What is heartworm, and how does it affect my dog?

Heartworm is a parasite that your dog acquires from a mosquito. A mosquito bites a dog, and it transmits 5 to 10 tiny worms into its skin. And those worms travel through your dog's body into the bloodstream, and eventually, they become adult worms that are about 12 inches long in the pulmonary arteries, the right side of the heart.

Signs are trouble breathing and a persistent cough, which can be moist or dry. Sometimes they lose weight, so you'll see ribs that are kind of much wider than the back end because they're losing muscle mass. The dog may even have blood coming from their nose and mouth.

Unfortunately, there are no signs during the early stages of heartworm, and that's why prevention and screening every year to catch it early is so vital. By the time we see signs, it's very, very advanced.

Dr. Ashly LaRoche
Animal Hospital of Statesville

What can be done to stabilize heartworm disease once they get it?

Once they get it, we test the dogs with what's called ELISA tests. They test for the adult female worm, which is the vast majority of the worms that affect the dog. Once we have a positive test, we look at the blood to look for baby worms circulating in the body. Those baby worms are what a mosquito picks up when it bites your dog to transfer to another dog eventually. If we have those two things, we have a positive heartworm dog, and then we start talking about treatment. There are three phases to treatment—the early phase. We start the dog on medication. It's also a heartworm preventative, a macrocyclic lactone; we use a couple of them. They can either be topical, by mouth, and some injectable products are used for prevention, not treatment.

These meds prevent the dog from getting a further infestation. These mosquitoes are biting daily. As we start treating the dog's infestation, the dog can get new worms, so we need to prevent that from happening. And then, we start an antibiotic called Doxycycline. The heartworm needs bacteria in its body to survive. It's a commensal or symbiotic bacteria. By treating that bacteria and killing it, we can start the treatment process. And it's essential because, if we don't do that treatment process, these worms have to be killed very slowly. After all, there are often many of them. And they're about 12 inches long. That's quite long in the right side of the heart and pulmonary arteries. You cannot kill these worms too quickly, as the dog has lots of complications if we do that. So we're setting up for pre-treatment. Eventually, we will give a medication called Immiticide or Melarsomine, a medication that goes in the muscle of the back. And we give a series of three injections that will eventually kill the heartworms.

How soon should a person bring their dog in to get tested for heartworm disease?

All dogs should be tested every year, but if you suspect heartworm disease, if you're seeing any signs of coughing in particular, difficulty breathing, you should have your dog examined. If your dog has not had a recent heartworm test or is not on prevention, one of the first things we're going to recommend is a heartworm test to help rule it out.

When do we start a dog on heartworm prevention?

Heartworm prevention for dogs that aren't infected is to prevent the disease. Puppies are started as early as eight weeks because mosquitoes can bite puppies as well. These mosquitoes bite a dog, deposit baby worms into their skin, migrate over the course of two months, then get to the bloodstream. So while they're in the skin, they're susceptible to being destroyed by these heartworm preventatives, which is why we start the heartworm preventatives. The dog gets infected, but we destroy the worm before it does any damage at this point. And that's how those preventatives work.

If you still have other questions and you'd like to reach out to us, you can call us directly at (704) 872-3625, you can email us, or you can reach out on Facebook. But please do reach out, and we'll get back to you as soon as we can.

Dog Heartworm Disease - FAQs

Dr. Ashly LaRoche
Animal Hospital of Statesville

How will you diagnose a dog with heartworm disease?

Usually, I'm going to run a test called an ELISA test. It's a blood test that we run here in the office. It is screening for adult female worms, which are 90% of the worms that dogs are infested with, if not more. This test is also recommended yearly because it screens for some of the tick-borne pathogens, Lyme disease, Ehrlichia, and anaplasmosis, which are also essential to screen for in your dog. If the dog is positive, we will also continue to look at the blood, looking for baby heartworms to confirm that diagnosis. We look in the microscope for those, and you can see them wiggling. They look like tiny, wiggly worms.

Why is early detection and diagnosis so important for a dog?

Because dogs tend not to show any signs until they have a very mature, advanced infestation. By that point, they often have had damage to their heart and lungs, so this is a disease we want to catch early before they have signs. We want to screen for this disease every year. If a dog has had heartworm for years, they're more apt to not do well through treatment because they've had this infestation for so long. There's often a lot of permanent damage to their lungs and heart.

In some cases, the damage is reversible. Usually, the earlier that you catch it, the better, but the more prolonged the infestation, the more apt the dog is to have complications during treatment, which can be fatal, quite frankly. And also, they can develop some permanent lung changes that lead to heart problems later on.

What are the complications of heartworm disease?

Death is one of the most significant complications. These dogs can go into respiratory distress syndrome. Also, these worms are 12 inches long. Pieces of them can break off while the dog is running and playing, and they form a blood clot that can go to the lungs or other parts of the body. And sometimes, even often, it's fatal. Fatality is one of the worst complications. Even with treatment, the dog will often develop pulmonary hypertension, which eventually leads to some right-sided heart failure.

