Heartworm Disease in Dogs

By T. J. Dunn, Jr., DVM

In many North American regions April is the time of the year when veterinarians begin to check dogs (and cats) for exposure to heartworm organisms that may have occurred during the previous mosquito season. If your pet was infected last mosquito season, evidence of the disease may be detectable then -- though it really is dependent of the time in which your dog was infected. Like any other pathogenic situation, the earlier a diagnosis is made and treatment is begun, the better the chances your pet will recover properly. Give your veterinarian a call early this spring so that your dog can be tested for heartworm disease. Current heartworm tests are more accurate than what was available only a few years ago.

The Life Cycle of the Heartworm

The key to understanding heartworms (Dirofilaria immitis) and how they affect your pet’s health is understanding the worm's life cycle. Once this is understood then it will make sense why it is wiser to prevent a heartworm infection than wait and treat the worm once it is present.

The heartworm goes through a total of four molts to mature into an adult worm. The first two occur inside the mosquito and the next two occur inside the final host. So how does the heartworm get into your pet?

Heartworm disease begins with an infected animal, known as the source, that has circulating microfilaria in its blood. A mosquito stops by for a meal and inadvertently sucks up a number of circulating microfilaria in the blood. Once inside the mosquito’s body, the microfilaria go through two molts over 14 days or longer depending on the environment’s temperature. They go through their first two molts and change from an L1 to an L2 and then an L2 to an L3 (third stage of development of the larva).

It is an L3 that they are able to migrate into the mosquito’s salivary glands, which enables them to burrow into your pet though a mosquito's small bite wound. Once inside an animal (host) where it continues development, the heartworm takes at least 6 to 7 months to go through the last two molts and to become sexually mature before the infection can be detected by a Heartworm test.

The L3 larva goes through its first molt to the L4 within the first 15 days and as early as 2-5 days after infection. The second molt from the L4 to the L5 occurs within the next 2 months. The L5 larva is considered a juvenile adult and works its way through the host’s tissues all the way to the heart as early as 70 days after first entering the host. The majority of L5 heartworm larvae arrive in the heart by 90 days, where they stay and grow rapidly in length and size. It will continue to live in the heart until it dies, typically between 5-7 years.

The heartworms actually continue to grow in size after reaching sexual maturity (about three months after entering the hear) and the females start to pass microfilaria into the blood. This is why some pets are infected with numerous worms. The mass of twisted and intertwined heartworms in dogs can serve as a significant mechanical blockage to the normal flow of blood. In fact, adult female worms have been known to grow up to 14 inches long; males, meanwhile, are generally shorter. 

Think of a garden hose. If pieces of debris block the hose, pressure builds up due to the obstruction of the flow of water. This is what happens to the heart and blood vessels when more and more heartworms congregate within the right ventricle. The smaller your pet (the host) is, the fewer worms it takes to cause a problem.

Once the heartworm becomes an adult it will continue to produce young for many years in the dog (a shorter duration for cats and other animals, which are not the usual host for the worm). Adults have been documented to live around 7 years in dogs (2-3 years in cats).

This horrific display completes the normal life cycle of the heartworm.

Another factor is some animals develop something similar to an allergy to the heartworms, or to the microfilaria, which results in the occult heartworm infections and can cause varying signs similar to allergies or asthma. This more elusive kind of infestation occurs most often in the cat. It is not uncommon for infected cats to suddenly die from the effects of just a few worms.

Signs of Heartworms in Dogs

Two major mechanisms result in the signs of heartworms in dogs. The first is due to the damage the worms cause to the arteries in the lungs (called the pulmonary arteries). The second is the mechanical obstruction of blood flow that results from the inflammation and the number of heartworms present.

When a dog is first infested with heartworm there are no visible or detectable signs. In fact, even a blood test will not detect heartworms initially. The changes in dogs begin when during the final molt of the heartworm larvae; it is then that the immature L5 larvae arrive in the right ventricle and neighboring blood vessels.

Within days, the artery lining is damaged. The body responds by inducing inflammation of the artery, called endarteritis, and other inflammation in the area to try to heal the damage.  Unfortunately, the heartworms cause damage at a rate faster than the body can heal.

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

The blood then re-routes to non-worm burdened arteries. This results in complete and partial blockage of blood vessels, causing fluid to accumulate around these blood vessels in the lungs and reducing the effectiveness of the lungs' ability to oxygenate the blood.

Due to the inflammation, blood vessel obstruction and fluid accumulation, your pet will begin to cough. S/he may display exercise intolerance, nosebleeds and shortness of breath, as well as a type of pneumonia secondary to the increase in lung inflammation (called pulmonary eosinophilic granulomatosis).

As immature L5 worms continue to arrive and mature in the heart and lungs, your dog's reactions become more significant and the signs worsen. Blood vessels and surrounding lung tissue are damaged, which 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.

Over time, the immune system enters into a state of overactivity. This puts extra proteins (in the form of antibodies) into circulation, which then settle in various bodily organs and cause inflammation, tissue damage and pain in areas such as the eye, kidney, and joints.

One of the most severe signs of heartworms in dogs (and cats) is called Caval Syndrome or Vena Cava Syndrome.  This is seen when there are large numbers of adult worms (usually around 100 or more) invade the heart. There is almost complete blockage of all blood flow.

Often there will be no signs of heart disease prior to the animal’s collapse. When fainting and collapse does occur, it is accompanied by severe shock, red blood cell destruction and often death within 1-2 days.

Sometimes the only chance for survival in these cases is for the veterinarian to surgically remove the heartworms from the heart through the jugular vein. If a sufficient number of heartworms can be removed to re-establish sufficient blood flow, there is a slim chance of survival.

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