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Heartworm in Dogs (Facts, Treatments & Statistics)

Heartworms. Just uttering the name of the critters is enough to strike fear into the souls of many a dog owner. However, while we know they are inherently bad, we may not know exactly why they are such a menace. Indeed, there is a lot to know about them.

Heartworms: A Brief Video Overview

There's a lot to digest regarding heartworms. This video does a great job of providing a quick overview of what makes this beast such a serious threat to your dogs. It's worth a viewing if only to help potentially spur you to action the moment that your dog starts showing signs of a potential heartworm infection.

What Are Heartworms?

As the name suggests, heartworms are worms that can take up residence in a dog’s heart, as well as his lungs and blood vessels. They look like semi-translucent spaghetti strands, and can grow up to about 12 inches in length. They pose a dangerous threat not only to dogs, but to cats and some 30 other mammal species, as well.

Heartworms are robust parasites, particularly when they latch onto a canine host. Its average life span in untreated dogs is 5 to 7 years. Conversely, their lifespan is just 2 to 3 years in an untreated cat.

In the United States, heartworms and heartworm diseases tend to be a more common problem along the Atlantic Ocean and the Gulf Coast, although the condition has been registered in all 50 states. Although April through November is typically considered to be “heartworm season” for dogs, more vets are recommending year-round treatments.

Where Do Heartworms Come From and What Do They Do?

Heartworms come from one source and one source only: the mosquito. This allows the worm to differentiate itself from other parasites such as roundworms or hookworms, which come from other sources. It should also be noted that treatments for the latter worms will have no effect on heartworms.

The fact that the mosquito is the sole carrier for heartworms does correlate with the corresponding disease’s tendencies. After all, mosquitoes tend to thrive in hot, humid climates. This particular weather-driven condition tends to be prominent in states where heartworm diseases are most prevalent.

There is no way to tell if a mosquito is carrying heartworms. This is particularly troubling when you consider that it just takes one single bite from a mosquito for the condition to flourish within a dog’s system.

Once the heartworm’s larvae enter the dog’s bloodstream via the mosquito bite, it takes about seven months for the parasites to mature into their adult stage. When they reach maturation, they can lodge themselves into a dog’s heart, lungs, or nearby blood vessels.

Upon settling into their new home, they begin to reproduce, and they can be quite prolific. A dog can have as many as 250 of these baby worms into his system. These worms, known as microfilaria, will circulate in the bloodstream, reach maturity, and lodge themselves into a different area of the pooch.

This proliferation cycle is why the heartworm is such a deadly parasite. It doesn’t attach to your dog’s flesh to suck his blood nor rob him of any potential nourishment. When allowed to grow in large numbers, it merely clogs the heart and major blood vessels.

It should be noted that heartworms are not transferable from dog to dog as some other parasites may be. Therefore, you shouldn’t be concerned about having your pooch be near other canines during the heartworm season out of fear of contracting the disease. As long as your pooch doesn’t get bit by a mosquito, he’ll be safe.

How Does a Heartworm Affect the Health of a Dog?

Because of the nature of the heartworm, the parasite doesn’t directly cause ill health to a dog per se. However, it is definitely a conduit that promotes bad health because of its cardiovascular-clogging ways.

Generally speaking, the act of clogging these main blood vessels will cause the blood supply to other organs to be lowered. This truncation of blood flow is particularly prominent in the liver, lungs, and kidneys. When this lessening of blood happens, the organs can begin to malfunction.

The signs that you may see within your dog can vary due to certain factors, none of which necessarily have to do with the breed type of your pooch. These signs are dictated by metrics like the number of adult worms a dog has in his system, how long the worms have been in the system, and how much organ damage has been caused.

The Four Stages of Heartworm Symptoms

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The symptoms that a dog with heartworms exhibits will not manifest all at once. Rather, it will be broken down into four distinct stages that can be picked up by a keen observer, although some organizations denote five stages. Not surprisingly, the symptoms do become stronger and more noticeable as they reach into the deeper stages.

The first stage of symptoms is so subtle you may not even detect any problem whatsoever. You may notice a slight cough now and again, but it may not necessarily be something to put the scourge of heartworm disease on your radar. Even with the cough, your pooch will still appear happy and healthy.

This stage is so subtle, in fact, that if you were to give your pooch a blood test, it might actually come back with a negative result for heartworm disease. If you end up coming to your vet to get your dog checked out for that occasional cough, a blood test may actually convince you that he’s clear of heartworms for the time being.

When your dog reaches stage two, however, a few moderate and relatively easy to spot symptoms start to manifest. That occasional cough that your pooch has been dealing with will morph into a lingering cough, and he may show signs of fatigue after exercise. If you take your dog to the vet at this stage, he may test positive for the parasite.