Are there any risks associated with heartworm treatment?

Blood clots are the most significant risk of treatment, and some of them can be fatal, which is why we assess our candidates while we're treating them. We do what's called staging to see how severe we feel the infestation is and the worm burden, as the disease is impacted by the number of worms that the dog may have. And it's an educated guess, frankly. We're looking at X-rays to look at changes in the lungs and at blood work to make sure the liver and kidneys aren't having any problems because this is a disease that affects the lungs' vessels, and hence, perfusion to the organs and things like that. We need to assess all of that stuff, and once we get to that staging, we can make an educated guess as to how severe and advanced this heartworm disease might be and how well or not well a dog might do through treatments.

When should a dog be tested for heartworm?

A dog should be tested once a year. All young dogs should start with testing at six to eight months of age using our current tests, only a test for the adult female worm. And those worms usually take six to eight months to mature, so we can't pick up those infestations until they're adult worms.

How is a dog tested for heartworm?

It's a little bit of blood that we take from your dog, sometimes out of the front leg and other times out of the neck. We mix it with a reagent and put it in this test, and it takes about 10 minutes in our office. Ultimately, it'll give us a positive or a negative.

If my dog's on heartworm prevention, why do I need to have them tested every year?

There are some rare cases of resistant heartworms. What that means is the potential to get heartworm disease despite being on these medications. Again, that's incredibly rare, and we recommend all dogs are on prevention. Also, missed pills happen, as people are busy. Sometimes owners miss giving the pills, or the dog goes behind the couch and coughs up a pill. With even one missed pill, or if you're six weeks late giving a pill, something like that, your dog still has a risk for heartworm disease. As we've been trying to hit home, we need to catch this disease early, not late. With a dog that we think is on year-round prevention, if somebody missed a pill and the dog got infected with heartworm, we haven't tested this dog for four years. That means the dog could potentially have had worms in it for four years, and that could have devastating consequences to the heart and lungs.

What is the most accurate test for heartworm disease?

The ELISA test is very accurate, but on occasion, if it's, say, a really weak positive, we call them, or it's just not a really strong positive, if there's any doubt about this dog's heartworm status, we will take more blood and send it to the lab. And we will do another test to confirm. If we have microfilaria (baby heartworms) circulating in the dog's blood plus a positive test, most of the time, you can be pretty assured that that dog does have adult heartworms.

Can heartworm in my dog be treated?

Yes. Dogs can be treated for heartworm, and we stage dogs. There are various stages. There are classes of heartworm disease. The dogs are classed based on the changes to the heart and lungs, X-rays, blood work, and the dog's symptoms. The class one and two dogs, which are earlier stages, tend to do pretty well. We start the heartworm preventative. We start that antibiotic we had talked about before, and then we allow a couple of months for those things to work. It sounds crazy that we would wait that long, but we also have to wait for immature worms in the body that can not be treated with conventional treatment. They're not susceptible to conventional treatment until they become adults. So, we got to wait for those to mature, and that takes a couple of months. We also need dideoxy cycling time to work to destroy the symbiotic bacteria that live in the heartworm.

And then, that antibiotic is also an anti-inflammatory in the lung, so dogs do so much better if we slowly prepare them for treatment.

About two months later, we give an injection called immiticide or Melarsomine, killing adult heartworms. We give it in the muscle of the back. But not one injection is effective, so we have to give another set of injections a month after that, so heartworm treatment is very drawn out. We have to kill these worms slowly. We can't kill them too quickly because, often, we'll have fatal pulmonary thromboembolus. Dogs do not do very well. They'll have many complications from treatment if we do not treat them slowly and prepare them to be treated.

Is treatment relatively the same for every dog that gets heartworm disease?

In the cases of advanced heartworm stages, where the dog is in heart failure or has portacaval syndrome, a terrible complication dogs can get with heartworms; we treat those dogs differently. They need to have their heart failure treated, and sometimes, they require surgical removal of those worms, and it carries a very poor prognosis.

How is heartworm prevented in dogs?

There are many products out there. They're called macrocyclic lactones, that's the class of medication, and they're very safe monthly pills. And many of them treat intestinal parasites as well. There's also an injection called ProHeart. There's ProHeart 6 and ProHeart 12 for people that have trouble remembering to get pills monthly. And there are topical products—Advantage Multi and Revolution are a couple of them out there that we apply topically to kill the worms. Many of these products are also effective against intestinal parasites, and some of them are combined with flea and tick products. The product we choose depends on the individual dog's lifestyle and the owner's needs.

How effective is this heartworm prevention?

