Heartworm disease in dogs is a serious, potentially fatal health condition. It can cause significant lung disease, heart failure, and other organ damage in your dog. The impacts of heartworm disease can be devastating for a dog and cause a lot of anguish for the pet parent. Heartworm is easy to prevent but difficult to cure. If your dog contracts heartworm disease the chances of death are high.  Heartworms in dogs is widespread and present in all 50 states.


Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. These worms can live inside, and be transmitted by, as many as 30 different species of mosquitos.  They are carried by the mosquitos and spread when a dog or other pet is bitten by an infected mosquito. The Microfilariae, an early lifecycle stage of this parasitic worm, must pass through a mosquito in order to become infective larvae. Once transmitted to a dog through a mosquito bite, they will mature into adults within about 7 months.  They will then mate and reproduce while living inside the dog.

Heartworm worms are called “heartworms” because the adult worms live inside an infected dog's heart, lungs, and corresponding blood vessels.  They can live inside your dog for as many as 7 years! 


Adult heartworms look like strands of spaghetti, with males reaching about 4 to 6 inches in length and females reaching 10 to 12 inches in length.  On average, dogs with heartworms can host about 15 worms, but there can be as many as 300 worms inside a dog's body!

Heartworms are not necessarily a threat all 12 months out of the year if the dog doesn't live in a warm climate.  However, if you travel with your dog to an area where the heartworm season is in progress, they can be at risk for heartworms if they are not taking heartworm preventative medication. 

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Heartworm disease, Heartworms, heartworm in dogs, common dog health problems, dog health, pet health, dogs
Heartworm disease; hard to treat, easy to prevent. It can be fatal to dogs

Heartworm disease is not contagious between dogs. A dog can't catch the disease from an infected dog.  The disease is only spread through the bite of a mosquito.  


Veterinarians routinely perform annual blood tests called the Heartworm Antigen test.  The test detects specific heartworm proteins, or antigens, that are released by adult female heartworms into the dog’s bloodstream.  Antigen tests detect infections with one or more adult female heartworms. The earliest the heartworm proteins can be detected in a dog’s bloodstream is about 5 months after it is bitten by an infected mosquito.  

A heartworm antigen test is usually required before a Veterinarian will prescribe heartworm preventative medication. It's important to perform the antigen test prior to administering heartworm preventative medication.  If your dog takes heartworm preventative medication while they have heartworms, they can suffer dangerous effects.

If a heartworm-positive dog begins taking a heartworm preventive medication, the dog will remain infected with adult heartworms. Heartworm preventives do not kill adult heartworms, and giving heartworm preventive to a dog infected with adult heartworms can be harmful or deadly.  If the offspring of an adult female heartworm are in the dog’s bloodstream, the preventive medication may cause the heartworm offspring to die suddenly, causing the dog to go into shock and may even result in death.


Heartworm disease can be a silent killer because the presence of heartworms in a dog can go undetected for a long time, depending on how many heartworms are in the dog's body and how long the dog has been infected for.  Obvious symptoms may not be present until the worms have gotten a significant foothold inside the dog's body.  Some symptoms of heartworm disease to be on the lookout for are:

🕱 Coughing: Occasional in mild cases, and persistent in more severe cases

🕱 Fatigue after moderate activity

🕱 As the disease progresses, the dog may have trouble breathing and show signs of heart failure

🕱 Weight loss and decreased appetite usually indicate there is a health concern for your dog. It can also indicate your dog has heartworms

If you suspect your dog may have heartworm disease, contact your Veterinarian immediately.


The treatment of heartworm disease in dogs doesn't always have a high success rate. It's also hard on the dog and can be costly.  There are several treatments available through your Veterinarian. One such treatment is a drug that contains arsenic and kills adult heartworms in dogs. It's given by deep injection into the dog's back muscles.   There is also another, less invasive drug that kills the Microfilariae (adult female heartworm's offspring) in the dog’s bloodstream.  This drug is a topical medication applied to the dog’s skin.  If your dog contracts Heartworm Disease, consult your Veterinarian for the best treatment option.

As you can probably tell, Heartworm treatments are potentially toxic to the dog’s body and can cause serious complications, such as life-threatening blood clots in the dog’s lungs.  Treatment is expensive because it may require multiple visits to the veterinarian, blood tests, x-rays, hospitalization, and a series of treatments.  Clearly Prevention is the best option! Heartworm preventative medications are available in several different forms, and there are a number of brands available.  However, you will need a prescription from your Veterinarian.


The American Heartworm Society estimates that roughly 1.1 million dogs were infected with heartworm disease in 2019. This estimate was based on thousands of U.S. Veterinary practices that report heartworm testing data to this organization. Note that not all dog and cat owners bring their pets to a Veterinarian for heartworm antigen testing. In fact, some pet owners don't bring their pets to a Veterinarian at all, so this number is probably somewhat conservative.


Heartworm disease is a serious matter and can have devastating repercussions.  Although treating heartworm disease in dogs is difficult and risky, preventing your dog from getting heartworm disease is simple!  There are a number of treatment options available, and most come in a flavorful chewable tablet. You will need a prescription from a Veterinarian to obtain preventative heartworm medication.  Considering the potential risks posed by heartworms it's well worth paying a few dollars a month to protect your pet from heartworm disease. There are a couple of natural prevention products but I haven't had enough input or any experience using them to opine on their effectiveness. Talk with your Veterinarian about options.