The third stage is when some pretty heavy symptoms start showing up on the scene. The cough that won’t go away and his penchant for post-exercise fatigue will still be present, but it will be punctuated by a greater sense of lethargy and difficulty breathing. He may also begin to cough up blood at this stage.

During stage four of the disease, the dog’s long-term health may start to get compromised. All of the other symptoms that have been present will become more severe, and will be joined by several unseen symptoms such as unusual lung sounds or an enlarged liver. If your dog is left untreated during this stage, he could die.

A Look at the Myths Behind Heartworms

Like a lot of things that can cause ill effects to a dog’s health, heartworms and heartworm disease has been shrouded in myths over the years. These myths have led to oodles of misinformation and false assumptions that, if followed blindly, could set a dangerous, if not potentially deadly, precedent.

The first myth has to do with the kind of dog you own and his indoor/outdoor habits. Here, it is assumed that if a dog is considered to be an indoor dog, they won’t be exposed to mosquito bites. Therefore, they aren’t at risk for heartworm.

While it is true that indoor dogs won’t be as exposed to mosquito bites as breeds that thrive on a ton of outdoor time, it would be foolhardy to think that they will never set foot outside for various periods of time, particularly to take care of things like bathroom breaks. Remember, it just takes one mosquito bite for heartworm issues to happen.

Another myth has to do with geography. Even though the disease has been reported in all 50 states – something that’s backed up graphically – it’s still assumed by some that it’s only a real issue in epidemic areas in the Southeast.

However, the advent of climate change has provided even greater havens for mosquitos to fly around and do their bloodsucking thing. What’s more, warmer year-round temperatures have done some funky things to the mosquitoes’ life cycle, thus leading to more and more vets advocating year-round protection.

A third myth surrounding heartworms is built on the assumption that treating heartworms is on the same level as preventing the disease from manifesting in the first place. This is primarily fueled by the assumption that your dog’s vet has some prescription or treatment method that will knock out the issue in a snap.

Nothing could be farther from the truth. Treating heartworms can be an arduous, grueling ordeal for your dog; one that may involve 60 days’ worth of steroidal medications, drug injections, and hospitalization. In some ways, this could be more of an ordeal for the owner, who has to stand idly by and watch his buddy suffer.

What You Need to do When You Suspect a Heartworm Issue

The very moment you suspect that your dog has a heartworm issue, drop what you’re doing and schedule an appointment with your vet as soon as you can. This is important because of how potentially deadly the disease can get if left untreated. It’s also because the longer the disease progresses, the worse your dog’s treatment will be.

Keep in mind that in the case of heartworms, the condition may register as asymptomatic during its early stages. As such, chances are pretty good that by the time you get your pooch to the vet for heartworm, it may have already progressed to stage two. As such, you’re most likely going to be dealing with a truncated window of time.

When your dog does get the dreaded diagnosis of heartworm, the course of treatment is going to be dictated by the stage of the parasitic infestation. If it’s in the latter stages of the condition, surgery may be an option to immediately rid the excessive clogging within your pooch’s body.

The Heartworm Surgical Procedure in Action (WARNING: Graphic)

It’s one thing to read about the potential menace of heartworms; it’s another thing entirely to see the scourge for yourself, which this video does. It’s graphic, so if you’re a bit squeamish, you may want to skip. However, if you can handle surgical footage of heartworm extraction, the video provides a pretty powerful lesson on heartworm danger.

What if the Disease is Caught Before Stage Four?

If your dog’s heartworm issue is caught before it gets to the final, deadliest stage, the first thing your vet will do actually has little to do with the parasite itself. Rather, he or she will order a battery of tests and x-rays to make sure your dog doesn’t have any underlying conditions that may worsen with the heartworm treatment.

This round of testing will also be used to assess the amount of damage done by the heartworms prior to your pooch’s arrival. This damage could be particularly prominent in older dogs whose long-term heartworm infections may have severely compromised lung, heart, liver, and kidney help. This could end up complicating the overall treatment.

Once cleared, the treatment typically starts off with a series of steroidal treatments, antibiotics, and preventatives. Once your canine has completed this series, he’ll have to endure a series of drug injections to kill the adult heartworms. You can expect the time interval of this treatment to last a minimum of 60 days.

As of right now, the Food and Drug Administration has only approved one drug, melarsomine, that is approved to kill adult heartworms. This particular drug is an organic arsenical compound that your vet will inject directly into your pooch’s back muscles.

Once the injections have been given, your dog must stay in the hospital for observation. This is to ensure he doesn’t develop any serious reactions to the treatment. Once the injections have been completed, your vet may also prescribe a lessening dosage of steroids for a finite period of time.

The heartworm treatment concludes six months after injections with a trip to the vet and another round of tests to make sure all of the heartworms larvae and adults are completely eradicated. If your pooch still shows signs of heartworms, he may have to go through the treatment again to wipe out the rest of the parasites.

