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The Dangers of Heartworms and How to Protect Your Dog

Being a pet parent comes with a lot of responsibilities. One of these responsibilities is learning how to protect your dog from parasites, including ticks, fleas, and heartworms.

In this article, we look at the dangers of heartworm and the best ways to protect your dog. Read on to learn more.

About Heartworms—Overview

Heartworms are parasitic worms that can live in your dog’s heart, lungs, and the surrounding blood vessels. In rare instances, they may also be found in other parts of your pet’s circulatory system.

Heartworms can live in your dog's body, lay eggs, and even reproduce. If not treated, your dog's body can host hundreds of worms that pose great danger to your dog's health.

Heartworms are transmitted through mosquitos. Your dog can get these worms if they get bitten by an infected mosquito.

Since it's impossible to tell whether a mosquito is infected, taking measures to protect your dog from heartworm is paramount.

The heartworm lifecycle starts in a dog's bloodstream. When a mosquito bites an infected dog, the mosquito becomes infected with microfilariae, or heartworm offspring. The heartworm offspring become infective larvae in 10 to 14 days.

The infective larvae can then be transmitted to your dog when the infected mosquito bites them. The infective larvae will take six to seven months to mature into adult heartworms inside your dog.

When the adult heartworms mate, the females release microfilariae (offspring) into the bloodstream which completes their lifecycle.

According to the American Heartworm Society, adult heartworms can live inside the host for five to seven years if not treated. Female heartworms can reach about 10 to 12 inches in length, while male heartworms can reach four to six inches.

The number of parasites inside your dog is called the worm burden. The number of worms in a dog can range from 1 to 250, with the average being 15 worms.

Understanding Heartworm Disease in Dogs

Heartworm disease is caused by Dirofilaria immitis. This is a large worm that completes its life cycle in about six to seven months. The worm lives in the host's heart, where it can spend several years. The worms clog the heart, leading to less blood supply to the rest of the body, which can lead to heart failure and other health complications (we'll look into this in detail later).

The symptoms of heartworm disease in dogs will depend on the severity of the infection and the organs affected. Common signs include:

  • Lethargy
  • Decreased appetite
  • Cough
  • Weight loss
  • Difficulty breathing
  • Exercise intolerance

Contrary to popular opinion, your dog is at risk of heartworm disease regardless of where you live. The American Heartworm Society reports that cases of heartworm disease have been reported in all US states, including states once considered non-endemic, like California.

Dangers of Heartworms in Dogs

Unfortunately, heartworm infections don’t present with detectable signs early on—your dog may be infected for years before you notice any symptoms.

Sometimes a dog may be too vulnerable for the treatment to be effective by the time you see the symptoms. That is why the best approach is to administer preventative treatment, even if your dog isn't showing any signs of heartworm infection.

In most cases, when you notice signs of infection the immature larvae may be impacting your dog's right ventricle and the associated blood vessels. This could damage the pulmonary arteries and affect blood flow.

The following are the damaging effects of heartworms in dogs:

Blood Flow Obstruction and Fluid Accumulation

The increased number of heartworms and inflammation in your dog's body can obstruct normal blood flow. Even a few worms in your dog's body can affect the normal blood flow depending on the size of the blood vessels.

Fluid accumulation can also result in blocked blood vessels in the lungs. Again, this will impact the lung's ability to oxygenate the blood.

Heartworms in your dog's right ventricle will lead to pressure build-up when the blood flow is obstructed. As a result, your dog will start to cough and experience other symptoms such as nosebleeds, exercise intolerance, weight loss, shortness of breath, etc.

Damage to Your Dog's Arteries

The presence of heartworms in your dog's body will eventually damage the artery lining. This will lead to artery inflammation (arteritis) when the body tries to heal the affected areas.

Since heartworms causes damage at a faster rate than your dog's body can heal, heartworm disease will begin to develop in your dog.

At first, the disease characteristics can be seen through X-rays. As the disease progresses, vessels become dilated, and in some cases, complete blockage of small blood vessels occurs.

Increased Blood Pressure Can Cause Heart Failure

Your dog will experience more severe symptoms as more worms find their way into the heart. As this continues, your dog's health will further deteriorate.

More mature worms in your dog's heart means more clogging in the blood vessels and lungs. This will subsequently increase pressure in the heart. If the situation is not rectified, heart failure can occur.

In short, the more worms there are in your dog's heart, the more severe the condition will be, and vice versa.

Caval Syndrome

Caval Syndrome is a serious condition caused by heartworm infection. This syndrome occurs when populous heartworms enter the vena cava, the right ventricle of the heart, and the right atrium of an infected dog.

Caval syndrome is an advanced condition of heartworm disease that can result in many serious issues affecting vital body organs, including the kidney and the liver. The condition can lead to jaundice, anemia, heart failure, and even death.

Although caused by the heartworm parasite, the symptoms of Caval syndrome in dogs differ from those of typical heartworm disease. The symptoms include:

  • Coughing
  • Lethargy
  • Difficulty breathing
  • Increased breathing rate
  • Appetite loss
  • Body weakness
  • Fever
  • Reduced activity
  • Weak pulse
  • Anemia
  • Pale mucous membrane
  • Hemoglobinuria
  • Murmur in heartbeat
  • Heart failure

Since Caval syndrome is an advanced stage of heartworm disease, the survival rate is minimal even after treatment. Your dog can have a better chance of surviving if the heartworm from the vena cava, right ventricle, and right atrium is removed immediately.

However, before any treatments are administered, your dog's liver and kidney must be analyzed to ensure they are properly functioning and won't be affected by drugs used to kill the heartworms.

