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Heartworm Facts

  • Heartworms are a parasitic worm (about the diameter of thin spaghetti) that normally live free floating in the right ventricle of the heart and nearby blood vessels.

  • The parasites are transmitted from one individual to another by mosquitoes.

  • Heartworm is diagnosed with blood tests, and/or X-rays, along with other tests.

  • Heartworms are not detectable with the commonly used antigen blood test until they are sexually mature (about 6 months after entering the patient). Female worms must be present for accurate test results.

  • Heartworm has been diagnosed in all 50 states and also worldwide.  In most areas of the country Heartworm is an important pet health care issue. 

  • In most areas of the USA, veterinarians recommend Heartworm prevention for dogs.

  • Preventative measures in cats is also becoming more common as veterinarians continue to learn more about the parasite in cats and become skilled at recognizing the disease in cats.

  • Previously it was thought that felines were not at a significant risk of getting Heartworm and that preventative was not needed.  We are learning this is not necessarily true.

  • The need for pre-exposure preventative medication in cats depends on the incidence of Heartworm in your area.  Your local veterinarian is your best source of information.     

  • Dogs should be tested FIRST… before starting heartworm preventatives unless they are less than 7 months old.

  • Dog’s over 7 months of age that are started on preventative without first testing for Heartworm are at an increased risk of developing severe reactions.

  • Puppies should be started on Heartworm preventative by 8 weeks of age  (depending on the product being used) and then blood tested at 7 months of age.

  • Dogs should be tested on a regular basis, yearly if any doses of preventative have been missed and once every 2-3 years even if no doses were missed and preventative is given year around.  

  • In the cat, the presence of Heartworm is difficult to detect with a blood test alone.  Preventative medication is often started without testing unless signs of a Heartworm infection are noticed.

  • In cats, one worm can cause sudden death or sudden (acute) respiratory signs that are indistinguishable from asthma without a medical work up. 

  • Any area where dogs can get Heartworm, cats can get them as well.  The current rate of diagnosis ranges from 5 to 20% that of dogs in the same area.

The rate of cat Heartworm diagnosis varies by geographic area.  As veterinarians continue to improve diagnostic techniques in cats, it is suspected there will be less variation from the canine incidence rate in the same area (5-20%). Until the last few years, it was thought that feline Heartworm disease was so rare that prevention was not needed in cats.
This view is changing as we learn that many cat Heartworm infections are overlooked since feline Heartworm disease does not commonly present with the same symptoms as dogs and the signs and symptoms look and act like other cat diseases.

The detection of adult Heartworms in cats can be difficult and tests are not 100% reliable.

