Heartworm in Dogs: Causes, Prevention, and Treatment

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I think it’s safe to say that we’re all too familiar with mosquitos, the pesky flying insects that tend to eat us alive all summer long leaving red and itchy bumps along our bodies. Mosquitos are one of the few downsides that come along with the warmer months and can truly ruin a fun summer day outside—not just for us, but for our dogs too. Did you know that mosquitos can bite your dog as well? However, instead of leaving behind itchy red bumps, they leave behind something much worse—heartworms. With summer right around the corner it’s important to understand exactly what heartworm is and how to prevent it from affecting your dog. Today, I’m sharing everything you need to know about heartworms in dogs: causes, prevention, and treatment. 

What are Heartworms?

Heartworm, scientifically known as D. immitis, is a deadly parasite that is transmitted only by mosquitos. Mosquitoes pick up heartworm larvae, known as microfilaria, from infected dogs when taking a blood meal and then transmit it to the next dog they bite. These microfilaria travel through the bloodstream and after a couple of months will settle in the dogs heart, where they continue to grow into mature worms, hence the name “heartworm”. 

Heartworms will continue to grow for around 6 months and can grow up to a foot long. They can live in a dogs body for seven years and will continue producing offspring (microfilaria) for the duration of that time. In extreme cases, dogs can harbor hundreds of worms in as short as only one year. 

If not caught quickly enough, heartworms can begin to inflame and damage your dogs heart, arteries, and lungs and will eventually be fatal. 

How Common is Heartworm Disease?

Heartworm is not uncommon and as of 2022 was affecting about 1 in 100 dogs in the U.S. each year. Because heartworms are carried by mosquitoes, the disease is much more prevalent in areas that border a body of water and warmer areas such as the south. In the United States, heartworm is most commonly seen along the Atlantic and Gulf coasts and along the Mississippi River. 

Symptoms of Heartworm Disease

Heartworm disease sounds extremely scary, so it may surprise you that for many heartworm positive dogs they will never show a single symptom. 

Dogs that are tested for heartworms on the recommended annual basis are most likely to come up positive, and be treated, before the microfilaria make their way to the dogs heart. In these cases, your dog will likely not show any symptoms of being infected.

For dogs that are not tested regularly, they may eventually show symptoms such as:

• Coughing
• Exercise intolerance
• Abnormal lung sounds
• Decreased appetite
• Weight loss
• Swollen belly (due to heart failure)

Dogs showing more extreme symptoms such as abnormal lung sounds, decreased appetite, and a swollen belly are much less likely to survive treatment than those that show no symptoms or mild symptoms such as a light cough. This is because once the worms have fully taken over the heart, it is significantly more difficult to get rid of them. 

How to Prevent Heartworm Disease

Luckily, there are several heartworm preventatives (HWP) on the market that are successful in protecting your dog against heartworm disease. Most of these preventions come in the form of a monthly chewable that contains a medication such as ivermectin or milbemycin. Your dogs vet will likely prescribe one of several different heartworm preventatives that are on the market. Some of the most popular include Heartgard, Interceptor, and Simparica Trio. All of these preventatives are prescription only but can be purchased through an online pet pharmacy such as Chewy. 

Depending on your dogs lifestyle and location, your vet may recommend giving preventative year round or only in the spring and summer months. For many dogs, keeping them on HWP year round is the safest option. 

In order to prescribe HWP, your dog will need to be tested for heartworms yearly. This is because giving HWP to an already heartworm positive dog can have negative side effects. If your dog is getting their prevention as prescribed, they are at a very low risk of becoming infected with heartworms. In most vet clinics, testing for heartworms is simple, only involving a blood draw and a quick in house test.

Treatment for Heartworms

If your dog becomes positive for heartworms, do not panic. Luckily, in most cases, heartworm is completely treatable in dogs. Unfortunately, the treatment is quite long and painful for you and your pet. 

Treatment for heartworms typically looks like this:

1. Heartworms are first diagnosed by a blood test. From there, further diagnostics may be run such as radiographs to check the heart and lung function, microfilaria count to confirm a worm count, or an echocardiogram to see inside heart chambers and visualize any worms. 
2. Once heartworms are officially diagnosed, your vet will encourage you to restrict activity with your dog. This is often the hardest part for us and our dogs, as it involves no playtime or exercise for up to 6 months. The reasoning for this is that exertion will increase the rate of damage to your dogs heart and lungs.
3. Killing the heartworms involves an FDA improved arsenic containing drug known as melarsomine dihydrochloride. The drug is administered in a series of injections that will take place over several months. The injections are painful as they have to be administered quite deep into your dogs back muscles. Many vets will place their patients on a pain medication during this time to help with some of the overall pain. 
4. About 6 months after your dog completes their series of injections, they will be retested for heartworm disease. At this time, most pets will be negative and free of any heartworms or microfilaria. From here, your vet will prescribe a heartworm preventative and highly encourage you to keep them on it year round. 

Heartworm treatment is expensive and painful for you and your pup and not worth going through when the disease is so easily preventable. If you don’t currently have your dog on HWP, talk to your vet today about getting your dog tested and protected before summer is in full swing.