The Heartworm lifecycle in dogs can be a bit complicated. I've shared a lifecycle chart below that helps clarify the steps in the lifecycle;  

⮚ A mosquito bites an infected dog, ingesting heartworm Microfilariae via a blood meal.  

 The Microfilariae then develops further for 10 to 30 days in the mosquito's gut and can enter it's mouthparts. At that point they become infective larvae. 

 The microfilariae can complete their maturation after they enter a dog's body via mosquito bite and  will begin causing damage to the dog's body.

I'm sharing the below Heartworm Disease Lifecycle diagram, created by the Drake Center For Veterinary Care

This Google link brought up several other diagrams of the Heartworm Lifecycle that you may find informative as well.

Where can I learn more about Heartworm Disease in Dogs (and Cats)?

For more information on Heartworm Prevention and Heartworm Disease in Dogs and Cats, visit the American Heartworm Society's web site    

Sources for this article:  My sources for the information in this article were; U.S. Food and Drug Administration, VCA Animal Hospital  Pets WebMd , and the Drake Center for Veterinary Care 

Thanks for visiting us today, I hope you found this information on Heartworm Disease in Dogs useful!

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How do you prevent Heartworm Disease in your dog, cat, or other pet?  We really appreciate your input, so please leave us a comment and share!  




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M. K. Clinton said...

Treating your dog for heartworms is so important especially down south where we have mosquitos all year long. The Bayou Boys had their monthly chews last week. Great article!

Tiffany said...

Thank you so much for sharing! So much great information here about heart worm. It’s so scary to think that some symptoms can be so mild and potentially go unnoticed - such a great reason to be mindful of heart worm prevention.

Britt K said...

This is SO important! Heartworm disease is too often overlooked with new pet parents and I think that's on us - the experienced pet parents, veterinary professionals and sources of information like pet bloggers. We need to talk about this more often, share information about prevention and raise awareness!

M Dawson said...

Oh wow! This heart worm business is utterly frightening! I can't imagine the worry and stress it would case to any owners, let alone the cost.

I am glad there is a preventative treatment and I sure as heck would want it for my dog. Does it occur in some parts of the country mare than others? I know this is the case with ticks, but is the same true of heart worm?

Dash Kitten Crew said...
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LaylasWoof said...

Important information and I have discussed this with my vet but as she says because Layla is not out as such or only in the dog park or friends yard I do not have to worry about it, but your post has taught me a lot, thanks so much

Dogs Luv Us and We Luv Them said...

Oh definitely! Now that we live in a warm climate, it's important all year round.

Dogs Luv Us and We Luv Them said...

I'm so glad you find the information useful. Heartworm disease is so important to protect pets from, and it's so easy to prevent.

Dogs Luv Us and We Luv Them said...

I totally agree Britt, it's very important information to share, especially for new pet owners. It can be so deadly and can go unnoticed for a very long time - then it can be too late.

Dogs Luv Us and We Luv Them said...

Yes, it really is frightening, but thankfully there are great preventatives that are so easy. Most are in chewable form and are pretty appealing to most dogs. Cats get hearworm too! I focused on dogs here since that's my area, but cats need to be protected too. Heartworms are found all over the U.S. They tend to be more prevalent in warmer climates - since it's spread only through an infected mosquito bite, in cold climates you have months where mosquitos aren't around. However, if you get even one warm day in Winter mosquitos could come out and infection is feasible.

Dogs Luv Us and We Luv Them said...

It's surprising that your Vet doesn't think it's an issue for Layla. I know heartworm disease occurs in every state and is transmitted by mosquitos. Interesting.

MattieDog said...

We really pay attention to keeping mosquitoes out of our yard. Fortunately we don't live in an area where they are known to live in large amounts so that's a big plus for our pups. We use herbs and sprays, and during the summer months give heartworm treatments. Thanks for sharing!

Chelle said...

Great post! Heartworms are something I've always taken seriously. I keep up to date on getting my pets tested yearly and keep them on preventatives year round.

DawgBlogger said...

Great minds think alike; just last week I updated Dr. Lorie's article she wrote for me on heartworm. Terrible parasite.

Beth said...

My vet has recently suggested that we treat our dogs year round, since the temperatures are so much warmer than they used to be. We fall under the "better safe than sorry" camp.

Dogs Luv Us and We Luv Them said...

I also try to keep mosquitos at bay, although it's not easy! I have potted Citronella and mint plants, I also use a cedar & black pepper candle in our screened in porch. I have wipes for the dogs that repel mosquitos but I'm not great at applying daily. I need to get an herbal spray for the yard as well. Thanks for sharing that!

Dogs Luv Us and We Luv Them said...

It really is so critical, heartworms can do such awful damage and even kill your dog or cat. I keep up w/ it each year as well. The preventions are inexpensive considering the horror of these worms.

Dogs Luv Us and We Luv Them said...

It really is awful! When I first researched it, I was shocked at the lifecycle and that it's so easily transferred by mosquitos.

Dogs Luv Us and We Luv Them said...

Oh I totally agree with that! Even in colder climates, there's always a short warm spell. That could be enough to bring out mosquitos and infection. I've used the same medication for years for all my dogs. I trust it.

Sweet Purrfections said...

I didn't realize cats could get heartworms until I was talking to the breeder about Truffle and Brulee. Since we live in the south and it's warm most of the year, we do have mosquitos. They are treated year-round.

Dogs Luv Us and We Luv Them said...

Yes, both cats & dogs can get heartworm! It's even more prevalent in warm climates. I'm so glad Truffle & Brulee are protected against those nasty parasites!

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