This ordeal doesn’t even touch on the fact that the treatment can be a bit of a financial burden. Treatment can cost upwards of $1,000, which could place an unwanted and unexpected weight on your wallet. Fortunately, there is a way that you can dramatically cut down on the potential to travel down that road.

The Importance of Preventative Medicine

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It should be fairly obvious that it’s in your best interest to take whatever preventative measures there are to have your dog avoid treatment in the first place. Your dog will unwittingly be beyond grateful that you were proactive in this regard.

The rule of thumb here is to make sure you have your dog tested for heartworm infection every year. This procedure can typically be done as part of your dog’s routine vet check-up, although you may want to discuss this test with your vet just to make sure it gets done. Never assume.

You may also want to talk to your vet about starting your pooch on a monthly heartworm preventative medication. You can start providing this medication to a pup under seven months of age without a corresponding heartworm test, since it takes about six months for the condition to manifest.

If you have a dog over seven months of age, and he is not currently on a monthly preventative medication, you need to test your pooch to make sure he doesn't have a heartworm infestation prior to administering the preventative medication.

Regardless of age, if your dog is starting out on a preventative medication, he needs to be re-tested six months after the initial prescription, and then six months after that. After he is cleared with the two six month intervals, he can be tested for heartworms on an annual basis.

It’s important that you follow through with the annual heartworm check-up even if he is taking a preventative medication. This is vital because even the best of dog owners are subject to human error. If you miss one dose of the medication, or even give it late by a day or two, such forgetfulness can expose your pooch to the parasite.

What’s more, even if you give your dog preventative medicine doesn’t mean it made it into his system. After all, your dog may have inadvertently thrown it up, or, if your vet prescribed a topical ointment, he may have rubbed it off. While these incidents may admittedly be rare, avoiding the ordeal behind treatment makes the annual precaution necessary.

If your absent-mindedness has really gone off the rails and you missed a couple months’ worth of heartworm prevention, you can re-start your dog on the medicine after a consultation with your vet. However, you'll need to make sure you re-test your dog six months later as opposed to waiting for the original annual re-test date.

You Need a Prescription for Preventative Heartworm Medicine

There are quite a bit of preventative heartworm medicines for you to consider. While there appears to be an abundance of choice, you must remember that you cannot pick up any of them without a prescription.

The Food and Drug Administration dictates that these medicines can only be purchased from your licensed vet or at a pet pharmacy with your vet’s blessing. This rule essentially forces you to get your dog in to see a vet for an annual heartworm test. Again, considering the difficult nature of treatment, this seems like a small price to pay.

Besides, it’s important that you work with your vet to ensure you’re getting the right type of preventative heartworm medication that suits your dog best. This may be a more important issue than you think, simply because some breeds may have some seriously negative reactions to certain meds.

Dog Breeds and Ivermectin

Perhaps the biggest example of these negative reactions revolves around Ivermectin. This is an active ingredient that is featured in a lot of heartworm preventative meds. In most dogs, this ingredient doesn’t do a thing to their overall health.

Yet some dog breeds suffer from a genetic mutation that tends to make them highly oversensitive to ivermectin, to the point where it becomes dangerous. Several herding breeds like border collies and Shetland sheepdogs have been known to carry this mutation, as well as some mix-breed dogs.

With dogs that carry this mutation, ivermectin crosses the blood-brain barrier. When this happens, the pooch can suffer neurological damage, to the point where it could kill them. Preventative heartworm meds are not the only place where this active ingredient can be ingested – dogs that eat the poop of heartworm med-treated livestock can also be exposed.

Acute symptoms pertaining to ivermectin toxicity can start manifesting pretty quickly – within 4 to 12 hours of ingestion, to be precise. Milder symptoms can also start cropping up within 2 to 3 days. Some of the symptoms include disorientation, lethargy, an inability to stand, slow heartbeat, and even sudden blindness.

Obviously, if you have a herding breed, it’s important that you talk to your vet to make sure he or she is ruling out any heartworm preventative medication containing ivermectin. Doing so will give your pooch an extra layer of protection against heartworms and more.

You Love Your Dog – Protecting Him is the Least You Can Do!

Heartworms are pretty ruthless critters. Their sheer volume can cause some significant and potentially deadly damage. Considering it just takes one mosquito bite to get them into your system, and the level of treatment that may be involved to get them out, it’s imperative that you don’t treat this potential issue lightly.

Fortunately, with the help of preventative medication and an annual trip to your vet, this potential issue can be closely monitored and nipped in the bud before it becomes a full-blown issue. Frankly, if you love your dog like a dog owner should, standing your ground against heartworms should come rather easily for you.


Laura Harris

Dr. Laura Harris is our resident dog health expert. She started to fact-check dog health-related information for HerePup during her internship and contributes since then. Her expertise is in dog nutrition, senior dog care, especially critical care medicine and internal medicine.

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