What Does Heartworm Disease Do to Dogs?

By PetMD Editorial. Reviewed by Hanie Elfenbein, DVM on Jun. 23, 2019

Reviewed and updated for accuracy on June 24, 2019 by Dr. Hanie Elfenbein, DVM, PhD

A few years ago, it was possible to say that your dog wasn’t at risk for heartworm disease because of where you live; however, today, that is a dangerous way to think.

According to the American Heartworm Society, cases of heartworm disease in dogs have been reported in every US state, including Hawaii and Alaska.

Heartworms may infect your dog for years or months before you even notice any symptoms, by which time your dog may be too sick to receive life-saving treatment. The best treatment is prevention that is given every month of the year—even if it’s snowing.

The key to understanding canine heartworms (Dirofilaria immitis) and the detrimental impact they can have on your dog’s health is understanding how your dog gets heartworms, what happens to their body once infected, and how rigorous treatment is.

Then you can see why it’s crucial to prevent heartworm infections rather than let your dog become infected and have her suffer the consequences.

How Dogs Get Heartworms

Heartworm disease begins with an infected animal, known as the source, that has microfilariae (immature larval heartworms) circulating in their blood. When a mosquito bites the animal, they will inadvertently also suck up a number of microfilariae.

The microfilariae migrate to the mosquito’s salivary glands, which enables them to burrow into your pet through a mosquito’s small bite wound.

After entering a host, the larva goes through its first molt over the course of 1-12 days. Most heartworm prevention is targeted at this early stage.

The second molt occurs within the next 45-65 days. After the second molt, the juvenile adult heartworm works its way through the host’s tissues and all the way to the heart as early as 70 days after first entering the host.

The majority of juvenile adult heartworms arrive in the heart by 90 days, where they grow rapidly in length and size. Males can grow to be 6-7 inches long, while females can grow to be 10-12 inches long.

The heartworms actually continue to grow after reaching sexual maturity (about three months after entering the heart), and the mated females start to pass microfilariae into the blood.

Once the microfilariae start circulating through a dog’s blood, they have become a host and are able to pass the disease back to mosquitos.

The heartworms will continue to live in the heart until they die—typically 5-7 years.

The Damaging Effects of Heartworms in Dogs

When a dog is first infected with heartworms, there are no visible or detectable signs. In fact, even a blood test will not detect heartworms initially.

The changes in dogs begin during the final molt of the heartworm larvae; it is then that the immature larvae arrive in the right ventricle and neighboring blood vessels.

When you do start seeing signs of heartworms in dogs, it’s due to two factors:

  • The damage the worms cause to the arteries in the lungs (pulmonary arteries)

  • The obstruction of blood flow that results from inflammation and the number of heartworms present

Another complication some animals develop is similar to an allergy to the heartworms, or to the microfilariae, which can cause varying signs that are similar to allergies or asthma.

Arterial Damage

Within days, your pet’s artery lining will start to sustain damage. The body responds by inducing inflammation of the artery, called arteritis, and other inflammation in the area to try to heal the damage.

Unfortunately, the heartworms cause damage at a faster rate than the body can heal.

Over time, the arteries develop certain characteristics that are typical of heartworm disease; often those changes can be seen on X-rays. The vessels become tortuous and dilated. Blood clots and aneurysms are common side effects, and complete blockage of small blood vessels can occur.

Blood Flow Blockages and Fluid Accumulation

The mass of heartworms in your dog’s body can cause significant blockage to the normal flow of blood. Depending on the size of your dog’s blood vessels, even one worm can cause significant damage.

The blood will reroute to arteries that are not burdened by worms, which results in complete and partial blockages of blood vessels. This causes fluid to accumulate around these blood vessels in the lungs and reduces the effectiveness of the lung’s ability to oxygenate the blood.

Think of a garden hose. If pieces of debris block the hose, pressure builds up as the flow of water is obstructed. This is what happens to the heart and blood vessels when more and more heartworms congregate within the right ventricle.

Due to the inflammation, blood vessel obstruction and fluid accumulation, you will start to see the “heartworm cough.” Your pet may also display exercise intolerance, nosebleeds, shortness of breath and weight loss.

The smaller your pet is, the fewer worms it takes to cause these problems.

Heart Failure

As immature worms continue to arrive and mature in the heart and lungs, your dog’s reactions will become more significant, and the signs will worsen.

The blood vessels and surrounding lung tissue are damaged, which, in turn, increases the blood pressure (hypertension) in the right side of the heart and vena cava—eventually causing heart failure.

The severity depends on the number of heartworms present and the dog’s reaction to the worms.

Caval Syndrome

Caval syndrome is a serious complication of chronic heartworm disease and is one of the most severe signs of an infection.