Important Definitions for understanding Heartworm

Life Cycle – The unfolding of one’s life from the beginning to the end including development, sexual maturity, environment, the ability to reproduce, and all that is required for a full cycle of development and maturity.
Host –
Is an animal or plant that harbors or nourishes another organism (parasite).
Parasite – A plant or animal which lives upon or within another living organism at whose expense it obtains some advantage.
Vector – A carrier, especially an animal or insect that transfers an infective agent from one host to another.
Biological Vector is an arthropod vector (in the case of Heartworm the mosquito) in whose body the infecting organism develops or multiplies before becoming infective to the final host.
Microfilaria – This is the name given to the microscopic, snake-like prelarval stages of the Filaroidea worm in the blood of man and animals and in the tissues of the vector.  Microfilaria are the offspring of adult heartworm reproduction.
Larva/larvae– An independent, motile, sometimes feeding, developmental stage (immature stage) in the life history of a worm or insect.  Usually undergoes metamorphosis or a molt to become an adult.  Heartworms have several stages of larvae.  L1 are the microfilaria in the blood of the infected animal and these are sucked up by the mosquito while it is feeding.  It molts from an L1 to L2 and again from an L2 to L3 in the mosquito before it can infect an animal.  L3 larvae are expelled from the mosquito’s mouthparts while the mosquito is feeding and burrow through the skin of the host. Inside the host it molts twice, from an L3 to an L4 and then from an L4 to an L5.  The L5 larva will become the adult once it matures.
Adult worm – A worm that is sexually mature enough to produce young. The worm may still continue to grow in length and size after reaching sexual maturity.  Adult heartworms live in the dog for up to 7 years.  In the cat they are thought to live a much shorter life, around 2 years.  Adult Heartworms prefer to live floating freely in the blood in the right ventricle of the heart and in the associated pulmonary blood vessels. The right heart ventricle receives the blood returning from the body that is low in oxygen.  The right ventricle’s function is to pump this blood through the lungs to absorb oxygen and to deliver the oxygenated blood to the left ventricle.  From this chamber blood is pumped back throughout the body.  The returning, poorly oxygenated blood flows from the large veins (the Vena Cava Vein) to the right side of the heart, to the lung arteries and capillaries (called the pulmonary arteries). The Heartworm organisms often move back and forth anywhere along this pathway from the Vena Cava to the right ventricle to the pulmonary arteries.  The signs an animal shows depends on the number of worms present, the animal’s reaction to their presence, and general health factors in the patient such as age, size, and obesity.
Aberrant migration or sometimes called abnormal migration – This term refers to the larval stage of the worm or other parasite where it wanders or deviates from the usual or normal course. The parasite ends up in locations that are different from where they would normally be present, such as under the skin, in the eye, or stomach.  This could occur in any animal but is more common in host animals that are not the preferred host for the parasite.  Think of aberrant migration as taking a wrong turn on the wrong road and ending up in an unintended location permanently.
Pre-patent Period – The time from initial infection with a parasite until the young adults are sexually mature and start to produce either larva or eggs.  In the Heartworm, this takes about 7 months in the dog and 9 months or more in the cat.
Patent Heartworm infection – Heartworm adults are sexually mature and producing microfilaria.
Microfilaremia – A condition where there are microfilaria circulating in the blood of an infected animal.
Occult Heartworm infection – The infected animal has sexually mature adult Heartworms present but for some reason no microfilaria are present.  The most common reasons are:  1) The worms are sexually mature but the infected animal’s immune system kills and removes the microfilaria as soon as they leave the female worm’s body, leaving none to be detected in a blood sample.  2) There is a single sex Heartworm infection and therefore no microfilaria are produced.  3) Aberrant migration resulted in no microfilaria production even though the worms are sexually mature.
Antigen – Any substance which under appropriate conditions is capable of inducing the formation of antibodies and of reacting specifically in some detectable manner with the antibodies induced.   Basically an antigen is anything the body can recognize as foreign and therefore stimulates its immune system to eliminate what has been recognized. A commonly used Heartworm test detects an antigen produced by the female heartworm that is producing microfilaria.

Heartworm Tests
There are several kinds of Heartworm tests performed on a blood sample.  The most common ones are mentioned below.

The most common one in practice is a Heartworm antigen test that looks for the presence of a protein given off by the sexually mature female worm actively producing microfilaria.  There are several manufacturers that produce tests of variable sensitivity.    They may miss infections if there are only 1-2 adult female worms present.

Other types of tests look for the presence of microfilaria in the blood sample.  One is a filter test that traps the tiny microfilaria in the filter. The treated blood is forced through a filter to trap the microfilaria for microscopic analysis.  This test and the Knotts test treat the blood with a chemical to break down the blood cells. In the Knotts test, the solution is then spun in a centrifuge, the top part poured away, and the contents in the bottom portion of the test tube are examined under a microscope for the presence of Microfilaria.  A positive diagnosis is made if microfilaria are present, however, Heartworm can be missed if there are very few microfilaria or the dog or cat has an Occult Heartworm infection.

A third type of test looks for the presence of antibodies to the larval stage of the Heartworm.  The down side of this test is that it only shows that the animal has had larval stages of Heartworm in its body at some time in the past.  The antibodies persist for a while after all worms are dead.   Therefore, a positive test is not proof of a current live Heartworm infection.  This test just confirms exposure to Heartworms at some time in the pet’s life.

Heartworm Infection -  versus -  Heartworm Disease

Heartworm infection – The host (dog, cat, or other animal) is parasitized by one or more stages of the Heartworm (Dirofilaria immitis).  This does not mean that the host is sick, or has adults in the heart.

Heartworm Disease – The host is sick.  Heartworm infection is resulting in any number of problems caused by damage due to the presence of Heartworms in the heart and nearby blood vessels.

Signs - versus - Symptoms

Signs - A sign is any objective evidence of a disease that is perceptible to the examining doctor or person. Therefore it is what someone other than the patient observes about the patient and it may indicate a disease process. An example would be hearing another person cough or sneeze and thinking they may have a cold.

Symptoms - A symptom is what a patient perceives and communicates to someone what they perceive. Therefore a symptom is something experienced and felt by the patient and described by them.

Technically, signs and symptoms are used in human medicine and only signs in veterinary medicine. It is not unusual to see them used interchangeably even though technically they are completely different.


We are not sure where they evolved originally but we do know they have been around for a long time.  The first written canine documentation was in the “Western Journal of Medicine” in 1847. The first documented cat infection was in the early 1900’s.   We have learned much more about them and now know that in areas where dogs contract heartworm, cats can become infected too.  Cats are simply less susceptible to becoming infected when exposed to the larvae deposited by a Heartworm-harboring mosquito.

