Heartworm In Dogs – What You Must Know

The very name of the ugly disease tells it all…heartworm…an infestation of parasitic worms that get into your dog’s heart and are transmitted through the bite of the mosquito. Yuck! Here’s what you need to know to protect your dog.

Years ago, it was a common practice to give heartworm medication to dogs in just the warmer months when mosquitoes were out and about, especially in the Northern U.S. where winter temps presumably took away all risk for infection. 

And yes, the Southern regions of the U.S. have long been associated with the disease, but now heartworm has been reported in all fifty states and climate and environmental change has contributed to the spread of the disease. Plus, the transportation of shelter and rescue animals all over the country has likely contributed to the spread of heartworm disease as well.

Heartworm (scientific name Dirofilaria immitis) does not spread from dog to dog, but must travel by what is known as an “intermediary” in this case the mosquito. The mosquito bites an already infected animal (a dog, coyote or fox) and picks up the microscopic worms on their bloodsucking mouthpieces. Then when the mosquito lights on your dog to get another blood meal, the parasitic worms are transmitted into your dog’s bloodstream

From your dog’s blood, the worms roam through the bloodstream and in about two months settle in the right side of the heart, where they continue to grow. The worms reach maturity in about six months but can live on in your dog’s body for an alarming seven years and reach a length of up to a foot long! They are constantly producing offspring and after a year of infestation, your dog may be infested with hundreds of these worms.

The worms do their damage by producing inflammation and can directly injure the heart as well as your dog’s arteries and lungs. 

Heartworm disease is divided into four classes, depending on the severity of the symptoms:

Dogs with Class one have either no symptoms or just exhibit a mild cough. With Class Two, the dog will start to become mildly intolerant of exercise and then develops a persistent cough. With Class Three disease, things become much more serious: exercise becomes harder, there are abnormal lung sounds and a weak pulse. 

The dog may faint, lose weight, have a decreased appetite and get a swollen belly due to fluid accumulating secondary to heart failure. Class Four disease is the most series and is life-threatening. This is known as caval syndrome which is accompanied by distressed and labored breathing, cardiovascular collapse, coffee colored urine and eventually organ failure and death.

So you can see why you want to do everything in your power to keep your dog from getting this disease in the first place. Fortunately the disease is prventable and is treatable if caught in time. In our next post, we will discuss diagnosis and treatment optoins. 

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By Ellen Britt

Dr. Ellen Britt has loved dogs since she was a child. She is particularly fond of the Northern breeds, especially Alaskan Malamutes. Ellen worked as a PA in Emergency and Occupational Medicine for two decades and holds a doctorate (Ed.D.) in biology.

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