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 adults, they actually live in the chambers of the heart and can affect cardiovascular function, respiratory function, and other organ systems.

Dr. Burgess
Countryside Veterinary Clinic

How would my dog catch heartworm?

Heartworm disease is transmitted by mosquitoes. So a mosquito actually bites a dog that is infected with heartworm disease, picks up the larval stages from them, and then goes to another dog, bites that dog, and infects it. So typically this disease is thought to be in coastal areas or areas that are very hot.

However, in the state of Maryland where we live, we're concerned about heartworm disease year-round because the mosquitoes can be around whenever they want, as our weather is very variable, and we are pretty coastal.

Can heartworm be prevented?

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

What are the signs on my dog that would indicate my dog has heartworms?

A lot of cases of early heartworm disease or mild low worm burden heartworm disease are asymptomatic. So we actually screen dogs at our clinic every year for the heartworm antigen, even if they're asymptomatic and even if they've been on heartworm prevention so that we can catch any cases that might pop up. In the early stages, however, your dog can have lethargy, coughing, and shortness of breath.

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

If you have a severe worm burden, your dog can have congestion in the lungs or in the liver. So you could have bulging ribs. You can have a bulging abdomen, you could develop a heart murmur because of those worms affecting the function of the heart.

Initially, as I said, we would do the antigen test, which tells us that there are adult heartworms 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, and that tests for the larva in the blood. That tells us you do have an adult heartworm infestation that we need to deal with.

How would heartworm disease be treated?

Heartworm disease is treatable, especially if it's in the early stages, but let me be clear—it is not very fun. You have to do an injection into the spinal muscles in the dog. 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, and we do 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 into the dog's bloodstream. And we never want those to cause any sort of clots or other issues. So, the dog's pretty restricted for a couple of months and has to come into the clinic and do all these things. So we would much rather your dog not get heartworm disease than have to be treated for it.

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

Your dog should come in as soon as possible. So if you have a puppy, we recommend bringing it in as soon 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 prior to starting for heartworm prevention. So we do recommend bringing in any rescues, or adoptees, or adult dogs that you might get as soon as possible. We would screen them for heartworm and then get them on heartworm prevention as well. As I said, preventing heartworm disease is much easier than treating it.

If you still have other questions and you'd like to reach out to us, you can call us directly at (410) 461-2400, you can email us, or you can reach out on Facebook. But please do reach out, and we'll get back to you as fast as we can.

Dog Heartworm Disease - FAQs

Dr. Burgess
Countryside Veterinary Clinic

How is heartworm prevented in dogs?

Heartworm is prevented in dogs by giving a monthly dewormer that kills the larva in the bloodstream and prevents them from developing into adult heartworms.

What are the different types of dog heartworm prevention?

One type of heartworm prevention that we have here is an oral Interceptor, which is an oral dewormer. It targets heartworm as well as intestinal parasites, roundworms, hookworms, and whipworms, and targets the larva and the bloodstream. We also have a topical called Revolution, which is put on the skin and gets in the bloodstream and acts similarly; however, it does not target the intestinal parasites. So we do recommend the oral, if at all possible.

When should I start heartworm prevention for my dog?

We recommend starting heartworm prevention at eight weeks for all of our patients. So when they come in for their puppy exam, we typically give a first puppy dose of heartworm preventative based on the weight and what the owner wants to do in the long-term as far as giving those preventative, and we discuss the options with you.

How effective is heartworm prevention?

When given every month, year-round heartworm prevention is very effective. Again, it kills the larval stages before they can develop into adult heartworms, and adult heartworms are really what cause the problem in dogs. The American Heartworm Society recommends monthly, year-round heartworm prevention for all dogs, especially in coastal and hot areas. We're sort of borderline here, but I don't think that we can count on mosquitoes that transmit heartworm disease to die reliably, so I don't think that there should be any break in heartworm prevention, and that's how it's going to be most effective.

Does my dog still need a heartworm test if they're on prevention?

Typically, yes. We recommend a heartworm test every year for monitoring. We want to make sure our products are continuing to be effective. We want to make sure there's been no lapse in coverage. We want to make sure that resistant heartworms aren't sneaking their way up the body and becoming a problem for our dogs. So we recommend a heartworm preventative every month, and we do recommend a heartworm test every year for the safety of your dogs and to make sure our products are continuing to be effective.

Are there any holistic or over-the-counter dog heartworm preventions?

No, there's really nothing that I know of that can prevent heartworm that is not a prescription FDA-regulated product. So I wouldn't recommend anything like that.

Can I do anything in my dog's environment to reduce the risk of heartworm?

Some people do treat their yard for mosquitoes, which can be helpful in reducing the mosquito population and, therefore, the risk. But all it really takes is one mosquito that carries heartworm larva to bite your dog and cause heartworm disease. So, again, even if you're doing the best treatment possible, even if you think there's a minimal risk of exposure to mosquitoes, I would recommend monthly heartworm preventative.

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

If your dog misses one dose, I am fine with restarting the following month. If you miss more than one dose, please give us a call so we can assess your dog's risk, how many doses were missed, and whether or not we need to repeat a heartworm test prior to resuming prevention.

If you still have other questions and you'd like to reach out to us, you can call us directly at (410) 461-2400, you can email us, or you can reach out on Facebook. But please do reach out, and we'll get back to you as fast as we can.