10 Fun and Not-So-Fun Facts About Heartworm

Monthly heartworm prevention is not a waste of money, and it is not unnecessary.

Bob suffered from one of the biggest health problems facing Southern stray dogs: heartworm. This is a real threat not only in the South, but all over the United States.
Bob suffered from one of the biggest health problems facing Southern stray dogs: heartworm. This is a real threat not only in the South, but all over the United States.

Good clients came in a couple of weeks ago with a black Lab shipped up from the South that they were “fostering.” Southern “Bob” was in the building!

And this great, lick-your-face Bob was only being fostered at this point. You know how that’s going to go. Before you knew it, Bob had his own bed, his own dish; in other words, Bob was here to stay. He was trading in his shrimp and grits for some clam chowdah.

Bob is a great dog, suffering from one of the biggest problems rampant in Southern stray dogs. He has heartworm, and the rescue group had been treating him by the “slow-kill” heartworm treatment method.

After a lot of discussion weighing the pros and cons, the health risks and the money, the good people took my advice and treated Bob with the quicker, more effective heartworm treatment, so Bob will get healthier faster and with less damage to his heart. Bob is a happy dog today. Here is some info on this awful parasite.

1. Heartworm is present in all 50 states.

This report (PDF) of heartworm cases in 2013 from the American Heartworm Society shows that no state is immune to heartworms.

2. Heartworm is a real threat if your dog is not on preventative.

Uneducated clients have accused me of recommending heartworm preventative in order to make money. This is not only insulting but misinformed.

Vets don’t like to treat heartworm. They want to prevent it. Period.

3. In endemic areas such as the South, almost 1 out of every 2 dogs will get heartworm if not on preventative.

The risk goes up the warmer the climate and the prevalence of mosquitoes.

4. Heartworm preventatives may not work like you think they do.

Heartworm medication works to kill all stages of an early infection from the previous month. Dogs in warm and temperate climates are at risk for heartworm all year round. But in New England, for example, most mosquitoes usually leave with the Halloween candy, and many people stop giving heartworm preventative too early.

Say you live in a colder climate and you stop giving medication on October 1, and your dog gets bit by a Halloween mosquito. Your dog will develop heartworm. Had you kept the dog on preventative through the winter months, this wouldn’t be a problem.

With temperatures trending upward nowadays, a few tough mosquitoes hang around for Thanksgiving and Christmas anyway, so most dogs should be kept on heartworm prevention year round — even in New England.

Click here to view a PDF explaining the life cycle of heartworm once a dog or cat has been infected by a mosquito.

By: Rebecca Wynn/USFWS
The hot and humid southern climate increases heartworm risk. By: Rebecca Wynn/USFWS

5. Most heartworm preventatives also give your dog extra protection against other parasites.

I used to see so many roundworms and hookworms in poop. Now, these pesky diarrhea-causing parasite eggs in fecal samples are rare, thanks to heartworm meds.

6. The earlier the infection, the better the prognosis.

This is true even though a young dog can have a serious infection if the environment has been ripe for heartworms to multiply like crazy. This is why your vet will want to “stage” a heartworm infection with bloodwork and X-rays.

Our friend Bob looked great on the outside, but his chest radiographs revealed that the heartworms had already caused some damage.

7. The slow-kill method has drawbacks.

Information is rampant in the dog world that if you just keep your heartworm-positive dog on preventative, the infection will go away. This is the slow-kill method. Beware of this information.

It can take up to 2 years for the adult heartworms to die off in your dog’s body. During that time, the worms are still circulating and causing damage to your dog’s heart and pulmonary function. This could cause unnecessary damage and debilitation.

For Bob, who already had some pulmonary and heart damage, I don’t think this would have been a good idea.

8. Resistance to heartworm is developing.

Using heartworm preventative as treatment has been a life-saver for many stray dogs in the South, like Bob, who have been helped out by rescue groups. But this has caused resistant strains of heartworm. This stinks!

In the Mississippi Delta, where heartworms are in paradise and many heartworm-positive dogs are being treated by this slow-kill method for financial reasons, there are new strains of heartworm arising that are resistant to the preventative drugs we have now. When parasites get crafty, they learn to survive.

These resistant strains of heartworm are eventually going to pack their bags, leave the Mississippi Delta, and look for new neighborhoods to infect.

9. But heartworm treatment is scary.

There is risk, but we know much more about the treatment and its risks now that protocols have been greatly improved.

We have your dog on protective medications before, during and after the treatments; insist on complete rest during the treatment period; and often use a protocol that breaks up the injections into 3 separate doses. If you follow directions, the risks are minimal.

10. Treatment is definitely not cheap, but there are ways to save money.

Okay, it can be expensive, but the treatment protocol does give you time to budget if that is possible.

Here’s my advice: Do some price-comparing at different hospitals if finances are a very difficult issue for you, or if you think your estimate is exorbitant. There may be more reasonable pricing around. The treatment is spread over several months, so you don’t get a huge whammy all at once.

From a recent surgery published in Clinician’s Brief, the heartworm staging ran $150–$500, and the medication given in a series of injections, which is dependent on the protocol used and the size of the dog, was $200–$700.

Cost of Heartworm Treatment (1)

So, dogs contracting heartworm is an unhappy topic. My goal here is to impress upon you that your monthly heartworm prevention is not a waste of money or unnecessary. Treatment is out there and can restore the vast majority of heartworm-positive dogs to a happy and healthy life.

Bob went home yesterday to enjoy the upcoming holidays.


This pet health content was written by a veterinarian.

Dr. Debora Lichtenberg, VMD

View posts by Dr. Debora Lichtenberg, VMD
Dr. Debora Lichtenberg, VMD, is a small animal and exotics veterinarian who has split her time between a veterinary practice in Pelham, Massachusetts, and her studio in New York City. Dr. Lichtenberg is a graduate of the University of Pennsylvania School of Veterinary Medicine with 30 years of experience. Her special interests are soft tissue surgery and oncology.

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