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Heartworm disease overview
Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs, cats and other species of mammals. Heartworms are classified as nematodes(roundworms) and dogs and cats of any age or breed are susceptible to infection. Heartworms are spread by mosquitos. When a mosquito bites an infected dog, the mosquito ingests a baby form of the heartworm called a microfilaria. The microfilaria then develop into "L3" larvae inside the mosquito and this is the larval stage that will then infect the next dog the mosquito bites. Once inside the dog, the larval heartworm develops through an "L4" stage and into an adult heartworm over a time period that can take as little as 52 days or as much as 120 days.
There are 2 ways to test for heartworm disease and both tests will only be positive if a dog has ADULT female heartworms present. There is currently no test for infection with just L3 to L4 stage larvae. The ELISA (antigen) test that we run at least once/year tests for the presence of adult female heartworms. The microfiliaria test that used to be the only test available looks for circulating "baby" heartworms. The ELISA is considered a better test because some dogs can have just female heartworms (called an "occult" infection).
Heartgard (and other preventives including Revolution, Iverhart, etc) contains a medication that kills ONLY the L3-L4 stage of the heartworm. Heartgard does not prevent a dog from becoming infected with heartworm larvae but it does prevent the larvae from developing into adults if given once every month. Heartgard does not kill juvenile or adult heartworms and this is why it is so important that each dog receives the preventive at the same time every month. On the day Heartgard is given, it kills any circulating L3-L4 larvae. Heartgard is only in the dog's circulation for 24 hours so it will not continue to kill off heartworm larvae after 24 hours. For the next 30-31 days after the last dose of Heartgard was given, the dog can become infected again with L3 larvae which will then be killed off by the next dose of Heartgard if it is given before the larvae develop past the L4 stage.
What Happens if I Miss a Dose?
Because Heartgard only kills the L3 and L4 larvae on the day it is given, a missed dose could lead to adult heartworm infection. On average it takes 2-3 months for the L3 larvae to develop into adult heartworms but it can take as little as 52 days. That means that if a dose is missed at day 30 and is not given again until day 60 there may already be juvenile and/or adult heartworms that will not be killed by Heartgard and will require treatment for heartworm disease.
Dr. Sarah Marzke
Topaz Veterinary Clinic
1804 East Southern Avenue, Suite 9
Tempe, Arizona 85282