Heartworm Disease (HWD) in Dogs

Heartworm Disease (HWD) in Dogs

Overview of Canine Heartworm Disease

Canine heartworm disease (CHWD) is a common condition in many regions of the world. It is caused by the filarial (threadlike) worm Dirofilaria immitis that lives in the pulmonary arteries (blood vessels leading from the heart to the lungs) in most infected dogs. The name “heartworm” is a bit misleading because only in very heavy infections do the worms actually reside in the heart itself.

Nonetheless, the presence of these worms causes strain to the heart and an intense reaction in the blood vessels, resulting in problems for the dog. D. immitis is injected into the dog by infected mosquitoes. For this reason, CHWD is more common in tropical regions of the world, where the conditions of warmth and moisture are ideal for mosquitoes to live.

Predispositions for CHWD include living outdoors. Male dogs may be more likely to be infected than female dogs. The most important predisposing factor is failure to receive heartworm preventative medication. All dogs living in an area where heartworm disease exists are at risk, even if they live entirely indoors.

Transmission occurs when a mosquito bites an infected dog and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.

Impact on the pet is variable. Dogs can be without symptoms if the infection is light or has occurred recently. In some cases HWD can cause severe debilitation and eventually may be fatal. Because heartworm disease is easily preventable, it has become a less common diagnosis in pets that receive regular veterinary care.

What To Watch For

Symptoms of heartworm disease in dogs don’t usually develop until damage has already occurred to the heart. Dogs can have a wide range of symptoms, with some dogs being completely asymptomatic (no symptoms at all). Symptoms usually occur because of heart failure. These include:

  • Coughing
  • Coughing up of blood (hemoptysis)
  • Heavy breathing
  • Unwillingness to exercise
  • Signs of right sided congestive heart failure, which include fluid distention of the belly, pulsation of the jugular veins in the neck when the dog is sitting or standing and heavy breathing.

Diagnosis of Heartworm Disease in Dogs

Two types of blood tests are used to establish the diagnosis:

  • Microfilaria screen. These are screening tests to detect the baby worms (microfilaria) circulating in the bloodstream (produced by the adult heartworms living in the pulmonary arteries). This test can either be a filter test, or a “Knott’s test” (spinning down a sample of blood mixed with formalin and looking for the microfilaria with a microscope).
  • Heartworm serology. This test checks for proteins in the bloodstream of the dog that are produced by the heartworms. These tests are very sensitive and accurate.

    Once diagnosed, to stage the heartworm disease:

  • Complete physical examination. A physical examination will detect signs of disease or may suggest a co-existing disease condition.
  • Laboratory evaluation. A CBC, biochemical profile and urinalysis may be run as a general health screen, and to assess stability of patient for treatment.
  • Chest x-rays. Looking at the radiographs (X-rays) allows your veterinarian to decide how severe the disease is, and to decide how to treat the dog, and to warn dog owners about the individual dog’s susceptibility to complications with treatment of the heartworms.
  • Evaluation of the heart. This specialty examination would include an ultrasound examination of the heart (echocardiogram) to look for enlargement of parts of the heart, or the presence of the heartworms within the heart itself. An electrocardiogram (ECG) maps out the electrical activity of the heart and determines the heart rhythm. An ECG may be recommended if there are any signs that the heartworm disease is advanced. The echocardiogram and ECG are not part of the routine evaluation of heartworm disease.

Treatment of Heartworm Disease in Dogs

  • Doxycycline is also used to treat Wolbachia, an organism that infects heartworms and is required for heartworm survival, in an effort to treat heartworm disease.

Killing of the adult heartworms (adulticide therapy)

There are two main agents administered by veterinarians for adulticide treatment:

  • Melarsomine (Immiticide®): This drug is given by injection deep into the muscles of the back as either a two-day series (one injection per day, two consecutive days) or as a split protocol (one injection, followed one month later by two consecutive injections one day apart for a total of three injections). This split protocol approach has become more common Immiticide® is currently the most commonly used type of treatment. Melarsomine is considered superior in its safety and efficacy.
  • Sodium thiacetarsemide (Caparsolate®). This drug is given by injection into the vein. The most widely accepted injection routine is two injections, eight hours apart on two consecutive days, for a total of four doses. Please note that thiacetarsemide (Caparsolate®) is currently unavailable/off the market.

    Killing of the baby worms (microfilaricide)

    This is done only in dogs with a positive filarial test. It is given one month after the adulticide treatment is completed:

  • Milbemycin (Interceptor®). A monthly heartworm preventative, effective at killing the microfilaria.
  • Ivermectin (Ivomec®). A concentrated form of this drug is given to kill the microfilaria.