The education of pet owners about Heartworms has increased significantly over the past twenty years.   The spread of the disease has accelerated due to the trend of pets, and especially dogs, to travel everywhere with their owners.  Pet owners often were unaware they were visiting Heartworm country, their pets became infected and the worm was brought back to their home environment unknowingly.  Thus territories that never experienced Heartworm infections in the past started to see them.   Heartworm disease is now wide spread and has been diagnosed in all 50 states in the USA as well as worldwide.  Actually, cats have been found NOT to be a big factor in the spread of the heartworm disease.


Most parasites have a specific host they prefer to infest. The normal host for Heartworm is the canine.  Heartworms can live in animals other than canines but often have a shorter life, vary in size, or end up in odd locations due to aberrant migration.   This is true of Heartworm infections in the feline.   Some of the other animals that can acquire heartworm include the wolf, coyote, fox, bobcat, jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse, orangutan, gibbon, sea lion, and man.



The key to understanding the importance of Heartworm to your pet’s health is to understand the life cycle of the worm and the damage that results due to its presence in your pet.  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 fancy name for Heartworm in dogs and cats is Dirofilaria immitis. 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. Heartworm disease begins with an infected animal that has circulating microfilaria in its blood.  This infected animal is called the source.  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).  As an L3, they have migrated into the mosquito’s salivary glands and during the mosquito’s next meal they burrow  into the victim  through the mosquito's small bite  wound.

Once inside an animal (host) where it continues development, the worm 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 it’s 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 larvae arrive in the heart by 90 days.  They stay here and grow rapidly in length and size.  The worm is home and will live here until it dies in 5-7 years.

Sexual maturity is achieved about three months after arrival in the heart.  The worms actually continue to grow in size after reaching sexual maturity and the females start to pass microfilaria into the blood.  Adult female worms have been known to grow up to 14 inches long and males are generally shorter.  Some pets are infected with numerous worms, all traveling from the site of the mosquito bite to their preferred home in the heart.  This mass of twisted and intertwined worms can serve as a significant mechanical blockage to the normal flow of blood.  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 worms congregate within the right ventricle.   The smaller the host the fewer worms it takes to cause a problem.

Once the worm becomes an adult it will continue to produce young for many years in the dog, and a shorter time in cats and other animals that are not the usual host for the worm.  The adults have been documented to live around 7 years in the dog and 2-3 years in the cat.  This completes the normal life cycle of the Heartworm.

Another factor to consider is that an animal can act as a source for future additional infesting mosquito bites, so over time unprotected dogs can develop a large number of adult worms in the heart.  Also, some animals develop something similar to an allergy to the worms, or to the microfilaria, which results in the Occult Heartworm infections and can cause varying signs similar to allergies.   This more elusive kind of infestation occurs most often in the cat.  Cats seem to harbor only small numbers of adult worms, the worms are usually males only, microfilaria are rarely found in circulation, and signs of Heartworm infection can present as an allergic condition similar to asthma.  It is not uncommon for infected cats to suddenly die from the effects of just a few worms.


Two major mechanisms result in the signs of Heartworm disease seen 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 worms present.

When a dog is first infested with Heartworm there are no visible or detectable signs.  The infection cannot be detected even with a blood test.  The changes in the victim start to occur when the final molt of the Heartworm larvae occurs and the immature L5 larvae  arrive in the right ventricle and neighboring blood vessels. The arteries do not do well with worms living inside them.  The artery lining is damaged within days, 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 worms cause damage at a rate faster than the body can heal.  The arteries over time 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 re-routes to non-worm burdened arteries.  The result is complete and partial blockage of blood vessels and fluid begins to accumulate around these blood vessels in the lungs.  This results in a loss of useful lung tissue and reduces the effective area of the lungs to oxygenate the blood for the body’s needs.   As a result of the inflammation, blood vessel obstruction, and fluid accumulation, coughing results.  The dog or cat displays exercise intolerance, nosebleeds, shortness of breath and a type of pneumonia may occur 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, the total number of worms at various stages of maturity increase and as they grow in size and number the above conditions take their toll. The host’s reactions become more significant and the signs worsen.  More and more blood vessels and the surrounding lung tissue are damaged and not useful to the dog and this results in an increased resistance to blood flow through the lungs. This “backup” increases the blood pressure (hypertension) in the right side of the heart and Vena Cava  due to the obstruction of blood flow.  With accumulation of even more fluid in the lungs, the end result is the signs of actual heart failure.  The severity depends on the number of worms present and the dog’s reaction to the worms.  The failing, weakened, stretched heart muscle results in rhythm abnormalities, fluid accumulation in the lungs (called pulmonary edema) and exercise intolerance.

