News & Articles

Why Year Round Heartworm Prevention is so Important!

Posted on: March 15th, 2016

heartwormDo you know what exactly heartworm disease is? It should be pretty simple, the name essentially describes the condition, but how many of you know how your pet can get heartworm?

Quick quiz!
Pick the correct answer for the question below

Dogs get heartworm disease from

A. fecal – oral contamination
B. ticks
C. mosquitos
D. drinking contaminated water

As the saying goes, when in doubt choose “C.” The mosquito plays an important role of transmission of immature heartworm larvae from an infected host to an uninfected animal. So where does the mosquito get these larvae from? The most common reservoir for the disease in nature is found in wolves, coyotes, foxes, and sea lions (for those pacific coast snow birds out there). If a mosquito feeds on an infected coyote then travels and feeds off an uninfected dog, the heartworm larvae will be passed into the bloodstream of its new host. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.

How big can these worms get? One inch? Four inches? How about this astounding fact, they can grow as long as 12 inches!! Picture a foot long worm attached to the wall of your pet’s vein – a very important vein that connects the blood flow from the lungs back to the heart. Now imagine up to 30 of these worms living in a vein as big as a pinkie finger in some of the average sized, 60 pound Labrador. These worms essentially cause the heart to work harder to pump blood, while acting as a parasitic clot in the pulmonary vein. In people this type of obstruction would require cardiac bypass surgery to help clear the clot.

So WHY does your veterinarian recommend year round heartworm prevention for those of us living in the Midwest, with our long bitter winters? Who knows what temperature it takes to restart the mosquito lifecycle? 50 degrees. Since we live in the Midwest, and we ALL know how our winters can fluctuate in temperature, we as well as the mosquitos, celebrate a beautiful 50 degree day in November, December, January, maybe not February (only if we’re lucky), and possibly March. That’s all it takes. How many of you have ever seen a mosquito flying around the ceiling of your bedroom or kitchen in the middle of winter? Our exposure to mosquitos are present 12 months out of the year, and it does not matter if you live north, south, east, or west of the Mason-Dixon line. Our practice has had several heartworm positive dogs this past year – it’s in our area.

The arguments that are given to your veterinarian about how your pet isn’t an outdoor pet aren’t valid excuses anymore. Generally speaking, owners take their dogs outside at least twice per day so that they can relieve themselves despite where you live. Pets that live in the city are at equal risk for contracting heartworm disease as dogs that live in the country. We’ve got to start putting ourselves in our pet’s shoes. I know during the prime mosquito season, the 60-second trip that I make from my front door to the mailbox in back, I’m lucky not to have gotten a mosquito bite. So why isn’t it possible for our pets to be susceptible to these bug bites in their brief trips outside?

I always love this statement when I’m in an exam room – we’ve never given our dogs heartworm prevention and if we get heartworm, we’ll just treat – That’s a great attitude, but the cost and risks of treatment greatly outweighs the benefit of prevention. The best medicine is preventative medicine – Treatment for a heartworm positive dog generally costs the same amount as 7-10 years of year round heartworm prevention. Even with treatment there is no guarantee that your pet won’t have life long damage to the heart or even survive the treatment (we worry about worms becoming dislodged and acting as a clot in the heart, or elsewhere in the body)– the total time of treatment from start to finish is almost 10 months!

Another quiz question!

True/False – Cats and dogs can both get heartworm disease.

True! In fact it only takes a fraction of the amount of adult heartworm present in a dog to affect a cat! Cats are what we would call an “atypical” host for heartworm disease – Often times heartworms do not survive to the adult stage, however if they do, as little as 3 mature heartworms can cause serious heart and lung changes in our feline patients! Even immature worms can cause damage to the hearts and lungs in the form of a condition know s as heartworm associated respirator disease (HARD). Unfortunately the medication we use to treat heartworm disease in dogs cannot be used in cats, so it is equally important to keep our outdoor feline patients on year round heartworm preventative as this is our only way to protect our cats from contracting heartworm disease? Even ferrets can be affected by heartworm disease.

For more information on heartworm disease, please check out the American Heartworm Society’s website at

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