Home Care


  • Strict exercise restriction. THIS IS THE MOST IMPORTANT PART OF THE TREATMENT PROCESS. Dogs who have been treated for heartworm disease should receive the absolute minimum of exercise for four weeks following the treatment. Ideally, cage rest should be provided. In very hyperactive dogs, a sedative may need to be used to help achieve this very important goal.

    Exercise restriction is so important, because as the drugs are killing the worms in the arteries, the worms can break off and travel to block parts of the blood vessels (pulmonary thromboembolism or PTE). PTE results in obstruction of the blood flow to parts of the lung (pulmonary infarction). Keeping the dog quiet allows the body time to slowly break down and absorb the dying worms.

  • Aspirin. Low dose aspirin is prescribed in some cases of heartworm disease as an anti-inflammatory to minimize the reaction in the lungs to the death of the worms.
  • Prednisone. Administered pre-treatment only if there is intense lung inflammation and severe coughing. Administered post-treatment only to treat the complications discussed below.
  • Watching for complications. The major complication to treatment is PTE. PTE is associated with signs of fever, heavy or fast breathing and coughing. Observation of any of these clinical signs would necessitate a visit to your veterinarian or local veterinary emergency center. In its worst form, PTE can result in sudden death. Back pain is often noted as a minor complication secondary to the Immiticide® injections into the muscles of the back.



Preventative Care

Prevention of heartworm disease is easy and should be undertaken in all dogs. These include monthly and daily preventatives.

Monthly preventatives include:


  • Milbemycin oxime (Interceptor Flavor Tabs®)
  • Milbemycin oxime (Sentinel Flavor Tabs®)
  • Ivermectin (Heartgard®)
  • Selamectin(Revolution®)
  • Moxidectin (Pro Heart®)

    Daily preventatives include:

  • DEC (Decacide®, Nemacide®). Use of these daily medications has almost entirely been replaced by the monthly preventatives. Although DEC is effective and the non-chewable forms inexpensive, it needs to be administered every day during the heartworm season, and if any doses are missed, a filarial test needs to be repeated before restarting.



In-depth Information on Heartworm Disease in Dogs

Canine heartworm disease is caused by infection with Dirofilaria immitis (D.immitis), a filarial worm living in the pulmonary arteries. Dogs who are not taking heartworm preventative medications become infected with heartworms when an infected mosquito carrying an immature form of D. Immitis (larvae) injects those larvae into the bloodstream of the dog. A portion of the larvae mature and migrate through the bloodstream and heart to arrive in the pulmonary (lung) arteries. In a period of 4 months after injection, the young form of the adult heartworm is present. It takes another few months before the worms are sexually mature and produce baby worms (microfilaria) that begin circulating in the bloodstream of infected dogs. Additional mosquitoes become infected by ingesting the microfilaria with the blood of the dog. The microfilaria are infectious to mosquitoes, not to other dogs. The microfilaria undergo a few changes in the body of the mosquito before they reach the larval form infective to the dog, completing the life cycle of the heartworm.

There are no known predispositions for heartworm disease other than living in a warm, moist environment favorable to the mosquitoes, spending the majority of the day and evening outside and the major predisposing factor of failing to receive heartworm preventative medication. Although some reports suggest male dogs are more likely to get heartworm disease, one study found spayed female dogs to be at higher risk. The modern heartworm preventative medications are highly effective and, if religiously administered, should prevent heartworm infection.

The severity of heartworm disease is variable depending on the number of heartworms present, the intensity of the reaction of the body to the presence of the heartworms, the duration of the infection, the age of the dog and the presence of other medical conditions. Heartworm disease is frequently detected in dogs without symptoms which are presented to the veterinarian for routine care. Left untreated, many dogs will display clinical signs. Clinical signs may include coughing, hemoptysis (the coughing up of blood), shortness of breath, unwillingness or inability to exercise or signs of right-sided congestive heart failure (RCHF). Signs of RCHF include accumulation of fluid in the belly (ascites), accumulation of fluid around the lungs (pleural effusion) and pulsation of the jugular veins located in the neck. Dogs with RCHF will appear fat or bloated, but are actually usually very thin and have difficulty breathing. Dogs who suffer a blood clot to the lung (pulmonary thromboembolism, PTE) will show signs of coughing, fast breathing and lethargy. In severe cases, PTE may be fatal.

A less common syndrome seen with HWD is “caval syndrome.” This syndrome occurs when the heartworms are in such large numbers that they are present in the right heart chambers and jugular veins. In this position, the worms form a mesh that causes some of the red blood cells in the heart to break up (hemolysis). The end result of the hemolysis is anemia (low red blood cell count) and jaundice (yellow color to the eye whites and gums caused by byproducts released by red blood cell breakdown in the bloodstream). Caval syndrome is very serious and requires mechanical removal of the heartworms from the heart using special instruments that are advanced into the heart through the jugular vein in the neck. This removal process is dangerous and sometimes results in death of the patient.

