Vet Blog

Signs and Symptoms of Heartworms in Dogs

April 08, 2022

Dogs are the preferred hosts of heartworms.

By the time you notice the symptoms in your dog, the disease will have spread throughout its body. Signs of heartworms begin showing up when the worms mature by the sixth month of their implantation.

The condition begins when a heartworm-infected mosquito bites your dog. The infection comes from the mosquito sucking blood from an animal with heartworms, transferring them to another animal. The repercussions of heartworm infection are severe. The treatments are expensive and emotionally draining.

The symptoms depend on the phase of the heartworm cycle. You can categorize them into four different stages. The following information will give you knowledge about what symptoms you should expect. The stages begin once the heartworms get into the heart.

Stage One

Your dog may be symptom-free during this stage as the heartworms settle in the heart. During this stage, the heartworms will not have started producing microfilariae. Your dog will also not produce enough antigens to detect the problem.

Stage Two

Persistent Coughing

Your dog will show intolerance to exercising and display constant coughing. A heartworm-induced cough is persistent and dry, unlike periodic regular coughs. Your dog may start coughing soon after they begin exercising. They do so because the heartworms create discomfort and blockage as they spread to the lungs.


Your dog may have heartworms if they have a sudden loss of interest in being active or going for walks. If you notice they appear tired while doing any activity, it is essential to take them for a checkup at the veterinarian at Goose Creek Veterinary Hospital.

Weight Loss

During this stage, the heartworm disease spreads, your dog loses interest in simple tasks such as eating, and you may notice a rapid loss of weight.

At this point, your veterinarian may detect heartworms using a blood test. The worms will have stayed long enough to produce microfilariae.

Stage Three

By this point, your dog will display the following:

  • Breathing difficulties
  • Protruding ribs

Stage Four

By this final stage, your dog will exhibit noticeable heart disease symptoms. Your pet will be very ill with similar symptoms as in the third stage but to a more severe degree.

You will notice their reluctance to exercise and constant coughing. Your dog may also struggle to breathe. Going to your veterinarian for testing may reveal the extent of the disease. The heart and lungs may produce abnormal sounds. Your veterinarian may also notice liver enlargement.


It is essential to get a blood test if you have not given your dog heartworm medication for over six months. Doing so helps ensure they do not have heartworms before starting preventative treatment. Your veterinarian will advise you to begin treatment when your dog is six to eight weeks.

Your veterinarian can also administer an annual injection to prevent heartworms. Doing so has proven to be a very effective prevention measure. You can take your pet when they turn 12 weeks old and take them for another booster when they turn six months.

For more about heartworms in dogs, visit Goose Creek Veterinary Hospital at our office in Ashburn, VA. You can also call (571) 291-9110 to book an appointment today. For emergency services, you can call 703-777-5755.