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Heartworm Disease

Updated: Jan 9, 2021

Heartworm is contracted when a mosquito feeds on your pet after feeding on a pet that is infected. Over a period of a few months, the parasite grows and enters the bloodstream. Eventually, the adult worms live in the blood vessels on the right side of the heart. As adults, the worms can reach 12 inches in length.


There is a difference between a dog being infected with heartworms and a dog suffering from heartworm disease. Here is a link to a useful website that discusses the common symptoms of this disease as well as the underlying damage that is done when a dog is infected.

Wendy Brook’s Pet Healthy Library on Heartworm disease.


Heartworm prevention is accomplished by providing a safe and effective dose of Heartgard Plus (or some other Heartworm Preventative) to dogs every 30 days. In areas like Chandler and Gilbert Arizona, the winter temperatures are mild enough that mosquitoes can easily survive the winter so pets in this part of the Valley need to be on preventative medications every 30 days all year long. Prior to starting prevention, and annually after giving Heartgard, we require testing to make sure that the pet is still free of this parasite.

There are two basic tests we perform to detect Heartworm disease in dogs. The first test is submitted to a commercial laboratory and the results are usually received the next day. The second method of testing for Heartworm disease is done with an “in-house test” and tests for Ehrlichia canis (Tick Fever) and Lyme disease as well. This can be done while you wait for the results (in about 15 minutes).

Why do I need to test if my dog has been on Heartworm prevention?

This is a common question. An  understanding of the method of transmission and treatment for heartworm disease is needed. When a pet takes Heartgard, a couple of the larval stages of heartworm disease are treated. This is a time-sensitive process. If Heartgard is given too late, and if an infected mosquito bites your pet at just the right time, the medicine will not be effective in preventing a full-blown infection. Missed or delayed doses of medicine are quite common. Even some of our extremely compliant clients have found an extra dose or two at the end of 12 months, causing them to wonder how the medication was missed. The current recommendations of the American Heartworm Society are that pets be tested annually. We recognize their guidelines as the most authoritative in our profession. In addition, if a pet ever does develop Heartworm disease and the client has been following our protocol faithfully (monthly dosing and annual testing), the cost of treatment for the infection is primarily borne by Merial, the manufacturer of Heartgard and Frontline. Even though this is unlikely, it gives us peace of mind that they stand behind us and your pet.

Another benefit in using Heartgard on a regular basis is the fact that you are simultaneously treating dogs for roundworms and hookworms. These are common intestinal parasites and can pose a health risk to other dogs and even to family members. Zoonosis means a disease contracted by contact with animals. Human infection with roundworms and hookworms are all-too-common in the United States as well as around the world.  Here are links to the Center for Disease Control’s website for more information on Roundworm and Hookworm infections.

Below are some useful links for more information about heartworm disease.

American Heartworm Society

Merial’s Heartworm information

Council on Parasite Control’s Heartworm information

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