What is Heartworm?

Chico Gilgulin_heartworm3Heartworm disease is caused by a parasite called Dirofilaria immitis that is spread to dogs through mosquito bites.  Cats can also get heartworm disease but they are more resistant to the infection.  Heartworms cause the blood pressure in the pulmonary arteries to increase. This increased pressure, called pulmonary hypertension, increases the workload on the right side of the heart which pumps blood to the lungs which can lead to right sided heart failure.

Heartworm symptoms include coughing, exercise intolerance, increased breathing rate or effort, abdominal distension with fluid, discolored urine as well as many other symptoms, depending on the severity of the disease process. Dogs with severe infection can be at risk for what is called caval syndrome – this is a life threatening condition where the adult heartworms back into the right heart, obstructing blood flow as it enters the right ventricle. Once there are mature adult heartworms living in the pulmonary arteries, Immiticide therapy needs to be administered after assessing the overall health of the pet. Immiticide, also known as melarsomine, is administered through injections given over a several month period.  During this time, all activity is restricted to short leash walks and when not supervised, dogs should be crated to prevent any excursion.  This helps to limit the risk of pulmonary thromboembolism and associated complications as the heartworms die off.  For those dogs with caval syndrome, immediate heartworm extraction is the best method of treatment.

How can Heartworm be prevented?

Administering a monthly heartworm preventative such as Heartgard easily prevents heartworm disease.  While we don’t see a lot of Heartworm in Colorado, dogs that come from other parts of the country can bring the disease here, and because it is spread through mosquitos, every dog is at risk of acquiring this disease.

A Real Life Story.

Heartworm disease is generally diagnosed and treated by a veterinary cardiologist through the course of an exam that includes an echocardiogram.  An echocardiogram is a non-invasive diagnostic tool that shows real time images of the  heart beating, blood flow, valve activity, etc. Rocky Mountain Veterinary Cardiology recently diagnosed and treated a patient with Heartworm disease. Here is Chico’s story.


Chico is a 12 year old Chihuahua who was adopted by his family in 2012. About a year later, he developed several concerning symptoms, including fainting, coughing, abdominal distension, and swelling in his legs.  In July, he was referred to Dr. Jeremy Orr, a board certified cardiologist at RMVC.  During the course of Chico’s exam, he was diagnosed with Heartworm disease and pulmonary hypertension, which was a result of the Heartworm disease.  Several adult heartworms were visualized during Chico’s echocardiogram in his pulmonary arteries as well as several worms within his right atrium which were not causing obstruction to blood flow.  As a result of his heartworm infestation and pulmonary hypertension, he was also in right sided congestive heart failure.


The first step was getting Chico’s congestive heart failure under control. After several weeks of medication, he was ready for his first series of Immiticide therapy which included a single injection followed by one month of strict cage rest. One month later, he received another two injections, twenty-four hours apart. He tolerated the treatment well, remained inactive as directed by Dr. Orr, and by February of this year, his heartworms were gone and his pulmonary hypertension symptoms were almost completely resolved with resolution of his right sided heart failure.  Chico is now very active and loves to play soccer!  He no longer is fainting and having labored breathing with activity.  His family is thrilled with his progress and his second lease on life.


Left untreated, Heartworm disease can be much more difficult to treat and with time, there can be irreparable damage to both the heart and pulmonary arteries leaving some animals with severe pulmonary hypertension and heart failure.  Therefore, prevention is key and for those animals who are infected, prompt and early diagnosis and treatment improves prognosis and outcome. Chico’s parents were on top of his symptoms and got him the care he needed and as a result, his long term prognosis is excellent!