Disease which might mimic signs of heartworm disease would be other diseases resulting in coughing (bronchitis, tracheal collapse, pneumonia), other diseases resulting in right-sided congestive heart failure such as:


  • Pericardial disease (diseases affecting the sac covering the heart)
  • Dilated cardiomyopathy (disease of the heart muscle)
  • Diseases affecting the right-sided heart valves (tricuspid or pulmonic valve disease)
  • Other diseases resulting in pulmonary thromboembolism (Cushing’s disease, etc.)
  • Other diseases causing red blood cell breakdown (immune-mediated hemolytic anemia, zinc toxicity, onion toxicity).

    Because there is a test for heartworm disease, diagnosis is not difficult. However, in a dog who is ill and positive for heartworm, one needs to determine if the signs the dog is showing are related to the heartworm disease or not.




Diagnostic Tests for Heartworm Disease in Dogs

To establish the diagnosis, there are two type of tests used.

  • Microfilaria tests. These tests can either be a filter test or a modified “Knott’s” test; these tests are indicated to see if an infection is “patent” resulting in circulating microfilaria, offspring of reproducing mature adult heartworms in the heart and pulmonary arteries.
  • Heartworm serology. This test should be done in all dogs to rule out heartworm infection, because heartworm infections are frequently occult (“hidden”). The term occult used here specifically means positive results on serology occurring without circulating microfilaria.

Causes of Occult Canine Heartworm Disease

  • The heartworms are young and sexually immature.
  • There is infection with a single sex: all male or all female worms.
  • The dog’s immune system is actively destroying the microfilaria as they are being produced.
  • The dog has been exposed to medications which kill microfilaria (milbemycin, higher dose ivermectin).
  • The dog has been exposed to medications which result in infertility of the female heartworms (this is reported in dogs taking monthly low-dose ivermectin). Finally, if a dog is positive on a filaria test but negative on the serological tests, this could suggest infection with Dipetalonema, a harmless blood parasite which resembles D. immitis. Treatment for heartworm would be inappropriate in this circumstance. A pathologist can identify Dipetalonema on a blood sample.

Tests To Stage Heartworm Disease in Dogs

  • Complete physical examination
  • Full blood evaluation. A CBC, biochemical profile and urinalysis are usually run to be sure the dog is otherwise healthy. Specific abnormalities that may be noted on the CBC with heartworm disease are mild anemia or an increase in eosinophils (a white blood cell type that is frequently present in the bloodstream of animals with parasitic infections or allergies). The combination of eosinophils with basophils (an uncommon type of white blood cell) is suggestive of heartworm disease. Some dogs may also have an increase in neutrophils, another type of white blood cell which increase in number related to the inflammation in the blood vessels related to heartworm disease.

    The platelet count may be mildly decreased in dogs with HWD. Platelets are particles made in the bone marrow which circulate in the blood to seal off any leaks in the blood vessels, controlling small bleeding problems. Platelets are decreased in dogs with HWD, due to the platelets clumping in the areas of inflammation in the pulmonary arteries. Mild liver enzyme elevations are often noted in dogs with HWD. Evaluation of the CBC and profile pre-treatment allows the veterinarian to be sure the dog is healthy and free of other diseases which could complicate the treatment of the HWD.

  • Chest X-rays. Radiographs (X-rays) of the chest are taken to examine the appearance of the heart, the pulmonary arteries and the lung fields. Dogs with mild infections may have normal X-rays, dogs with more severe infections may have a change in appearance of the pulmonary arteries, enlargement of the pulmonary arteries and the heart and changes in the lung texture. Looking at the radiographs allows your veterinarian to decide how severe the disease is, to decide how to treat the dog and to warn dog owners about the individual dog’s susceptibility to complications with treatment of the heartworms.
  • Evaluation of the heart. An echocardiogram is done on patients who have signs of congestive heart failure on physical examination, on patients who appear to have heart disease (abnormal heart sounds on physical examination or heart enlargement on the chest X-rays) or on patients showing signs of caval syndrome (anemia, jaundice, right-sided heart failure). An electrocardiogram (ECG) is indicated in any patient with signs of an arrhythmia (abnormal heart rhythm). The ECG may help identify patients with severe right heart disease. An echocardiogram and ECG are not routinely done in healthy young patients with normal heart appearance on chest X-rays. Because heartworms actually live in the smaller pulmonary arteries, they are not typically seen on echocardiograms of most infected dogs. In dogs with caval syndrome, the worms are readily imaged in the vena cava and right heart using echocardiography.

Treatment of Heartworm Disease in Dogs

After evaluation, treatment is generally recommended.

