What is Heartworm Disease?


blog post photo
photo credit: Dr. Amara Estrada


Since an article on heartworm disease and dogs was first reported in a medical journal over 100 years ago, this infection has been branded as a worldwide clinical problem. It is a dangerous disease that can potentially kill an infected dog — but what exactly are heartworms and how is the disease contracted?

Heartworm disease has been diagnosed across the U.S. and around the world. The parasites are transmitted to mammals through an infected mosquito. Once inside a definitive host — such as a dog, cat, sea lion, or wolf — the heartworms mature and reproduce. The potential for heartworm infection has increased over the years due to mammal relocation, human environmental influence, and natural climatic shifts (like temperature, pressure, wind, rainfall, and humidity).

Recently I connected with Dr. Amara Estrada, DVM, to better understand heartworm disease and how it affects canine populations in particular. Dr. Estrada is currently an Assistant Professor at the University of Florida College of Veterinary Medicine.

“Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the pulmonary arteries and the right side of the heart of dogs and other species,” Dr. Estrada explained to me. “The L3 penetrate local connective tissues and a systemic vein and are carried by blood flow through the right heart to the terminal branches of the pulmonary arteries.”

The spreading of the heartworms in an animal are determined by blood flow. They can end up in a dog’s pulmonary artery or even the eye, liver, or brain. According to the American Heartworm Society, the heart and lungs are the organs with the greatest pathology in dogs.

Within days of infection, heartworms can severely damage a dog’s pulmonary arteries. Larger arteries can expand and twist, while smaller ones may become obstructed and negatively affect blood flow in the animal. Additionally, the right side of the heart may become enlarged, possibly leading to congestive heart failure or even death.

An infected dog may show no signs during the early stages of heartworm disease. As the worms grow and multiply, the animal may have a cough, appear sluggish or reluctant to exercise, or demonstrate a loss of appetite.

Dogs infected with heartworm disease have a good chance at improved health if given appropriate and timely treatment. The Heartworm Society states that the goal of heartworm disease treatment is to “improve the clinical condition of the animal and to eliminate all life stages of the heartworms — microfilariae, larval stages, juveniles and adults — with minimal post-treatment complications.”

Dr. Estrada, DVM explains further that “more severely affected animals may have the potential to benefit the most from treatment. At the same time, any animal can have complications with treatment, as this involves actually killing the worms while they are in the pulmonary arteries — the more severe the disease, the greater the risk for complications. Animals with severe disease have the potential for life-threatening complications both with and without treatment, a dilemma many veterinarians often have to face. In some animals with very heavy infections, surgical removal of the worms is recommended in addition to adulticide therapy.”

blog post photo
  photo credit: Dr. Amara Estrada

Dogs are generally tested for heartworm disease by a veterinarian every six months or when switching preventative products. Testing is usually through a simple blood test; however, animals that receive a positive result may also have an X-ray or ultrasound examination.

Learn more about heartworm disease and how it affects dogs at the American Heartworm Society’s informational website.

Head back to the Nat Geo Dogs blog to learn how to prevent heartworm disease in your dog.