Why Your Dog Needs Heartworm Prevention

And yes, needs.

In my twenty-plus years as a veterinary technician, I’ve encountered a lot of misconceptions about heartworm disease, and it’s time to set the record straight. So let’s get started.

What’s the deal with heartworm, anyway?

Basically, it’s just what it sounds like. It’s a worm that lives in the heart of a dog. The adult worms do, anyway. They live their happy little lives growing up to a foot long, and when they get too big for the heart, they stretch out into the blood vessels that lead to the lungs. They also reproduce there, and then release their larvae, the baby heartworms, out into the bloodstream. When a mosquito comes and bites that dog, it sucks up some larvae along with the blood. The larvae live in the mosquito until it bites another dog, then are transmitted into the new dog’s bloodstream. Eventually those larvae make their way to that dog’s heart, mature, and the cycle starts all over again.

But my dog’s never around other dogs. How would he get heartworm?

It doesn’t matter if your dog never ever has contact with any other dog. Heartworm spreads via mosquito, so there only needs to be an infected dog living nearby. And since mosquitos regularly fly up to three miles (and some species, up to seven miles) in search of a meal, “nearby” doesn’t necessarily mean “in the neighborhood.” And even if you live in the middle of nowhere, with no other dogs for miles around, wild canines like coyotes, wolves, and foxes carry heartworm too. So if you think your dog’s not exposed, trust me, he is.

But I live in (insert place with cold winters)!

Look, I live in Minnesota. We regularly have multi-day streaks where the temperature never gets above zero degrees Fahrenheit. And at my clinic, we regularly diagnose dogs with heartworm. It’s out there, people. Heartworm has been diagnosed in every one of the fifty states. And while heartworm is a little more prevalent in the southern states, a lot of rescue organizations will ship dogs around the country to try to find homes for them. So southern dogs don’t necessarily stay in the south, and they bring whatever infections they have with them.

But I’ve never worried about heartworm, and none of my dogs have ever had a problem.

If this is true, you’ve been incredibly lucky. But I’m going to go out on a limb and say that none of your dogs have ever had a problem that you know of. I use as an example my own grandfather, a farmer who had a string of farm dogs his whole life, and didn’t believe in routine vet care. When he passed, my aunt and uncle took in his current dog. That dog, who had been “slowing down” as he got older, not only had heartworm, but had such a heavy worm burden that treating it would have likely killed him (we’ll talk about treatment in a little bit). This poor dog had to be euthanized. The insidious thing about heartworm is that it doesn’t cause signs right away--sometimes not for a few years--and by the time you do see signs, the dog is very sick. And this whole time, your dog is serving as a source of infection for any other dogs that live within several miles.

Okay, so what if my dog gets heartworm? I’ll just give him a dewormer.

Sorry, no can do. Dewormers are meant to kill the worms that live in the dog’s gut--roundworms, hookworms, tapeworms, whipworms. Heartworms live in the circulatory system, and they’re a lot harder to get rid of. Treatment first involves staging the disease--doing lab and x-ray tests to see how severely your dog is affected, in order to determine the safest course of treatment. The treatment itself involves pre-treatment with drugs to weaken the heartworms before giving injections deep into the dog’s back muscles to kill the worms. Depending on staging, at least two and usually three injections, over the course of a month, will be needed to ensure worm death. Which brings us to our next topic…

Wait, you’re killing the worms in my dog’s heart? These worms are a foot long! What happens to them?

Once they die, the worms break down and the dog’s white blood cells start to absorb them. The worms are significantly larger than the viruses and bacteria the white blood cells normally eat, though, so this process takes a while. In the meantime, there are basically pieces of dead worm in your dog’s bloodstream. I know, yuck. It’s also dangerous for the dog. Your dog will need to be kept very calm--think cage rest--for several weeks. We don’t want to get their heart rate too elevated or do anything to put stress on the circulatory system while this process is happening.

So, before treatment your dog needs to be on drugs that can cause nausea, vomiting, or diarrhea; the injections themselves are quite painful; there’s a whole lot of inflammation in the body due to treatment; and your dog needs to be kept strictly rested for a month after treatment. And I haven’t mentioned the expense of this whole ordeal yet--depending on the staging, we’re talking $1000 to $1500.

Yikes! Yeah, this doesn’t sound fun at all. How do I prevent this?

The good news is, easily. You give a little chewable pill once a month. It’s not a time-release drug; the dog eats it and it’s in and out of their system within 48 hours. But it kills any heartworm larvae in their system they’ve picked up in the past thirty days, before those larvae get to the heart and mature.

Here’s where the caveat comes in. Once heartworm larvae enter your dog’s system, they molt several times in the process of maturing. The preventative only kills certain stages of larvae. Basically, they’re only susceptible to the preventative for, at most, 45 days. Once they’ve finished their molt, the preventatives no longer affect them and we’re looking at those injections we talked about earlier. So if you miss even one dose of preventative, you might be out of luck.

Okay, you’ve convinced me! I’ll pick up some heartworm preventative next time I’m at Target/Costco/Fleet Farm/other big box store.

Well…. You can’t just buy heartworm preventative off the shelf. All (legitimate) heartworm prevention is prescription only. There are a lot of different brands out there. You don’t need to get it from your vet (although they’d appreciate it if you did), but if you’re not getting it from a pharmacy, it won’t prevent heartworm.

So what kind should I get?

That’s a question for your vet! I don’t know your dog (unless I’m lucky enough that they’re a patient of mine, in which case call the clinic and we’ll chat), so I don’t know their health situation or which medications are best for them. Someone who’s examined your dog is in the best position to advise you on this.

So there you are! Heartworm in a nutshell. Hopefully you’ve learned a little something reading this article--but most importantly, hopefully after this your dog will never need to worry about heartworms again!