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Dog Heartworm - Preventing, Treating, and Diagnosing Heartworm Disease in Dogs

What is heartworm disease and how can it affect my dog?

Heartworm disease is a condition where a dog will be afflicted with a heartworm parasite that lives in the right side of the heart. Imagine if you had a bunch of long worms living in the right side of your heart. So what it does is it causes problems with the blood flow. It can cause problems with the sealing of the valves, opening and shutting on the right side of the heart. And the most common thing that we see is a respiratory type of symptom because blood goes from the right side of the heart to the lungs. If heartworm disease affects those pulmonary arteries, which it does, and those worms sometimes try to migrate into the lungs, that can cause a lot of respiratory distress as well—difficulty breathing, coughing, those kinds of things.

Dr. Broussard
The Waggin' Train Veterinary Clinic

How would my dog catch heartworms?

The only way for dogs to get heartworm disease is through a mosquito bite. Mosquitoes are a mandated intermediate host. The short version of what happens is a mosquito bites a dog who has heartworms, picks up a, not infective larvae, but immature larvae from that dog. It has to live and develop in the mosquito itself for about two weeks and then it turns into the infective larvae. So if that mosquito, which now has heartworms and is ready to go, bites a dog that's not on prevention, that is how a dog will develop heartworms.

How can heartworm disease be prevented?

There are a bunch of preventatives out there. I won't spend a whole lot of time talking about all the different makes and models and name brands and all that good stuff but, in general, there are injectables that last six months. There's even one that lasts a year. There are topicals you can put on them. There are so many options out there, but the take-home message is this: in the scenario I just explained to you guys when the mosquito bites and tries to implant the infective stage of heartworm disease, which is called L3 larvae, that is what the heartworm preventatives work against. You can't reliably prevent your dog from getting bitten by mosquitoes but, if one of those mosquitoes has heartworms, you can definitively and reliably prevent that infective stage from living and from migrating to the heart and becoming full-blown adult heartworm.

What are the signs in my dog that would indicate they may have heartworm disease?

I briefly touched on it. Probably the most common sign that we see are respiratory issues. When an infected dog (particularly an active dog) gets excited, they'll start hacking and coughing or gagging and things like that. Another common one is exercise intolerance. A dog that used to be really fit and could run for miles or hours or whatever it is, now they get winded quickly. Things like that. Those are the most common signs because the majority of the heartworms that we diagnose here on a blood test don't have clinical signs, so the owner would never know. If we didn't pull a drop of blood and run it on the test and show them that, yes, he's positive and explained to them how and why they would never know in the early stages.

Sometimes we see dogs that are diagnosed in the latter stages and you’ll see those things that I previously mentioned. I'll throw in one more in that in a bit more severe case you can have not only heart murmurs, which is somewhat common, but you can also have edema, or swelling of the legs. The extremities and even the abdomen can fill up with fluid. Those are much more advanced symptoms, but those do happen as well.

What are some middle to late symptoms of heartworm disease?

I stole my thunder there I guess. So middle to late signs would be those I just mentioned. The much more advanced signs are a right-sided heart murmur, abdominal fluid, or ascites if you will, pitting edema, generally on the extremities, the legs, sometimes even on the neck and face. It can get really nasty sometimes. In the case of very end-stage instances, you will have what's called jugular pulses. The jugular vein is a vein—it should not pulse. It's just passive blood flow. Sometimes the heart gets so backed up with worms, and therefore blood, you can actually see the jugular veins pulse with every beat of the heart. That's not a good sign for your dog. So jugular pulses are a very advanced sign.

They can also experience hemoglobinuria in which they’ll have this almost red wine-looking coloration to their urine. That's a very, very end-stage process with advanced heartworm disease as well.

What can be done to stabilize my dog's heartworm disease?

We need to have your dog tested and staged. Blood work and x-rays also help us get a really good idea of how advanced the heartworm disease is. In my opinion, every animal should be on a heartworm preventative, even if they’ve been diagnosed. Why? If we don't put them on it, they're going to continue to get more heartworms and it's just going to make the problem worse. Yes. I know. If you talk to enough veterinarians, you're going to get some that say, no, no, no. You’ve got to treat the dog first. And I agree with that. I think you should treat the dog first, but I live in the real world and some owners can't or won't treat their dogs. I personally don't feel comfortable letting them sit around and just get more and more heartworms.

Number two. If the dog is starting to show clinical signs, there are things like steroids (prednisone), an anti-inflammatory, that I like to give. It's not going to cure anything, but it will calm down some of that inflammation in the heart and pulmonary arteries that the disease can cause and it can often help.

And probably the third thing I would think to give as well is an antibiotic called doxycycline. Sometimes we'll use minocycline, as they're equal in their effectiveness. That kills bacteria that live inside of heartworms. Let me be extremely clear and I'm going to say it again. Antibiotics do not kill heartworms. They kill a bacteria that lives in the heartworm and that benefits the dog and us when we're trying to treat it simply because it decreases the inflammation that's caused when that worm dies off, as it shortens the lifespan of the worm.

How soon should I bring in my dog to see a veterinarian for heartworm prevention?

I like to start prevention early. I'll be honest with you guys, I start on the first visit. Granted, we do live in South Louisiana. I don't know where you live, but in South Louisiana, mosquitoes are everywhere almost year-round. We might get a short break in the winter, but not long. So because of that, six weeks is when I start them. When they come in for their first checkup and perhaps vaccines, that kind of thing, I will typically send them with a dose of heartworm preventative or even give it to them that day in the office. But I start them early and then from there, it's every month for the rest of their life.

