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What You Need to Know about Heartworm Disease

The Disease:

Heartworm disease is a very serious and potentially life-threatening illness that results from an infection of heartworms (Dirofilaria immitis).  In the United States, dogs are most commonly infected, but cats and ferrets may also become infected.

When a mosquito bites a dog that is infected by heartworms, the mosquito picks up baby worms (called microfilaria) that are circulating in the infected dog’s blood stream.  A few weeks later, when the mosquito bites another dog, it may then transmit the growing heartworms into the new dog.  If the dog is not treated or has not been receiving heartworm preventative, it can develop a heartworm infection.

As you would expect, dogs are more likely to get heartworm disease in warm weather when mosquitoes are present.

Diagnosis and Treatment

Canine heartworm disease is diagnosed with blood tests and chest x-rays.  Treatment early in the infection is very important because the longer a dog is infected, the more worms they will have when treated.  The long, stringy, spaghetti-like mass of worms can cause extensive damage to the heart and lungs.  For this reason, veterinarians recommend yearly heartworm testing for dogs.

There are a number of treatments for heartworm disease; unfortunately, all of can also be hard on your dog.  Dogs can develop complications (ranging from mild to life-threatening) from the medications, or as a result of the dying heartworms still inside the dog’s body.


The best option for dealing with heartworm disease is to prevent infection in the first place, because it can be very difficult to treat. Even if treated successfully, affected dogs can suffer irreversible damage to their heart and lungs.

Thankfully, there are a number of very safe and effective medications available to prevent heartworm infections.  Please discuss these options with your veterinarian!

For Additional Information:

American Heartworm Society

Veterinary Partner

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