any of the following Heartworm symptoms:


  • heartworm symptomsExcessive Coughing
  • Dog appears lethargic
  • Dog refuses physical activity

Whilst all the above are heartworm symptoms, they are also common symptoms for a host of other illnesses. The only way to determine if your dog is infected is to visit the veterinarian for testing.

Your Vet will test for the presence of larvae, if this test is positive an x-ray will be performed to determine the scope of the infection. The presence of adult worms will result in a positive test; however, it does not indicate how many are in the dog’s heart. It is very important to know this to determine the appropriate treatment and care and it is for this reason that the x-rays are needed as heartworm symptoms may not be obvious until a late stage in the infection.

If the dog does not have any adult worms but larvae were detected in the bloodstream, an oral medicine is administered to kill the larvae. Additional testing must be performed to ensure treatment was successful and the veterinarian will recommend a preventative medicine to prevent any future infection.

If the dog has adult worms, treatment must commence as soon as possible. The dog is injected with an arsenic-based medication to kill the adults. Two injections are required within two days. The goal is to kill the worms gradually. Once the worms are dead, phagocytosis occurs and they are expelled from the heart in tiny pieces.

For six weeks, the dog must be kept calm and physical activity must be limited. If the pieces of the dead worms are expelled too quickly, they can travel to the lungs, where they can cause a blockage (or embolism) which may result in death. After six weeks, the dog returns to the veterinarian for an oral dose of medication to kill the heartworm larvae.

While heartworm symptoms are difficult to detect, regular visits to a veterinarian and a schedule of preventative medicine for dogs will keep your pet healthy and safe from infection.


Heartworm Symptoms — 2 Comments

  1. After the treatments…either constipitation or diarrhea…awful for the dog and the owners when it’s middle of the night!!! We just rescued this dog a week ago and the test was +. Poor boy, he has collie fur and it’s over 100 F. He layed outside under the trees, where the grass was damp because of a low flow hose that I run for the birds/animals into a sandy area. He just laid there most of the day…growled at me awhile ago. Sure not in the brochures about this part!!!

    • Hi Karen
      If your dog is suffering diarrhea after heartworm treatment, you need to talk to your vet – quite urgently in this hot weather – if your dog gets dehydrated because of not taking in enough fluids to combat the diarrhea, kidney damage could result especially as his system is already under pressure from the immiticide treatment. Hope he feels better soon.