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Everything you need to know about heartworm in dogs & cats

In this episode:

Pet Joke of the Week: The Dog Dictionary Part II.
Headline pet news: Skittish cat stuck in airport x-ray machine/L.A. cracks down on pet licensure.
Personal comment: Everything you need to know about heartworm disease.

Transcript of personal comment from this episode of The Web-DVM:

My personal comment tonight deals with a topic that far too many pet owners surprisingly know too little about, heartworm disease. This is clearly evident in even my home state of Florida, the perennial heartworm capital of the USA, where veterinary clinics accomplish only 20% compliance for heartworm preventive in canine patients, and less than 8% for feline patients. So tonight I will tell you everything you need to know about heartworm disease in dogs and cats so that you may take it a seriously as it should be taken.

The heartworm is a parasitic worm that colonizes the heart of the definitive host, the domestic dog (I will address its effect on cats later). It is transmitted by the bite of mosquitoes, as the microscopic infective larvae live in the gut of the mosquito. Once in the bloodstream of the dog, the larvae mature to adult worms and then colonize the right ventricle of the heart. The worm also spends part of the time in the lungs, traveling back and forth from the heart and lungs via the pulmonary artery.

The result of the presence of these worms is inflammation of the heart muscle pulmonary vessels leading to scar tissue formation. The mass of the worms also leads to less ability of the affected heart chambers to completely fill, making the heart have to work harder to pump the same amount of blood over time. The combination of these factors leads to heart enlargement, narrowing of the heart chambers, leaky and scarred heart valves and, eventually, heart failure. At the level of the lungs, the worms cause allergic reaction, leading to inflammation, scarring, and chronic bronchitis, all this as the result of a bite from an infected mosquito to an unprotected pet.

The typical clinical heartworm disease patient presents with a severe, sometimes debilitating cough. Diagnostic findings often include fluid in the lungs and enlarged heart visible on chest x-rays, electrical disturbances of the heart in EKG findings, anemia, and high white blood cell count.

For cats, the disease is a little different, less common, but a big problem make no mistake. Since they are not the definitive host of the parasite, worms do not thrive well in the feline body, colonizing the heart as they do in dogs. Instead, the feline immune system attempts to wall off the worms, sometimes leading to the formation of cysts in the brain, lungs, kidneys, and liver. The presence of these cysts causes disease of the organs in which they form, which can include seizure, liver failure, kidney failure, or sudden death. So in cats, while heartworm is not as common as the dog, it can be every bit, if not more, dangerous.

Treatment of heartworm disease is accomplished with an arsenic known as melarsamine, administered by 2 deep intramuscular lumbar injections, each given 24 hours apart. Arsenics can cause secondary liver and kidney toxicity and dangerous allergic reactions to the dead heartworms following treatment can severely compromise the patient’s breathing and circulation. In cats, sensitivity to the arsenic and tendency to throw clots after treatment, has led to death by complication rates as high as 60%-70%, making treatment not an option for cats, but prevention all the more important.

With regard to prevention, this is the easy part. Not only are heartworm preventive medications administered only monthly, but the actual drugs have very favorable safety profiles and are also highly effective, with good client compliance obtaining protection rates as high as 95% or better. For a list of heartworm preventive medications, how they are administered, and which other parasites they protect against, see the Supplemental Information section of this post. If you are not watching this video from the blog or one of our blog syndicates, our blog home page is
For our last heartworm item, I will address the most common question pet owners present to me on this topic, which is, “If my dog is on heartworm prevention year round, why you make me get a heartworm test every year to get more heartworm medication?” There are actually a few reasons for this requirement.

First, while these preventives are very effective, they are not infallible, with failure with treatment still possible as much as 5% of the time. Keep in mind that heartworm preventive medications only kill immature infective larvae, but if they can manage to survive to adulthood, preventives won’t even touch them. Second, pets can engage in behavior that makes the heartworm preventive less effective or even useless, such as vomiting up the pill, spitting the pill out, or not having it absorb optimally because of diarrhea or eating grass. The third reason is human error – it is not uncommon for pet owners to be late on their pet’s heartworm preventive, even under the best intentions to be on time. For these reasons, some manufacturers will not guarantee the safety and effectiveness of their products without up to date yearly heartworm tests on record. On the other hand, if you comply and your pet still gets heartworm, they will pay for all diagnostics and treatments necessary to clear the heartworms and make your pet well.

That is our show for tonight, please help us to continue our discussion by posting your comments at our blog at But, we believe in being democratic here at The Web-DVM, so we are making a little change. I will still address comments posted at our blog on my live BlogTV broadcast sunday night 9:00 pm EST, but I will now also include comments from our faithful commenters at YouTube, as well as those posted by our new friends at the HubPages. I will not have time to address them all, but will be selecting the most thoughtful, unique, and/or well written comments for inclusion in my live show.

Please join us next week when I will be discussing, Spork, the vicious wiener dog. Folks, you are not going to believe this!

Supplemental Information:

Heartworm preventive products

Heartgard – Monthly chewable flavor tabs; protects against heartworm disease, three species of intestinal parasites.

Interceptor – Monthly tablet; protects against heartworm disease, four species of intestinal parasites.

Sentinel – Monthly tablet; protects against heartworm disease, four sepcies of intestinal parasites, and fleas. In my experience, not great against fleas.

Advantage Multi – Monthly topical treatment; protects against heartworm disease, four species of intestinal parasites, fleas, ear mites, and mange.

Revolution – Monthly topical treatment; protects against heartworm disease, four species of intestinal parasites, fleas, and mange. In my experience, not great against fleas, and has been reported to be inconsistent in protecting against heartworm in dogs.

One thought on “Everything you need to know about heartworm in dogs & cats

  1. Stephen Bush says:

    The dogs are more susceptible to heartworms than the cats. These noninfectious worms are killing. In fact, if these blood parasites present in your pet’s body for a longer period of time, then it can make your pet extremely sick. I would like to tell that it’s an ailment which is mainly caused by the bloodsucking worms that can live in the lungs, heart, and blood vessels. There is no such medium through which this virus can spread except by an infected mosquito bite. Intolerance to exercise, cough, breathing difficulty, cardiac problems are the symptoms by which you can assume that these parasitic worms are present in your pet’s body. Well, in that, it is crucial to take a reputable vet consultation if your four-legged family member is exhibiting any of these signs.

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