Heartworm Disease in Dogs
and Cats
In certain parts of the world, especially in my beloved Southeast United States, Heartworm is the most common heart disease in dogs.

You simply must  take this disease seriously; if you live in an area of world with heartworm disease, you need to put your dog on heartworm preventive...after first testing....see your vet.

Heartworms are also causing respiratory and heart disease in cats.

This page will hopefully answer all your basic questions about this common disease.

Thanks, Roger Ross, DVM
FoxNest Veterinary Hospital, Seneca, SC

Heartworm Disease and Prevention
in Dogs:

Some basics about the disease:

The life cycle of the heartworm begins when an infected dog, carrying tiny immature heartworms (microfilariae) circulating in its blood, is bitten by a mosquito. The mosquito takes in microfilariae (larvae) when it feeds.

During the next two to three weeks, the larvae develop within the mosquito into the infective stage.

When the mosquito feeds again, it can transmit infective larvae to the healthy dog. The larvae deposited into the skin of the dog (or cat) migrate through the tissues and develop over the next few months, eventually becoming adult heartworms and eventually reaching the dog's heart, or more often, the pulmonary artery which is the vessel between the heart and the lungs.

Once in the dog's heart, the worms can grow to as long as 14 inches and cause significant damage to the heart, lungs and other vital organs. If left untreated, heartworm disease causes significant lung disease and quite often results in death.


Luckily prevention is easy.  If you've been to a vet with a dog in the last 20 years you probably already know about the importance of heartworm preventive. 

There are several choices, all of which work well:

Filaribits and other types of daily pills containing a medication known as "D.E.C." :  This medication is pretty much obsolete but still occasionally prescribed.

Heartguard Plus: Monthly chewable tablets that also help control round worms and hook worms.  (But not tape worms, and not very effective against whip worms)  There are now several generic brands available as well.

Interceptor: Monthly chewable tablets that also help control round, hook, and whip worms.  (But not tape worms)  In the Southeast and in other places where intestinal parasites are prevalent, Interceptor (or Sentinel) are excellent choices for parasite control.
Sentinel is Inteceptor combined with a medication that prevents fleas from reproducing

ProHeart Injection:  This option is an injection given at the clinic that gives good protection for 6 months.  It costs the same or slightly less than a 6 month supply of Heartguard Plus or Interceptor at our clinic.  ProHeart is not readily available anymore in the United States

Revolution:  My favorite option.  Revolution is a monthly treatment....drops on the back of the neck...that does a good job of preventing heartworms but is also excellent for fleas, flea eggs & larvae, sarcoptic mange, and ear mites.  And it does a fair job of controlling ticks.

Advantage Multi:  Good to excellent flea control, great at preventing heartworms, and very good at controlling intestinal worms; Like Revolution, Advantage Multi is applied to the back of the neck once a month.

Generic Monthly Tablets:  There are now several brands of generic monthly tablets similar to Heartguard Plus.  They work fine and are slightly less expensive.

Trifexis:  New in 2011.  Chewable tablets. Near perfect flea control for 3-4 weeks.  Excellent prevention and control of heartworms and intestinal worms.

Heartworm Disease and Prevention in Cats:

What is heartworm disease in cats?

Heartworm disease in cats is a serious and potentially fatal condition caused by Dirofilaria immitis. This is the same parasite that causes heartworm disease in dogs but new research shows a potential for more severe reactions and even sudden death in cats.

How do cats get heartworm disease?

Cats get heartworm disease the same way dogs get it.  By mosquitos biting an infected animal, then passing the infection on to other animals they bite.

Where are cats at risk for heartworm infection?

Wherever dogs are at risk. Even cats that live indoors.

What are the signs of heartworm disease in cats?

Common signs of infection are:


·Breathing Difficulty

Asthma Like Symptoms



·Weight loss

Other more acute signs are:



·Sudden death

So don't be surprised if your vet recommends a heartworm test or chest radiograph if your cat demonstrates any of the above symptoms.  Of course, these signs may also be seen with other feline diseases.
(Note: Heartworm screening tests for cats are not as reliable as they are for dogs; in cats,  chest radiographs are often a better diagnostic tool)

Ask your veterinarian about your cat's risk for heartworm disease.

Prevention of heartworm disease in cats is simple:

Heartguard monthly tablets are now available for cats.  They're inexpensive, easy, and effective.

Revolution is more expensive, but in addition to being an excellent heartworm preventive medicine is also excellent for control of fleas, ear mites, intestinal worms, and mange.

Interceptor monthly tablets may be available for cats soon.

Treatment of Heartworm Disease

At present, there isn't an approved treatment for heartworm disease in cats.  Your vet may or may not recommend risking an unimproved treatment.  It will depend on the individual case.

For dogs, there is only one approved treatment in the United States.  It's an injectable product called Immiticide.  Almost as important as the medicine though, is the supportive care your vet will provide during the treatment.  Most vets will recommend hospitalization of your pet during treatment to ensure against reactions and complications from the dying heartworms.

Treatment also involves antibiotics and short term steroids.  It also involves using heartworm preventive medications both as part of the treatment as well as to prevent new heartworm disease.

In other countries, with Japan, I believe, being the first to introduce the technique, heartworms are being removed surgically using micro-invasion techniques through a catheter introduced through a vein!

My comments about what to expect if your dog has heartworm disease

There seems to be a lot of misinformation about heartworm disease out there.  Here's a simplified explanation about what to expect if your vet detects heartworm disease in your dog.

Heartworm disease is certainly a very serious and often fatal disease.

But, not all heartworm positive dogs die of the disease; about 25% of untreated dogs with heartworms end up living a pretty normal life. 

On the other hand, without treatment, the other 75% either die within a few years of the disease or live a pretty miserable life due to inflammatory lung and other diseases associated with the effects of the heartworms.  The most common problem secondary to heartworm disease is pneumonia, hacking up blood, and so forth, but kidney failure is also common.

Sometimes heartworm kills quickly without any warning.  But usually the disease progresses slowly over a few years.

Basically, what happens is that while the heartworms are alive in your pet's body, they tend not to be recognized by your pet's immune system.  Many, if not most, heartworm positive pets in the early stages of the infestation act perfectly normal.

But the heartworms only live about 18-30 months in your pet.  The big trouble is likely to occur when the worms die.  They are dislodged from the heart and get pumped into the lungs.  All of a sudden the immune system recognizes all this foreign protein ...heartworms are fairly big... and the trouble starts: coughing and hacking, lung infections, kidney disease, allergic tissue reactions, and sometimes sudden death and sometimes a slow death.

Yes, treatment is risky, but it usually works out fine and it's certainly less risky than not treating.

If you're bright enough to be reading this, then it's probably dawned on you that prevention would be a whole lot better.
Please go to the trouble of giving your dogs (and cats) once a month heartworm preventive if you live in a heartworm area.

Treating Heartworm Disease:
more info coming soon.
On This Page:

Introduction to heartworm disease in dogs and cats

The Prevention of Heartworm Disease in Dogs

The Prevention of Heartworm Disease in Cats

Treatment of Heartworm Disease

My comments about what to expect if your dog has heartworm disease

About Other Heart Problems in Dogs and Cats on Other Pages:

Heart Disease in Dogs and Cats:  What To Expect From the Vet

About Specific Heart Diseases

Our Misc Heart Page and a Little About The History of Cardiac Medicine

Other Diseases spread by Mosquitos

A Review of Flea Control Products