How will a veterinarian diagnose if my dog has heartworm disease?

So the initial diagnosis and, in fact, the only real definitive diagnosis, is going to be given by using blood work, a blood test. For most, we use a SNAP test that you can get a result in about eight minutes or so, depending on the test, eight to 10 minutes. But that's done in the office. It looks for an antigen released from the uterus of a female heartworm. It's pretty dependable unless you had a hundred percent male heartworm infection, as, in that case, it would tell you your dog was negative. So there are loopholes in there, but the chances of that happening are pretty slim.

So the short answer is that diagnosis is given with a blood test that's run to look for adults and if that comes back positive, oftentimes we would do a secondary test to help us stage it. And that's called a direct smear, or some people call it a difil test. That’s looking for microfilaria, which is the baby stage of heartworms. If they have adults and those adults start reproducing, they will produce babies, microfilaria, and that is what that secondary test is helping us find.

Why is early detection and diagnosis of heartworm disease so important?

Early detection and diagnosis are huge because if you can diagnose them earlier, you can prevent a lot of the clinical signs that we just discussed in the previous questions. Heartworm disease doesn't go away on its own. Period. It can go away with some conservative methods, which I will not get into right now. The gold standard is to treat them with drugs like Immiticide. It does require cage rest.

Why treat it early? So you can avoid all the clinical signs I mentioned—the coughing, the weight loss, the big belly, the swelling—because oftentimes when those things happen, you're really fighting a tremendous uphill battle. Not to mention, the inflammation in those heart valves and pulmonary arteries doesn't always subside completely. So I've had several dogs that we'll treat for heartworms, they get better, they don't have heartworms anymore, and yet they never stopped coughing. It might improve, but we're still battling a cough months or even years later because they had such a bad infection that stayed there for too long.

If you still have other questions and would like to reach out to us, you can call us directly at (337) 230-9247, contact us on Facebook, or email us. We'll get back to you as soon as we can!

Dog Heartworm Disease - FAQs

Dr. Broussard
The Waggin' Train Veterinary Clinic

Is heartworm disease common in dogs?

Unfortunately, yes, depending on where you live. Let me answer it this way—if you have mosquitoes where you live, then yes. They are how the disease is spread so, if you live, say, in Canada where there are no mosquitoes, it's not going to be a big deal to you guys. But here in South Louisiana, the Gulf Coast, and the majority of the United States, I would say it's a pretty big issue.

What are the complications of heartworm disease in dogs?

Why do they call it heartworm? Because the worms live in the heart. I'm not saying that to be silly, but anything that lives in the heart is obviously going to be detrimental. These worms can get pretty good-sized. Believe it or not, they can reach six to eight inches long, so that's a pretty good size. If you get one or two, it’s probably not a huge deal. If you get 10 or 12, it’s starting to become a big deal now. If you get more than that, well, you can imagine. Common sense would tell you that's going to begin to affect how efficiently and effectively that the heart can pump. Can those valves seal off? Is blood flow going to be impeded in going to the lungs to get oxygenated? All that stuff is a factor, so there are some very significant signs associated with the disease itself. And some of those changes are irreversible if they've been there for long enough, even if the dog is treated.

Are the risks associated with the treatment for each state of heartworm different?

That's a tricky question to answer. I guess you could look at it in three ways. So number one, heartworm prevention is extremely imperative. I recommend it to everybody. Very seldom do I see complications with just preventatives. And that kills the infective stage of the heartworm that the mosquito tries to give the dog. So the risk isn’t zero but it’s close.

If an animal is positive for heartworms, that means they have adult worms there. And if we go through with that treatment, there's risk associated with that because you are killing these worms that live, again, in the right side of the heart. And as mentioned, they're pretty long (six to eight inches) so killing them is half the battle, but they're still there. So now the body and immune system have to break down that dead worm and dispose of it, for lack of a better word. Well, what happens if they don't do it properly? What happens when that dog's too active after treatment? Then that dead or dying worm, or even partly disintegrated worm, can still migrate and go to the lungs and cause problems there.

And the drug itself, while I seldom see reactions or problems to it, it's a pretty harsh chemical. It's got to kill a bunch of big, active heartworms. Any time you put a chemical in the body of a living creature, things can happen that you don't want to happen or don't expect to happen. Do I see it commonly? No, I do not. But they can have pain at the injection site, soreness, and I’ve seen some dogs that are wobbly on their backend.

The third and final portion of this long answer is that sometimes when dogs have adult heartworms, those adults will reproduce and cause microfilaria, or baby heartworms. That takes a different modality to treat it. Some preventatives treat that nowadays, but oftentimes we'll use a high dose of ivermectin or something to kill that. Can you have side effects from that? Yeah. It's a high dose of ivermectin. Ivermectin is commonly used as a dewormer or heartworm preventative, but not usually at these levels.

You can have dogs that might have a sensitivity to ivermectin, which can cause neurologic signs. You wouldn't want to give that to a Collie breed type of dogs, so Border Collies, Collies, Australian Shepherds, or any of those kinds of herding breeds. You’ve got to be careful when giving high doses of ivermectin. So all these complications are very rare, but they can happen.

What are the risks if dog heartworm disease is left untreated?

The risks are pretty significant because if heartworm disease is left untreated and essentially unprevented, it's probably not going to end well for that dog. It may not be next week or next month, but at some point, that disease will rear its ugly head, and usually by the time it does, it’s not good. They basically die of heart failure.

If you still have other questions and would like to reach out to us, you can call us directly at (337) 230-9247, contact us on Facebook, or email us. We'll get back to you as soon as we can!

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