In a Nutshell

Parasite Dirofilaria immitis
Transmission Mosquito
Susceptibility to infection Very high - virtually 100% of dogs exposed to infective larvae become infected
Longevity of worms 5-7 years
Ectopic infections occasionally
Number of worms not uncommon to find more than 30
Single-sex infections in meso- to high-endemic areas unusual
• persistent
• very common (80%-90%)
• can last years, even after death of adult worms
Organ with greatest pathology heart and lungs
Diagnosis relatively simple
• 2 compounds approved
• we recommend Immiticide®; Merial
Compounds for prevention
• 4 compounds approved
• we recommend Heartgard®; Merial

Canine Heartworm Disease

Dogs are considered the definitive hosts for heartworm infections. In experimental infections of D. immitis, the percentage of infective larvae developing to adult D. immitis in dogs is 40% to 90%. However, the percentage of experimentally-infected dogs from which adult worms are recovered is virtually 100% of infected dogs. The worm burden in dogs is usually high ranging from 1 to 250 worms. The life span of the worms in dogs appears to be about 5 to7 years. The average prepatent period (the time elapsed from when the larvae enter the host until the adult female worms begin to produce microfilariae) in dogs is about 6-7 months.

Microfilaremia is relatively common in dogs. Not all heartworm infections result in circulating microfilariae that can be found upon examination of the dog's blood, however. These are known as occult heartworm infections and can be the result of a number of factors. Single sex heartworm infections, host immune responses affecting the presence of circulating microfilariae and the administration of heartworm preventives can be factors which produce occult infections in dogs.

The onset and severity of disease in the dog is in part a reflection of the number of adult heartworms present. Dogs with higher numbers of worms are generally found to have more severe cardiac and pulmonary disease changes. Until the number of heartworms exceeds 50 in a 25-kg dog, nearly all of the heartworms reside in the caudal pulmonary arteries. Higher numbers of heartworms will result in their presence in the right ventricle, and possibly the right atrium, of the dog's heart. The most common pathological changes created by heartworms are due to inflammatory processes that occur in and around the caudal pulmonary arteries in response to the presence of heartworms.

Canine heartworm infection is widely distributed in the United States. Heartworm infection has been found in dogs native to all 50 states, and is considered at least regionally endemic in each of the contiguous states and Hawaii. All dogs regardless of their age, sex, or habitat are susceptible to heartworm infection. The highest infection rates (up to 45%) in dogs not maintained on heartworm preventive are observed within 150 miles of the Atlantic coast from Texas to New Jersey and along the Mississippi River and its major tributaries. Other areas of the United States have lower incidence rates (5% or less) of canine heartworm disease, while some regions have environmental, mosquito, and dog population factors that allow a higher incidence of heartworm infection. Endemic regions have heartworm infections diagnosed in dogs as young as 1 year, with most areas diagnosing infections primarily between the ages of 3 and 8 years of age. The infection rate in male dogs is as much as 4 times that of female dogs and dogs housed outdoors are 4 to 5 times more likely to be infected than indoor dogs. Although there are differences in frequency of infection for various groups of dogs, all dogs in endemic regions should be considered at risk and placed on surveillance/prevention programs.

Clinical Signs

D. immitis infection may cause multiple system dysfunction affecting the pulmonary circulation, heart, liver and kidneys. Heartworm disease may have an acute onset, but it is usually insidious, resulting from a chronic infection with a combination of underlying pathophysiologic responses. Dogs with low worm burdens that receive little cardiopulmonary exercise may never have overt signs of heartworm disease. The heart and lungs are the major organs affected by heartworms in dogs, and they can have no clinical signs, or can present with mild, moderate or severe clinical signs.

Clinical Signs Associated with Canine Heartworm Disease

Acute No abnormal clinical signs observed
Mild Disease Cough
Moderate Disease Cough, exercise intolerance, abnormal lung sounds
Severe Disease Cough, exercise intolerance, dyspnea (difficulty breathing), abnormal lung sounds, hepatomegaly (enlargement of the liver), syncope (temporary loss of consciousness due to poor blood flow to the brain), ascites (fluid accumulation in the abdominal cavity), abnormal heart sounds, death


The diagnosis of canine heartworm disease depends upon an accurate patient history, the recognition of varied clinical signs, and the use of several diagnostic procedures that may include radiology, angiography and ultrasonography (echocardiography), serologic testing, microfilarial detection and differentiation, clinical laboratory tests and, in the worst case scenario, necropsy.


