Heartworms In Dogs


Heartworms are basically parasites transmitted by mosquito bites.  The heartworms take up residence in the lungs of the dog causing inflammation of the blood vessels which can result in damage to the heart and an early death.  Treatment for heartworms is very difficult for a dog.  It is not always Heartworms in dogssuccessful causing serious health risks and is potentially fatal so prevention of heartworms is important.

Year-round prevention is recommended by most Veterinarians including The Companion Animal Parasite Council who are experts in parasitology and veterinary medicine.  No one knows when and where mosquitos will be present so it is critical to get a heartworm check every year.  Most heartworm medications also treat intestinal parasites which can also be transmitted throughout the year.

Heartworm disease is caused by a worm parasite known as Dirofilaria immitis.  There are over 70 species of mosquito’s that carry the heartworm larvae from an infected animal to a new animal host.  When they arrive in the new animal host, they begin to grow into adult worms living in the blood vessels that support the lungs and heart.  This places a tremendous amount of stress on the dog’s heart and causes inflammation producing severe complications.  It depends on how many worms are present or when treatment actually kills the heartworms.  Some animals can be infected again and again and most dogs are highly susceptible to heartworm infection because the majority of larvae develop into adult worms.

The severity of heartworm infection depends on four things:

  1. Number of worms present
  2. The immune system of the infected dog
  3. Duration of the infection
  4. Activity level of the dog

If the heartworms are not killed from treatment and medication, the constant irritations they inflict lead to scarring and reduced flexibility of the blood vessels.  Heartworms can live in a dog for over five years however small dogs do not tolerate heartworm infections or the treatments as well as large dogs.  This is due to the fact that small dogs have smaller heart chambers and blood vessels so they can tolerate fewer worms meaning less damage.


The severity of the signs depend largely on the dog’s activity level however common signs of heartworm infection include:

  • Coughing
  • Trouble breathing
  • Intolerant to exercise or walking
  • Discoloration of the skin – often blue or purple in color
  • Fainting
  • Nosebleeds
  • Swelling of the abdomen

Dogs that are sedentary show very few signs or none at all whereas active dogs like hunters or performance dogs show more dramatic signs of infection.  Most Veterinarians use the antigen detection test to verify infection of heartworms – it is considered to be the most sensitive method available however some use blood tests and chest x-rays based on the history of the dog.


It is important for your Veterinarian to select the most appropriate and effective treatment which will depend on several factors including if the dog is dealing with any other medical issues.  The only drug available to kill heartworms is an arsenic compound given in a 2-dose or 3-dose protocol intravenously in the dogs back, alternating sides between treatments.  Some dogs experience pain, swelling and soreness with movement or in rare instances, abscess at the injection site.

The 2-dose protocol is done 24 hours apart while the 3-dose treatment involves an initial injection followed by 2 more injections one month later, 24 hours apart.  Many Veterinarians prefer the 3-dose method because it is safer for the dog and more efficient in killing all parasites and the heartworms.

Once the heartworms are dead, they can cause some respiratory problems, especially if it is too difficult to keep the dog quiet and inactive.  This can last for a few days to a few weeks.  Some dogs experience coughing, spitting up blood, rapid or labored breathing, fatigue and fever.  Oxygen and anti-inflammatory drugs usually work well to alleviate these problems within 24 hours after treatment however the key is to keep the dog confined in a calm environment during the entire process as much as possible.  Once treated, they should be placed on heartworm preventative and tested after six months to make sure all the heartworms were killed.  If they do test positive again, then another round of treatment may be necessary.


There are several medications that are both safe and effective in preventing heartworm infection beginning at 6-8 weeks of age. In older dogs, it is necessary to do an antigen test to make sure they are not already infected before starting a preventative program and retested 6 months later to ensure the dog is not infected with heartworms.

The most important thing pet owners can do to prevent heartworms is administer the medication at the right time each month.  If forgotten, it can have serious consequences for the dog.  It is smart to post a calendar on the refrigerator or wherever you can to mark the dates each month.  If a dosage is missed, call your Veterinarian right away for recommendations regarding administering the proper medication.  Your dog depends on you to keep them protected from heartworms.

Photo:  Courtesy of Tom via Flickr (CC BY ND )

Sherry is a Nutritionist, Writer, National Speaker, Ghostwriter of books for Natural Medicine Doctors and an Author of 2 healthy cookbooks. She is a Nationally Certified Fitness Instructor and Personal Trainer in Pilates, Yoga, Body Pump, STEP and Aerobics with over 20 years experience. She served as the On-Air Nutritionist for QVC television in the United States and the UK and hosted her own weekly “Healthy Living” segments for PBS. Sherry is passionate about helping animals and worked with “Helping All Animals” in Palm Springs, CA. in their rescue efforts, and is a member of the ASPCA and the Humane Society of the United States. Her experience working as a Veterinarian’s Assistant for many years’ aids in her passion for helping animals lead healthy and happy lives. For more information on Sherry, visit www.sgtotalhealth.com or write to Sherry at sgfit12@aol.com - call 517.899.1451