Heartworm Disease

Heartworm infection in dogs has been diagnosed around the globe. The domestic dog and some wild canids are natural hosts of heartworm disease. The worms are called “heartworms” because the adults live in the heart, lungs, and associated blood vessels of an infected animal.    

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How is heartworm disease transmitted? 

The worms are spread through the bite of a mosquito. The dog is the definitive host, meaning that the worms mature into adults, mate, and produce offspring while living inside a dog.  The mosquito is the intermediate host, meaning that the worms live inside a mosquito for a short transition period in order to become infective (able to cause heartworm disease).

Heartworm disease is not contagious, meaning that a dog cannot catch the disease from being near an infected dog.  Heartworm disease is only spread through the bite of a mosquito.

The Heartworm Life Cycle

In an infected dog, adult female heartworms release their offspring, called microfilariae, into the dog’s bloodstream.  When a mosquito bites the infected dog, the mosquito becomes infected with the microfilariae.  Over the next 10 to 14 days and under the right environmental conditions, the microfilariae become infective larvae while living inside the mosquito.  Microfilariae must pass through a mosquito to become infective larvae.  When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite wound.  In the newly infected dog, it takes about 6 to 7 months for the infective larvae to mature into adult heartworms.  The adult heartworms mate and the females release their offspring into the dog’s bloodstream, completing the lifecycle.  

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When should my pet be tested?

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How do I prevent heartworm disease? How does prevention work?

All dogs should be tested annually for heartworm infection, and this can usually be done during a routine visit. Following are the guidelines on testing and timing: 

Puppies under 7 months of age can be started on heartworm prevention without a heartworm test (it takes at least 6 months for a dog to test positive after it has been infected), but should be tested 6 months after your initial visit, tested again 6 months later and yearly after that to ensure they are heartworm-free.

Adult dogs over 7 months of age and previously not on a preventative need to be tested prior to starting heartworm prevention. They, too, need to be tested 6 months and 12 months later and annually after that. 

You need to consult your veterinarian, and immediately re-start your dog on a monthly preventative and then retest your dog 6 months later. The reason for retesting is that heartworms must be approximately 7 months old before the infection and be diagnosed. 

Whether the preventive you choose is given as a pill, a spot-on topical medication or as an injection, all approved heartworm medications work by eliminating the immature (larval) stages of the heartworm parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the following larval stage that develops inside the animal.

All dogs should be tested annually for heartworm infection, and this can usually be done during a routine visit. Following are the guidelines on testing and timing: 

Puppies under 7 months of age can be started on heartworm prevention without a heartworm test (it takes at least 6 months for a dog to test positive after it has been infected), but should be tested 6 months after your initial visit, tested again 6 months later and yearly after that to ensure they are heartworm-free.

Adult dogs over 7 months of age and previously not on a preventative need to be tested prior to starting heartworm prevention. They, too, need to be tested 6 months and 12 months later and annually after that. 

You need to consult your veterinarian, and immediately re-start your dog on a monthly preventative and then retest your dog 6 months later. The reason for retesting is that heartworms must be approximately 7 months old before the infection and be diagnosed. 

DISCLAIMER: All information has been taken from the American Heartworm Society website and from U.S Food and Drug Administration website.