Radiographic abnormalities develop early in the course of the disease. Radiographs of the heart and lungs are the best tool available to evaluate the severity of the disease. Typical changes observed are enlargement of the following structures: right-side of the heart, main pulmonary artery, pulmonary arteries in the lobes of the lung. Blunting and thickening of pulmonary arteries, along with tortuosity is often noted. Evidence of inflammation in the lung tissue that surrounds the pulmonary arteries is often found.

Angiography and Ultrasonography

These forms of imaging are rarely used in the diagnosis of canine heartworm infection outside of referral practices and teaching institutions. Angiography is a technique that permits visualization of blood vessels in the body by taking radiographs within seconds after injecting a contrast material (dye) into those blood vessels. In canine heartworm infection, angiography is used to study changes to the pulmonary arteries. Ultrasonography (echocardiography) has been used to evaluate right ventricular enlargement and to look for the presence of heartworms in the right ventricle or main pulmonary artery.


While necropsy is probably the most definitive diagnostic test, most veterinarians will assure you that this procedure is not regarded as a practice builder. Heartworms are often readily found in the right ventricle of the heart or in the major pulmonary arteries. The worms can also be found in the farthest branches of the pulmonary arteries. Ectopic heartworm infections are rare, with worms detected occasionally in organs other than the heart and lungs or in body cavities.

Clinical Laboratory Tests

There are no definitive changes to the complete blood count, serum chemistries or urinalysis of dogs with heartworm disease. Increased number of eosinophils and basophils found on a complete blood count are regarded as supportive of, but not diagnostic for, heartworm infection.

Microfilarial Detection and Differentiation

The identification of D. immitis microfilaria from a blood sample is indicative of infection with adult heartworms in over 99% of individuals. Microfilaria identification is equally accomplished through either of 2 concentration tests: the Knott's test (a technique requiring centrifugation of the test sample) or the filter test. Practitioners will often do a quick examination of a blood smear to look for the presence of microfilariae, but this procedure is not sensitive enough to serve as a final rule out for the absence of microfilariae in a sample.

Physical Exam

The physical examination may be perfectly normal in heartworm-infected dogs with mild disease, or severely-affected dogs may present recumbent, in right-sided heart failure. Labored breathing or crackles may be auscultated in dogs with severe pulmonary hypertension or pulmonary thromboembolic complications, and a history of chronic cough and exercise intolerance are some of the earliest detectable abnormalities. Tachycardia, ascites and hepatomegaly indicate right-sided congestive heart failure. Hemoptysis occasionally occurs and indicates severe pulmonary thromboembolic complications. There have been reports of anorexia, cachexia, syncope and jaundice in severely-affected dogs. Occasionally, heartworms are reported in unusual locations such as the eye, abdominal cavity, cerebral artery and spinal cord (ectopic locations). Clinical signs and disease experienced in ectopic infections depend largely on the location of the worm. In dogs, the primary response to the presence of heartworms occurs in the heart and lungs.


Antigen tests detect specific antigens from adult female heartworms, and are used successfully to detect canine heartworm infection. Currently, tests are available as in-clinic tests, as well as at many veterinary reference laboratories. Most commercial tests will accurately detect infections with 1 or more mature female heartworms that are at least 7 or 8 months old, but they generally do not detect infections of less than 5 months duration.

Antibody Tests

In the late 1970's and early 1980's, several canine heartworm antibody tests were developed and introduced. These tests relied on detection of antibodies using a crude mixture of heartworm antigens from adult worms, and while they were very sensitive (i.e. they were able to detect very early infections and infections with small numbers of heartworms), the tests were non-specific and were believed to cross-react to the common gastrointestinal parasites found in dogs. These tests were never very popular as screening tests in dogs.

An IFA detecting antibodies to microfilarial cuticular antigen has been used for the diagnosis of occult infections that result from immune-mediated clearance of microfilariae in dogs. The presence of sterile worms, worms of only one sex or the absence of a host response to microfilarial surface antigen does not produce a diagnostic titer. A variation on this test includes lysing the microfilariae to assess antibody to somatic antigens. This somatic IFA is nonspecific and not clinically useful in dogs.


