Heartworm In Dogs

By Chyrle Bonk, DVM May 07, 2020

One of the easiest internal parasites to prevent is also one of the hardest to treat. Heartworm is a common issue for our canine companions, and it’s one that can be deadly if not handled properly. For more information on how to keep your pup healthy and heartworm free, let’s explore the subject further.

What Is Heartworm In Dogs?

Heartworm disease is caused by a worm named Dirofilaria immitis. Heartworms are long, skinny worms resembling cooked spaghetti. Males range from four to six inches in length, while females can reach 12 inches long.

These worms live in the heart, lungs, and blood vessels of infected dogs. An average worm load is around 15 worms, while some dogs can harbor worms in the hundreds. They are capable of causing heart failure, severe lung disease, and potentially fatal damage to other organs in the body.

Heartworm is found in all areas of the United States, but is most prevalent along the Atlantic and Gulf coasts.

How Do Dogs Get Heartworm?

Heartworm, like other internal parasites, requires an insect vector for transmission. In the case of heartworm, it’s the mosquito. Adult heartworms in a dog lay eggs called microfilariae that circulate in the blood stream. The microfilariae are picked up by mosquitoes when they bite and draw blood. Once inside the mosquito, the microfilariae become infective larvae. This is something that only happens within a mosquito. Microfilariae aren’t capable of infection while living inside our furry friends.

Infected mosquitoes then transmit the infective larvae to the next susceptible animals that they bite. Once the larvae are inside their new canine host, they mature to adults in about six to seven months and produce their own batch of microfilaria.

Adult heartworms have a lifespan of five to seven years, giving them plenty of time to produce many, many microfilaria. During each mosquito season, dogs can get re-infected year after year with new infective larvae that add to their adult heartworm numbers.

Is Heartworm Contagious To Other Animals And Humans?

Since heartworm requires a mosquito as the transmission mechanism, it is not contagious from animal to animal. That’s not to say that your dog couldn’t infect a mosquito that then infects other animals. But animals by themselves can’t pass heartworm through contact.

Dogs, cats, ferrets, and wild dogs like wolves and coyotes, are all susceptible to heartworm disease from an infected mosquito bite. In some cases, humans are also susceptible, but this seems to be a very rare occurrence.

What Are The First Signs of Heartworm?

Since heartworm larvae take five to seven months to reach maturity inside a dog, the first signs of an infection are usually nothing at all. This can be a problem because a dog can be exposed well before anyone has a clue that something is wrong, giving the larvae a chance to grow, become stronger, and reproduce. 

The dogs that are heavily infested with heartworm and dogs with other health conditions are typically the first ones to show symptoms. Moreover, they generally will have a more severe reaction.

What Are The Symptoms Of Heartworm?

There is a wide range of symptoms of heartworm. The severity of those symptoms will depend on the worm load the dog is carrying, the length of time that they have been infected, and how their body responds. The symptoms of heartworm disease can be divided into four stages.

  • Stage one

No symptoms or the occasional cough. This occurs during the time from infection to when the larvae become adults.

  • Stage two

Occasional coughing and some inexplicable tiredness following moderate physical activity.

  • Stage three

A persistent cough, tiredness after very little physical activity, and trouble breathing.  Signs of heart failure, such as a swollen belly, weight loss, and exercise intolerance is also common.

  • Stage four

Caval syndrome. When the worm burden gets high enough, it can physically block the flow of blood in major vessels. Dogs with caval syndrome may suddenly collapse, have labored breathing, pale gums, and bloody or dark brown urine. Caval syndrome is a very serious condition that is usually fatal.

Even if a dog with heartworm doesn’t experience caval syndrome, the reduced blood flow and presence of worms can lead to permanent damage of the heart, lungs, liver, and kidneys.

Again, active dogs, those with a heavy worm load, or those that have been infected for a long time will be the first to show signs and show the most severity. This is because physical activity requires extra work from the heart and lungs, something that is difficult for them to do if they’re overrun with heartworms.