The heartworm preventatives are 95% effective, if not more. There are a few cases of resistant heartworms that have become more recent. They used to be 100% effective. Now, they're about 95%, I would say. And that's still pretty good.

Also, some people are under the misconception that they don't have to give preventives in the winter. That's not true. Especially here in North Carolina, we have temperatures where mosquitoes can replicate and infect dogs all year. We recommend year-round treatment in most places in the United States now.

Are there any side effects associated with heartworm prevention?

An occasional dog can have diarrhea, but it's very rare. And with some of the topical products, the products placed on the skin, you have an occasional dog sensitive to the carrier or whatever it is in that topical product. That dog might have some itching and hair loss in that area. Again, that's pretty rare, though. These products are safe for the Collie breeds. You'll read about that sometimes, and people worry about that. Sometimes, Collies have trouble with certain medications, but these products are all very safe.

Are there any holistic or over-the-counter preventions for heartworms in dogs?

No, there are none that we would recommend. In dogs with heartworms and baby worms circulating in their blood, some of these heartworm preventatives are not appropriate in that situation, as dogs can have an allergic reaction if they have baby worms circulating in the blood with certain products. We avoid those products in the initial treatment of a positive dog because of that.

Can we do anything in our environment or in our dog's environment to reduce the risk of heartworm?

We can. Vector control or mosquito control is probably the most important. Mosquitoes breed in standing water, so try to eliminate standing water in your area. Flower pots can be a source of water for them to replicate in. Also, there are certain high feeding times for mosquitoes—morning is one, and then at dusk. Avoid having your dog hanging out outside during those times, and it might decrease your dog's risk of mosquito bites. However, that alone is not a preventative.

Even dogs in the house get infected with heartworm. Mosquitoes can get in the house and bite your dog and inject five to 10 baby worms into your dog.

What should I do if I miss a dose of my dog's heartworm prevention?

Give the next regularly-scheduled dose. Once a dog gets infected, it takes seven months for that infestation to mature. Keep giving your regular heartworm preventative as directed, and then, seven months later, get another test. And if it's negative, great, your dog is probably fine.

How are heartworms transmitted?

Heartworms are transmitted via a mosquito bite. And then, that mosquito injects baby worms into the dog's skin. And they migrate into the dog's skin for about two months till they hit the bloodstream. And then, they end up in the pulmonary arteries or arteries of the lungs seven months later. That tissue stage, or those first two months, is where the heartworm preventatives destroy that infection before it gets any further. That's the only time that those are an effective heartworm control or prevention.

How does my dog's lifestyle affect their chance of getting heartworm?

Dogs that are outside or in pens all of the time can be inundated with mosquito bites compared to a dog that's more indoors. Of course, your field dogs, your hunting dogs, things like that probably have a higher risk. All dogs have risks despite their lifestyle, but certain dog lifestyles are higher risk, such as predominantly outside and athletic dogs.

If your neighbor doesn't treat for heartworms, you're also at a bit more risk than if everybody around you treats for heartworms, which you don't know.

What are the possible signs of heartworm disease in a dog?

In a dog, typically, you're going to see coughing. And that's the most common sign. Sometimes, weight loss, you'll see that as well. Those are the two most common signs. Labored breathing, sometimes, as well. And then, as it advances, you can get heart failure and some other things, but those are the big signs. Anytime we have a dog coming in, coughing, gagging, and that's not on heartworm preventative, we highly recommend a test.

How soon after infection will a dog show signs of heartworm?

A dog can show signs of heartworm as early as about six months. They typically don't show any symptoms before that, as the infestation isn't mature enough at that point.

Is heartworm disease painful?

It's not painful, per se, but they feel sick, uncomfortable, and they're likely having difficulty breathing. They're not perfusing very well, so they don't feel well. Is it overt pain? I'm not sure.

Are heartworms visible?

Heartworms are visible via ultrasound in some cases. You will not see them because they live inside the heart and the pulmonary arteries, so you'd need some advanced imaging to be able to see them.

How common are heartworms in dogs?

Heartworms are on the rise, and it depends on the area. In the Southeast United States, they're pretty common in dogs not on preventatives. You can look at theheartwormsociety.org, and they have incident maps there that are interesting. And you can look and see how it has increased over the years. I believe the incidents in our area are up about 12% or something like that, but I might be wrong. And it changes all the time. If you're near a heat stamp or water or something like that, rural areas can be hot spots for heartworm. Some cities are warmer, so they promote mosquito growth for more extended periods. And how many positive reservoirs are running around? It depends on many things, but it's relatively common in dogs not treated or on preventative medication.

Theheartwormsociety.org is an excellent resource, and the Companion Animal Parasite Console also has a lot of great information if we're looking at regional incidents.

If you still have other questions and you'd like to reach out to us, you can call us directly at (704) 872-3625, you can email us, or you can reach out on Facebook. But please do reach out, and we'll get back to you as soon as we can.