Most dogs infected with heartworm can be successfully treated. The goal of treatment is to kill all adult worms with an adulticide and all microfilariae with a microfilaricide. It is important to try to accomplish this goal with minimal drug toxicity and a tolerable degree of complications created by the dying heartworms. Heartworm infected dogs which are normal or have mild disease have a high treatment success rate. Patients with evidence of more severe heartworm disease can be successfully treated, but the incidence of post-adulticide complications and mortality are greater. Certainly there are patients whose severity of heartworm disease, or presence of other concomitant life-threatening diseases, prevent treatment of the heartworm infection.

Adulticide Therapy

There are currently 2 drugs approved by the FDA for use in dogs for the elimination of adult heartworms. Both drugs are organic arsenical compounds. Dogs receiving these drugs will typically have had a thorough pretreatment evaluation of their condition and will be hospitalized during the administration of the drugs.

Melarsomine dihydrochloride
Melarsomine (Immiticide®; Merial) is the first new adulticide for canine heartworm to become available in the U.S. in over 25 yrs. It has demonstrated a higher level of efficacy and safety than what has previously been available. It is administered by deep IM injection into the lumbar muscles. An important new feature of this drug is the potential ability to minimize post-adulticide complications in dogs with more severe heartworm disease.

Thiacetarsamide sodium
Thiacetarsamide (Caparsolate®; Merial) must be administered by intravenous injection. Leakage of the drug around the vein can result in intense perivascular inflammation. The incidence of side effects due to the systemic toxicity of this drug can interfere with its use in many patients. Thiacetarsamide's efficacy is dependent upon a full dosage schedule being administered. Male worms are more susceptible to treatment than female worms.
Post-adulticide Complications

The primary post-adulticide complication is the development of severe pulmonary thromboembolism. Pulmonary thromboembolism results from the obstruction of blood flow through pulmonary arteries due to the presence of dead heartworms. If heartworm adulticide treatment is effective, some degree of pulmonary thromboembolism will occur.

When dead worms are numerous, widespread obstruction of arteries can occur. Clinical signs most commonly observed include fever, cough, hemoptysis (expectoration of blood). These patients require strict reduction in exercise and anti-inflammatory doses of corticosteroids.

Elimination of Microfilariae

Microfilaricide treatment is typically administered 3 to 6 weeks following completion of the adulticide treatment. This allows an adequate amount of time to pass for the adult worms to die and to have ceased production of additional microfilariae. The most effective drugs for this purpose are the macrolide anthelmintics - ivermectin and milbemycin. These drugs are the active ingredients in the commonly use heartworm preventives. However, their usage in this application has not been approved by the FDA, but there are no approved microfilaricidal drugs currently recognized. It is recommended that microfilariae positive dogs being treated with these drugs be hospitalized for 8 hours following treatment for observation of possible adverse reactions resulting from rapid death of the microfilariae.

A microfilaria concentration test is performed 3 weeks after microfilaricide treatment. Pets that are microfilaria negative have completed the treatment for canine heartworm infection and are ready to start a heartworm preventive program.

Confirmation of Adulticide Efficacy

The goal of adulticide treatment is the elimination of all adult heartworms. However, clinical improvement in dogs treated for heartworm infection is possible without completely eliminating the adult heartworms. Heartworm antigen testing is the most reliable method of confirming the efficacy of adulticide therapy. If all the adult worms have been destroyed or very few survive, heartworm antigen should be undetectable by 16 weeks post-adulticide. Dogs that remain antigen positive at that time should only be considered as candidates for repeat treatment with an adulticide after a full review of each case.


While treatment of canine heartworm disease is usually successful, prevention of the disease is much safer and more economical. There are currently 4 products approved for use in the US for the prevention of canine heartworm infection. Prevention of heartworm disease in dogs can be accomplished through the routine administration of either diethylcarbamazine or one of the macrolide anthelmintics. These drugs are highly effective when administered at the appropriate doses and intervals. Rottweiler Rescue of Michigan recommends that all rottweilers be kept on preventative year-round. Before starting a preventive program, all dogs that could possibly be infected with mature heartworms should be tested for the presence of circulating antigen.

Ivermectin (Heartgard®; Merial) was the first in this family of drugs to be approved for use in preventing heartworm infection. Infection with larvae as long as 2 months prior to initiation of ivermectin treatment will be blocked from development.

Milbemycin (Interceptor®; Novartis) has benefits which are similar to ivermectin.

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