Based on an article that first appeared at

As veterinarians, what we’d love to talk about all day is puppy breath, good nutrition, and pet toys we recommend. Unfortunately, we have to also talk about the tough stuff to keep our pet and human clients safe, and one of those topics is heartworm disease. April is Heartworm Awareness Month. The American Heartworm Society reports that more than one million dogs are currently living with heartworm disease. This insidious disease threatens animals in all 48 contiguous states and Hawaii, and other temperate regions of the world. The good news is it's also preventable, so we’ve gathered some frequently asked questions about preventing, diagnosing, and treating heartworm disease below and, for the sake of this article, we’ve covered the answers for dog heartworm disease.

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

Heartworm is a parasite that affects the heart. It goes into the bloodstream and, when you have adult heartworms, they live in the chambers of the heart and can affect cardiovascular function, respiratory function, and other organ systems.

How would my dog catch heartworm?

Mosquitoes transmit heartworm disease. A mosquito bites a dog infected with heartworm disease, picks up the larval stages from them, and then goes to another dog, bites that dog, and infects them. Because of mosquito transmission, this disease tends to be more prevalent in coastal areas or areas that are very hot. However, as we mentioned, it is a factor in every state.

Can heartworm disease be prevented?

Yes! Heartworm disease can be prevented by using a monthly larvicide—parasite prevention that kills the larval stages before developing into these unfriendly adult stages.

What are the symptoms that would indicate my dog has heartworms?

Unfortunately, many cases of early heartworm disease or mild low worm burden heartworm disease are asymptomatic. That’s why your veterinarian will want to screen your dog every year for heartworm antigens, even if they're asymptomatic and they've been on heartworm prevention. That way, we can catch any cases that might pop up.

In the early stages of heartworm disease, some dogs may exhibit:

  • Lethargy
  • Coughing
  • Shortness of breath

What are some middle to late-stage symptoms of heartworm?

The symptoms of heartworm disease tend to depend on the level of worm burden.

If your dog has a severe worm burden, you might see the following symptoms:

  • Congestion in the lungs or the liver
  • Bulging ribs
  • Bulging abdomen
  • A heart murmur because of those worms affecting the heart’s function

How does my veterinarian diagnose heartworm disease?

Initially, we would do the antigen test, which tells us that adult heartworms are giving off a substance that's screened for in the bloodstream. If you wanted to confirm that diagnosis, you do something called a microfilaria test, which tests for the larva in the blood. And that tells us your dog has an adult heartworm infestation.

How is dog heartworm disease treated?

While heartworm disease is treatable, especially if detected in the early stages, it is not a fun process for your dog. We have to do an injection into the spinal muscles. We typically do three injections—one on the first day, then one a month later, and one a day later. These injections can be painful, but we put them on steroids to reduce inflammation. We also have them on strict activity restriction because as these worms die off, they might give little pieces off into the dog's heart and the dog's bloodstream. And we never want those to cause any sort of blood clot or other issues. The dog being treated is pretty restricted for a couple of months and has to come into the clinic to do all these things. That’s why veterinarians hammer the importance of heartworm prevention so much, as we’d much rather prevent heartworm disease than put your dog through this painful and drawn-out treatment.

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

Your dog should come in as soon as possible. If you have a puppy, we recommend bringing it in as quickly as possible after you get it. We recommend starting puppies on heartworm prevention at eight weeks or, again, as soon as you have them. Any dog over six months should have a heartworm test before starting for heartworm prevention.

We recommend bringing in any rescues, adoptees, or adult dogs that you might get as soon as possible, and we would screen them for heartworm and then get them on heartworm prevention as well. We cannot overstate the importance of preventing heartworm disease as opposed to treating it.

As veterinarians, we are happy to have things like Heartworm Awareness Month to shine a light on such a crucial issue. If your dog isn’t yet on heartworm prevention, please contact your primary care veterinarian and set up a time to take them in!