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.
Diagnosis of Heartworm in Dogs
Any patient presented because of the suspicion of heartworm disease will have a thorough medical history taken and undergo a complete physical exam, chest X-rays and routine blood chemistry tests.
The definitive diagnosis (proof of diagnosis) is usually made through the use of a heartworm antigen test. Whole blood is drawn from the dog, stabilized so it will not clot, and tested for the presence of a protein shed by the female worm as she passes microfilaria. This test is very reliable and will detect heartworm burdens of 2-3 worms or higher.
Another common test that can be used in conjunction with the antigen test is the Knotts test or Modified Knotts test. This is where whole blood is drawn and treated to cause the blood cells to break open. Then the sample is spun in a centrifuge, the top portion is poured off, and the bottom sediment is looked at under the microscope for the presence of microfilaria.
Usually once a diagnosis is made via a blood test, then X-rays, CBC (complete blood count), chemistry profile (evaluates the function of the body’s organs), and urinalysis are evaluated to determine the impact of the heartworm infection on the dog’s health. Pets displaying signs of heart disease side effects may have a complete cardiac evaluation, or evaluation of any other area of the body that is indicated by the initial test results.
Your dog will be staged for heartworm disease as part of the evaluation. It assists your veterinarian in choosing the best method of treatment for eliminating the heartworms:
Lowest risk... young healthy dogs with minimal heartworm disease evident on X-rays and all other tests are normal.
Moderately affected dogs... some coughing is noticed, some difficulty breathing, changes are seen on X-rays, and blood work may reveal some kidney and/or liver damage.
Severely affected dogs... the patient has weight loss, coughing, difficulty breathing, more damage visible on x-rays, and blood tests shows kidney and/or liver damage.
Vena Cava Syndrome or Caval Syndrome... the dog is collapsing in shock, all of the above abnormalities are more intense and the dog is dying. They are initially treated with surgical jugular removal of some worms if possible. There is no guarantee this treatment will be successful and many patients with Caval Syndrome die in spite of treatment.
Treating Heartworms in Dogs
By now, it is clear that the treatment varies from dog to dog. Each animal’s personal condition is evaluated and the treatment protocol tailored to best effect a full recovery with the least side effects. Therefore, the following will be very general regarding the medications used for treating heartworm disease and the more common side effects. In the end, you and your veterinarian will choose the best treatment protocol for your pet.
Treatment involves two basic areas:
1. Patient evaluation and stabilizing for treatment procedure.
2. Elimination of all forms (adult, larvae, and microfilaria) of the heartworm parasite.
Patient Evaluation and Stabilization
The veterinarian evaluates the overall health of the animal by conducting X-rays and blood and heart tests, then determines how to best proceed with treatment. Part of this evaluation is staging the severity of the heartworm disease in the dog (see above).
Some animals need to have certain conditions stabilized before heartworm treatment can proceed. Those in third stage heartworm disease, for example, may require deliberation to decide if it is best to try surgical removal of some worms through the jugular vein before any other steps of parasite elimination are considered.
Elimination of the Heartworm Parasite
This is a two-step process. The adult worms and the microfilaria are eliminated separately. No one medication kills both. The adults are treated first, then a different treatment is used to kill the microfilaria and migrating larvae.
The most serious side effects usually occur with the treatment of the adult heartworms. As the worms die they lodge in the lung arteries and block even more blood vessels than before treatment. Besides the usual inflammation caused by the presence of the worms, the inflammation is amplified due to the decomposing worms within the blood vessels.
The worm destruction also releases foreign substances in to the dog’s circulation as the worms break down and are eliminated from the dog by the immune systems. A large amount of inflammation and swelling generally occurs during this period.
Before treatment begins, it is very important to ask your veterinarian any questions you may have about the treatment and what to expect. Some veterinarians will keep the dogs in the hospital during treatments for observation. Your doctor will make the decisions on an individual basis regarding what would be best for your dog.
The prescription medicine used to treat the adult heartworms are called adulticides. The two adulticides used most commonly are derivatives of arsenic. It is not known exactly how these medications work to kill the heartworms. We just know they do work.
(NOTE: New medications may be available at any time; this listing of treatments may not be complete!)
The first one is thiacetarsamide. It has been used for at least half a century and is effective but can be toxic to the liver, kidneys, or cause severe irritation if the solution gets outside of the vein. The second medication is called Melarsomine dihydrochloride. With fewer side effects than thiacetarsamide, it is also an arsenic derivative and is administered by a careful intramuscular injection. It appears to be as effective and possibly more so in dogs than thiacetarsamide. It also has potential for significant side effects, so close veterinary monitoring is paramount.
Side effects from Melarsomine dihydrochloride can be immediate or take up to 2 weeks to appear. One aspect of the side effects are due to the destruction of the adult worms and the resulting blood vessel blockage and inflammation.
No matter what adulticide is used, it is very important to keep your dog very quiet and follow all of your doctor’s instructions. If you have any doubt about what to do or what is going on, do not hesitate to call your veterinarian ASAP.
As the inflammation peaks after adulticide treatment at 5-10 days, sometimes anti-inflammatory medications are used. However, some anti-inflammatory medications can reduce the effectiveness of the adulticide, so a veterinarian will recommend when it is best to use the anti-inflammatory medication, if at all. Some patients even require a second set of adulticide treatments since the very immature L5 heartworms and young female adults are more resistant to the treatment.
After the adulticide treatment and its side effects are resolved (usually at about 1 month post treatment), the microfilaria are then eliminated with one or another of two common heartworm preventives, Ivermectin or Mibemycin oxime. This will be done approximately one month after the adulticide treatment, depending on your veterinarian’s final decision regarding when it can be done.
Approximately four months after adulticide therapy, the dogs are retested for the presence of heartworm(s). This will determine if a second treatment is needed.
Heartworm Prevention in Dogs
It is much simpler to prevent heartworm disease from occuring than treating it afterward. The most common preventatives on the market kill the immature heartworm larvae before they molt to the L5 stage. As long as they are given to the dog every month (or according to the medication instructions), they are very effective in preventing heartworm infection and subsequent development of heartworm disease.
The choice of which type of prescription heartworm prevention to use will be determined by a discussion with your veterinarian and what is best for your pet. Ideally puppies are started on monthly heartworm preventatives by 8 weeks of age. They should have a heartworm blood test at around 7 months of age and then be retested on an annual basis or according to the veterinarian's recommendations.
The latest recommendation by the American Heartworm Society is once every 2-3 years in dogs that NEVER miss a dose of preventative. Any missed preventative doses should be communicated to your veterinarian and re-testing should be scheduled accordingly.