Over time, the immune system becomes “turned on” at a rate higher than normal.  This puts extra proteins (in the form of antibodies) into circulation and they can settle out in the various organs of the body that are delicate in nature such as the eye, kidney, blood vessels, and joints.  This causes inflammation, more tissue damage, and pain.

One of the most severe signs of heartworm is called Caval Syndrome or Vena Cava Syndrome.  This is seen when there are large numbers of adult worms (usually around 100 or more) in the heart.  There is almost complete blockage of all blood flow.  Many times there will be no signs of heart disease prior to the animal’s collapse.  When fainting and collapse does occurred 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 worms from the heart through the jugular vein.  If enough worms can be removed to re-establish sufficient blood flow, there is some chance of survival.


Any patient presented because of the suspicion of Heartworm Disease will have a thorough medical history taken, a complete physical exam performed, x-rays of the chest taken and routine blood chemistry tests done to evaluate the entire patient.  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 worm 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. Animals 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.

Canine patients will be staged for Heartworm Disease as part of the evaluation.  This helps the practitioner decide which method of treatment would be best for the elimination of the Heartworms:

  • Stage I Lowest risk... young healthy dogs with minimal Heartworm disease evident on X-rays and all other tests are normal.

  • Stage II 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.

  • Stage III  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.

  • Stage IV  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 attempts to treat.


The bottom line:  Heartworm is a significant disease in dogs and cats.  The treatment involves managing the heart, vascular and systemic disease present as well as eliminating the parasites.  The goal of treatment is to eliminate the worms one way or another so the animal’s body can rebuild itself and return to the best possible post-infection health.  This sounds simple but it can be very complicated depending upon the number of worms present, the dog’s reaction to their presence, the patient’s general state of health, handling the side effects from the medication and the effects on the patient of the dead worms within the circulatory system.

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, this discussion of heartworm treatment will be very general regarding the medications used and the more common side effects.  The specific treatment protocol for your pet will be left up to your veterinarian since there is no way to predict how each animal will react to Heartworm treatment.

  1. Treatment involves two basic areas:
    Patient evaluation and stabilizing for treatment procedure.

  2. Elimination of all forms (adult, larvae, and microfilaria) of the Heartworm parasite.

Patient evaluation and stabilization

This involves X-rays, blood tests, heart evaluation, and any other tests indicated to completely evaluate the pet.  The veterinarian evaluates the over-all health of the animal, then determines how to best proceed with treatment.  Part of this evaluation is staging the severity of the Heartworm Disease in the dog. Some animals need to have certain conditions stabilized before Heartworm treatment can proceed.  Those in third stage Heartworm disease 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 worms.  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.  This worm destruction 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 to watch them closely.   Your Doctor will make the decisions on an individual basis regarding what would be best for your dog.

The prescription medications 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 worms. 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 (Caparsolate).  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 (Immiticide).  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 has potential for significant side effects and close veterinary monitoring is very important.

Side effects from the medication 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.  The veterinarian will determine at the time what to use after evaluating the severity of the reaction.  Some anti-inflammatory medications can reduce the effectiveness of the adulticide.  Therefore it is a judgment call regarding what is best for the pet’s health at the time.

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 preventatives, Ivermectin (HeartGard) or Mibemycin oxime (Interceptor).  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.  This will determine if a second treatment will be needed.

In Summary

Once the Heartworm is eliminated from the dog, then preventative medication is continued as prescribed by your veterinarian.  Each dog’s response to Heartworm treatment is different so the information presented here is a general guide to help you understand the basics of Heartworm eradication.  Your veterinarian will communicate more of the specific information as it relates to your pet’s particular circumstances and your pet’s probable response to treatment.  Our goal is to help you better understand that the process is involved, the medication alone can be toxic, and every animal reacts differently.


Preventing Heartworm Disease is definitely easier on the dog and is now much simpler than it used to be.  The most common preventatives are given once a month by the pet’s caretaker.  Preventatives kill the immature Heartworm larvae before they molt to the L5 stage.  As long as they are given every month, they are very effective in preventing Heartworm infection and subsequent development of Heartworm Disease.

The choice of which preventative to use will be determined by a discussion with your veterinarian and what is best for your pet.  Besides Heartworm prevention, Interceptor and Revolution are effective against some intestinal parasites and Revolution is also an effective product for flea control.

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.

Thanks to for this valuable information!

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