Heartworm (Dirofilaria immitus) is a devastating and potentially fatal condition that affects thousands of dogs in North America each and every year. The infection starts when an infected mosquito bites a dog and subsequently larvae are injected into the dog where they proceed to the large arteries surrounding the heart and develop into adult heartworm. That process takes approximately 6 months to complete and the infection will progress if not treated, as adult Heartworms will produce offspring (Microfilaria).

Clinical Symptoms

Clinical signs are highly variable and based on the size of the dog and size of the worm burden. Typically, dogs with Heartworm disease will show signs of mild cough exercise intolerance and fatigue, weight loss and decreased appetite. Later stages of the disease would include heart failure and possibly death if not treated. It is important to note that dogs early in the course of infection may not have any symptoms at all.

Testing & Prevention

As Heartworm infection does not show clinical signs in the early stages, it is very important to screen/test your dog periodically as recommended by your veterinarian (depending on location, most veterinarians will recommend yearly testing/screening). Testing is the only way to confirm infection and therefore determine which treatment plan should be utilized to prevent the progression of the disease. Most heartworm tests are quick, easy to perform and cost effective.

The good news is that there are many safe Heartworm preventative medications available. Most medications are given on a monthly basis throughout the Heartworm season (varies based on location) and are either administered via a flavored oral tablet or a topical application that is applied directly on the skin. While using a preventative medication, it is highly recommended to perform a Heartworm test prior to starting the medication as if there is an active Heartworm infection the medication could result in shock and a life-threatening situation.

It is important to note that dogs on preventative medication may also test positive and this would be a result of either drug failure or improper administration.


Most stages of Heartworm infection can be treated successfully. Adult Heartworms are killed using a specific medication that is injected into the deep muscle layers. The treatment usually consists of multiple injections at different times and is costly and hospitalization is usually required so that your pet may be monitored closely for any signs of reaction.
All vets would agree – you would much rather prevent Heartworm infection than treat for it!

Summary Notes

•    Heartworm infections are a result of being bitten by an infected mosquito and pose a serious risk to your dog’s overall health.
•    Clinical signs are highly variable and may not be noted for many months after infection.
•    Make sure to test your dog for Heartworm yearly, or as recommended by your veterinarian.
•    Most preventative medications will also prevent fleas, intestinal worms and other parasites.
•    Heartworm treatment is possible but is costly and may be associated with many adverse effects.
•    Frequency of testing and the dosing schedule of preventative medications are usually a result of geographical location.
•    Make sure to discuss an appropriate preventative plan with your veterinarian.

Dr. Brad Hinsperger, DVM
Kingsdale Animal Hospital