Dogs with a heavy worm load will also have more damage to the cardiovascular system and a greater chance of experiencing a blockage of blood flow. Dogs that have been infected for a long time will also experience more damage to vital organs, and show more severe symptoms.

How Long Can A Dog Live With Heartworm?

If left untreated, heartworm can kill a dog. The length of time that a dog can live with heartworm will depend a lot on their worm load and other health conditions. If a dog is infected with just a few adult heartworms, chances are they can live a fairly normal life. The may not win any races or be the best retrievers, but if left unchallenged those worms can remain fairly harmless. As those worm numbers increase, the dog’s longevity decreases. The more worms a dog has, the more damage can occur to vital organs.

Should Your Dog Be Tested For Heartworm?

As with any disease, the earlier the detection, the better chance for a successful treatment. There are many heartworm tests available that detect specific proteins released by adult female heartworms and ones that detect microfilaria in the bloodstream. The trouble is that they can only detect adult heartworms, meaning your pup will come up negative if the heartworms are in their larval phase. Therefore, it will take five to six months for a dog to test positive for heartworm after they’ve been bitten.

With this in mind, dogs should get tested annually for heartworm. Even if your dog is taking a preventative year-round, dogs should get tested for heartworm because preventatives aren’t always 100% effective. Moreover, dogs should get tested if there has been a lapse in taking a preventative.

Before starting on a heartworm preventative, every dog needs to be tested. The preventive medications kill the microfilaria and not the adult heartworms. So giving a preventative to an infected dog would allow the adult heartworms to live on, while potentially causing a mass die-off of microfilaria that can cause your dog to go into shock.

What Are The Treatment Options For Heartworm?

If your pup is showing symptoms and tested positive for heartworm, there are treatment options available. Your veterinarian’s first priority will be to make sure your dog is stable enough to undergo treatment. They may also perform additional tests to confirm the diagnosis, as they don’t want to put your pup through something that they don’t have to.

Your veterinarian will more than likely restrict your dog’s exercise in order to decrease the work load on the heart and lungs. They may also prescribe medications to help combat heart and lung problems and reduce your dog’s chances of forming blood clots.

Once your dog is stable, treatment can begin. This includes multiple injections of a medication that kills adult heartworms. Additional medications are given to kill the microfilaria in the blood stream. Frequent blood work, x-rays, hospitalization and follow-up testing may also be part of a treatment plan.

The severity of a dog’s heartworm disease will correlate to the treatment’s success. Those dogs in stages one, two, or three have the best prognosis, while stage four is more grime.

Treatment for heartworm is not without risk. Killing adult heartworms and microfilaria can cause a dog to go into shock. The medications used to kill these worms can also be toxic. That is why prevention is the best treatment.

How Is Heartworm In Dogs Prevented?

The treatment for heartworm can be long and hard on a pup. Rather than risk putting your dog through treatment, opt for prevention instead. Fortunately, heartworm preventatives are easily given and readily available from a veterinarian. There are many effective products out there such as monthly topicals that go on the skin or oral pills. There are even injectables that only need to be given once or twice a year. Many of these products also take care of other internal and external parasites, so it’s a win-win.

While year round prevention may be the most effective form of prevention, cost and convenience can be an issue for some dog parents. It’s important to discuss your dog’s risk of heartworm disease in order to figure out the best method of prevention. Those at increased risk would be dogs living on the Atlantic and Gulf coasts or those traveling to those areas. It’s also important to remember that dogs (and mosquitoes) are relocating all of the time, bringing heartworm to areas that were previously not thought to be a problem. Because of this, prevention all year long is the safest option.


Having heartworms can be a nasty situation for any dog to be in. Rather than risk going through a lengthy, expensive, and harsh treatment process, use heartworm preventatives instead. It’s also important to remember that heartworms can cause permanent damage to many important organs, meaning other problems can persist even after the worms are gone.

Petozy is a brand dedicated to pet and pet parent happiness. Learn more about us here.