  • Doxycycline is also used to treat Wolbachia, an organism that infects heartworms and is required for heartworm survival, in an effort to treat heartworm disease. Eliminating the Wolbachia disrupts the heartworms’ life cycle. It is also believed that treatment with Doxycycline also results in less inflammation in the lungs when the heartworms are killed with an adulticide.

Adulticide Therapy

Killing of the adult heartworms. One hopes to accomplish complete death of the heartworms, but slowly enough to allow the body time to break down and absorb the dying heartworms.

  • Melarsomine (Immiditicide®). This arsenic-based drug is given by injection deep into the muscles of the back as either a two-day series (one injection per day, two consecutive days) or as a split protocol (one injection, followed one month later by two consecutive injections one day apart, for a total of three injections). This split protocol allows for slower death of the heartworms, which may cause fewer complications in dogs with more severe infections. Immiticide is the most commonly used adulticide.
  • Thiacetarsemide (Caparsolate®). This drug is given by injection into the vein. The most widely accepted protocol is two injections given eight hours apart on two consecutive days, for a total of four injections. It is extremely important that this drug not spill outside the vein or there will be severe inflammation and injury to the tissues, resulting in swelling of the leg and possible death of muscle and skin in the area. Caparsolate causes liver toxicity in many dogs. Before each injection, the veterinarian checks to be sure that the patient is eating and feeling well, and doesn’t have any yellow color to the eyes and gums (icterus or jaundice). If any of these occur, the treatment must be temporarily suspended and tried again one month later. Please note that thiacetarsemide (Caparsolate®) is currently unavailable/off the market.

Microfilaricide Treatment

Killing the circulating baby worms. This is unnecessary in dogs with occult heartworm infections. Microfilaria can be killed in a few ways:

  • Milbemycin (Interceptor®). Will kill microfilaria within a few doses.
  • Ivermectin (Ivomec®). Ivermectin is administered in a concentrated form either orally or by injection under the skin in low doses to kill the microfilaria. The dose of ivermectin used to prevent heartworm infection (Heartgard®) is not high enough to kill the microfilaria.

    Sometimes microfilaria treatment needs to be repeated (one month after the first treatment) before complete elimination is achieved.

Follow-up Care for Dogs with Heartworm Disease

Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up can be critical.

Administer all prescribed medications as directed. Alert your veterinarian if you are experiencing problems treating your dog.

STRICT EXERCISE RESTRICTION is the most important aspect of care at home after treatment for heartworm disease. This is imperative for the first four weeks after treatment. During this time, exercise or activity may result in an increased likelihood of pulmonary thromboembolism (PTE). PTE is interruption of blood flow to a portion of the lung due to blockage of a pulmonary artery with dead or dying worms, and associated blood clot and inflammation. The body can take care of the dying worms by breaking them down and absorbing them, but this takes time. Although PTE can occur in a dog that is being quiet, it is much more likely to occur in a dog who is exercising. The PTE episodes can be very severe, even fatal.

Owners need to be very alert for the signs of PTE: coughing, fast or heavy breathing or fever. If these signs are noted, the dog should be presented for examination as soon as possible. If PTE is identified, it is usually treated with the administration of the anti-inflammatory drug prednisone. Oxygen may need to be administered until the dog is breathing more easily.

Some very mild coughing is to be expected after heartworm treatment. However, if the coughing is occurring frequently during the day, or if the dog coughs up blood, the dog should be examined. Any coughing should be reported to your veterinarian.

The heartworm tests need to be repeated after treatment, to ensure the treatment has been successful in eliminating the heartworms. The filarial test is done one month after the microfilaricide is given. If it is positive, microfilaricide is repeated. Cases with occult heartworm infection have repeat serology three months after treatment. If the serology is still positive, it should be repeated three months later. At that time, if it is still positive, the dog should be re-evaluated for repeat treatment. Luckily, adulticide treatment is usually successful the first time!

All dogs with heartworm disease should be started on heartworm prevention immediately after the adulticide for occult infections, and after successfully killing the microfilaria in the other cases.

In dogs determined to be medically unsuitable for heartworm treatment, an appropriate heartworm preventative will be prescribed by the veterinarian to prevent the dog from acquiring additional heartworms. Some of the heartworm preventatives available are dangerous to administer in dogs already infected with heartworms.

The American Heartworm Society (AHS) recommends that all dogs in areas endemic for heartworms should take a year-round preventative. Annual testing will ensure that an infection is caught in plenty of time to effectively manage it. Testing is also recommended when a pet owner switches between preventative medications.

number-of-posts0 paws up

Previous / Next Article

Previous Article button

Diseases & Conditions of Dogs

Common Dog Disorders and Diseases Dog Owners Should Know

Next Article button