How to Protect Your Dog from Heartworm

The best way to protect your dog from heartworm disease is to prevent heartworm infection from occurring in the first place. There are several FDA-approved products that your vet can prescribe to protect your dog from heartworm infection.

Heartworm prevention products are available in many forms, such as tablets, topicals, chewables, and even injectable versions. In fact, some of these products can also help protect against tick and flea infestation to give your dog complete protection against parasites.

Here are some of the products that your vet may prescribe for heartworm prevention:

Tri-Heart Plus for Dogs

Tri-Heart Plus is available on prescription only. This chewable tablet can help eliminate the tissue stage of heartworm larvae. It may also be effective in the treatment of hookworms in your dog. The product has three dosage strengths to cater to dogs with varying weights.

Heartgard Plus

This once-a-month chew product for dogs is available by prescription only.

Heartgard Plus Chewables for dogs don’t only protect from heartworms but are also effective in treating roundworms and hookworms for one month.

However, Heartgard Plus won't protect your dog from adult D. immitis, so your dog should undergo a heartworm test and treat adult heartworms before starting this treatment. Unlike Tri-Heart Plus, Heartgard Plus has no weight limitations during usage.


Your vet may prescribe REVOLUTION Topical Solution for Dogs to treat internal and external parasites. This product can help prevent heartworm disease, kill adult fleas, and prevent fleas from hatching.

This one-month protection can be administered to puppies six weeks old and above and is safe for heartworm-positive dogs and pregnant and nursing females.


Simparica, a once-a-month chewable product, is FDA-approved to protect your dog against parasites. Simparica Trio Chewable Tablets won't only protect your dog from heartworm disease but also roundworms, hookworms, fleas, and ticks. The product requires a veterinary prescription and is ideal for puppies eight weeks old and above.


Another product that your vet may prescribe is Sentinel Tablets for Dogs or Sentinel Spectrum Chewable Tablets for Dogs. This once-a-month prescription can help prevent heartworm disease and flea infestation in puppies and dogs. The product may also help control adult hookworms, whipworm, and hookworm infections in dogs.

In addition to the highlighted products, your vet can also prescribe other heartworm preventives for dogs. Consult your vet to help you decide which medicine is best for your dog's condition.

Heartworm Diagnosis in Dogs

If heartworm disease isn't detected early, it can pose serious health risks to your dog. That said, the earlier you diagnose a heartworm infestation in your pet, the better you can protect your four-legged friend.

Unfortunately, there are no early signs to show that your dog has a heartworm infestation. The easiest way to detect whether your dog is infested is to have your vet perform a heartworm test.

The test involves taking a small blood sample from your dog and testing for the presence of heartworm proteins. Heartworm proteins become detectable in your dog's bloodstream several months after an infected mosquito has bitten your dog; at least five months, to be more specific.

When Should Your Dog Be Tested for Heartworms?

Many factors will determine how often you should test your dog for heartworms. However, your pet should be tested annually for heartworm infection. That said, here are factors to guide on heartworm testing and timing:

  • When did you last administer heartworm prevention to your dog? If you have a puppy under seven months, you can simply start them on heartworm prevention (the heartworm test isn't necessary). After that, have your pet tested after six months and then perform annual tests in the future.
  • How old is your dog? If your dog is seven months or older and hasn't received heartworm prevention, prior testing is necessary before starting on heartworm prevention. After that, have your pet tested after six months and 12 months and annual tests afterward.
  • Have you traveled with your dog lately? You should also have your dog tested for heartworm if you traveled to an area more prone to heartworm infection.

You still need to test your dog for heartworm annually, even when they are on heartworm prevention, to ensure that they are safe and heartworm free.

How to Treat Heartworms in Dogs

Receiving news that your dog has tested positive for heartworms is devastating. However, if the disease is diagnosed early, it can be successfully treated.

If your dog is showing signs of the disease, your vet will first focus on stabilizing your dog before killing the immature and adult worms. Also, care should be taken to ensure that the treatment doesn't put your dog at further risk.

Heartworm treatment in dogs is a long process involving many steps. The treatment can take up to six months before you experience positive results, and testing is necessary to confirm whether the treatment is effective.

Here is what the heartworm treatment process in dogs involves:

Diagnosis Confirmation

Additional tests are necessary to ascertain whether your dog has heartworm. Heartworm treatment is complex and expensive and should only be administered when necessary.

Exercise Restriction

Restricting routine exercise for your dog is a crucial component of the heartworm treatment process. Physical exercise can increase blood pressure which can increase damage to the heart and lungs by the heartworms. As such, it is best to maintain a lower level of activity during the treatment for better recovery.

Dog Stabilization

Your vet may use appropriate therapy to stabilize your dog's condition before the treatment. Sometimes, your vet may prescribe antibiotics and steroids to kill bacteria inside heartworms, make the treatment more effective, and reduce the risk of an allergic reaction.


Following the above steps, your vet will recommend the best treatment for your dog, depending on the severity of the disease. Melarsomine dihydrochloride is a common FDA-approved drug containing arsenic (a compound used to kill adult heartworms in dogs). Your vet may inject this drug into dogs with stabilized class 1, 2, and 3 heartworm disease.

Surgery may be necessary for severe cases of heartworm disease, although once the disease has progressed to that point the survival rate is unfortunately low.


Your vet will need to test for microfilariae in your dog's bloodstream 30 days after the injection. Another test nine months after treatment is necessary to determine whether adult heartworms and juvenile heartworms were eliminated.

After this, your vet will prescribe the best heartworm prevention for your dog to prevent the disease from occurring again in the future.

As always, is home to the best pet medications, supplements, and foods. Visit our site today to order top quality products for your dog from a family-owned company that truly cares.




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