Symptoms of caval syndrome include:

  • Acute anorexia

  • Respiratory distress

  • Weakness

  • Anemia

  • Hemoglubinuria

  • Hepatic and renal dysfunction

  • Signs of forward and backward heart failure

With caval syndrome, there is almost complete blockage of all blood flow, resulting in sudden collapse. This severity of heartworm disease is deadly, even with emergency care.

Positive Heartworm Diagnosis

Usually once a diagnosis is made through a blood test, the veterinarian will order X-rays, a complete blood count (CBC), a chemistry profile (evaluates the function of the body’s organs) and a urinalysis to determine the impact of the heartworm infection on your dog’s health.

Heartworm-positive dogs displaying signs of heart disease may have a complete cardiac evaluation done, or evaluation of any other area of the body that is indicated by the initial test results.

After evaluating your dog, the veterinarian will assess the severity of the infection to determine which one of the four heartworm classes your dog falls into. By determining the class of heartworm disease, your vet can choose the best method of treatment.

Class I: Lowest Risk

These dogs are young and healthy with minimal heartworm disease that’s evident on X-rays, but all other tests are normal.

Class II: Moderately Affected

In Class II, dogs have some coughing and difficulty breathing. Changes are seen on X-rays, and blood work may reveal some kidney and/or liver damage.

Class III: Severely Affected

Dogs cough, have difficulty breathing and experience significant weight loss. There is visible damage on X-rays, and blood tests show kidney and/or liver damage.

Class IV: Caval Syndrome

The dog is collapsing in shock. All of the above abnormalities are more intense, and the dog is dying. Once a dog reaches Class IV, they are treated with surgical removal of some worms, if possible. But there is no guarantee that this treatment will be successful. Many patients with caval syndrome die during or in spite of treatment.

The Effects of Heartworm Treatment

Without treatment, a heartworm-positive dog will rapidly progress through the stages of heartworm disease until they reach caval syndrome. A dog with heartworms will not live a long or healthy life—treatment is required for their survival.

The adult worms are treated first, then a different method is used to kill the microfilariae and migrating larvae. They have to be eliminated separately, as no medication kills both.

The treatment of heartworms in dogs is a long, multi-step process. It takes over six months to complete a heartworm treatment and then test a dog to confirm that it has worked. During this time, this is what your pup will have to go through:

Exercise Restriction

The first part of treatment is mandatory exercise restriction. This is done to keep your dog’s heart rate and blood pressure down in order to reduce their risk of dying or having dead worms cause an allergic reaction.

This restriction will continue throughout your dog’s treatment until success can be proven. It is essential that you adhere to it because it can prevent very serious and fatal cardiopulmonary complications.

Antibiotics and Steroids

After a confirmed heartworm-positive diagnosis, your veterinarian will prescribe an antibiotic and a steroid.

The antibiotic helps to kill a bacteria found inside heartworms. This weakens the worms and makes them more susceptible to the treatment medications. The steroid helps to reduce the risk of allergic reaction from dying worms.

Your veterinarian will also start your dog on monthly heartworm prevention to help prevent new infections.

Adulticide Injections

The prescription medicine used to kill adult heartworms is called “adulticide.” The only adulticidal drug approved to treat heartworms is melarsomine dihydrochloride.

Melarsomine dihydrochloride is an arsenic derivative that is administered by a careful intramuscular injection. A heartworm-positive dog will typically have to get three of these painful injections.

They will receive their first injection 30 days after they finish their round of antibiotics and steroids. After another 30 days, your dog will receive their second injection, followed by the third the very next day.

Melarsomine dihydrochloride has the potential for significant side effects due to the destruction of the adult worms and the resulting blood vessel blockage and inflammation.

Close veterinary monitoring is paramount. Side effects can be immediate or take up to two weeks to appear.

As the inflammation peaks after adulticide treatment at 5-10 days, anti-inflammatory medications are sometimes used.

However, some anti-inflammatory medications can reduce the effectiveness of the adulticide, so a veterinarian will recommend when it is best to use them, if at all.

Approximately four months after the adulticide therapy, your dog will be retested for the presence of heartworms. This will determine if a second treatment will be needed.

Heartworm Prevention

Heartworm prevention should be given year-round, even if you don’t think mosquitos are active.

It is much simpler to prevent heartworm disease from occurring than to treat it afterward and have your dog go through the pain of the disease itself and also its treatment.

As long as you give it to your dog every month (or as prescribed), heartworm prevention is very effective in preventing heartworm infection and disease.

Your veterinarian can determine which type of prescription heartworm prevention to use for your dog. Ideally, dogs start on monthly heartworm preventatives at 8 weeks old.

All dogs should also have a heartworm blood test at around 7 months of age and then be retested on an annual basis (or according to your veterinarian’s recommendations).

Any missed preventative doses should be communicated to your veterinarian, and retesting should be scheduled accordingly.

Heartworm disease is a serious health issue with deadly consequences, but prevention is easy.

By: T.J. Dunn, Jr., DVM

Featured Image: iStock.com/Capuski

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