Heartworm Disease in Dogs, an Explanation - Diagnosis and Treatment

I can remember being devastated as a second grader to find out that our family dog had heartworms and was dying and there was nothing we could do. That was 1966. Thankfully, much has changed since then. Today, treatment for heartworms is available and better yet, it can be prevented. Heartworms are an internal parasite of both dogs and cats. These worms live in the bloodstream and cause life threatening disease. Heartworms are a leading cause of lung and heart disease in dogs and dogs are dying of this disease every day. Heartworm disease is caused by the parasite, Dirofilaria immitis. It is transmitted by mosquitoes and has been found in every state in the US. It only takes one mosquito bite for your dog to get heartworms and, without prevention; there is a 100% chance your dog will get it.
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The way your dog becomes infected is the stuff of nightmares. You see, dogs are the reservoir of infection. This means that there are many dogs out there that are infected and when a mosquito bites an infected dog, it ingests immature heartworms known as microfilaria. Microfilaria is the larval form of heartworms that change within the mosquito to reach the third stage. It is the third stage that can infect your dog. When the infected mosquito bites your dog, it deposits third stage microfilaria on your dog's skin and the microfilaria enters the dog through mosquito's bite wound. The microfilaria then travel or migrate through the tissues (meaning skin, muscle, connective tissue) and mature to become a juvenile adult or fifth stage larvae. This process takes about 50-70 days in the dog. Sometime between 70 and 110 days after the dog is infected, the larvae reach the blood stream and end up in the arteries of the lung (pulmonary arteries). These juveniles then grow up to produce microfilaria of their own in about 6.5 months after the dog is infected.
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Heartworms cause the most harm just by being present in the major arteries of the lung, the pulmonary arteries (pa). They wreak havoc in these arteries and the entire dog suffers. It happens on many levels and here is a list of the damages.
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1. Injury to the lining of the pa (pulmonary arteries).
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2. Increased pressure within the pa, known as pulmonary hypertension, due to the presence of the heartworms, the thickening of pa walls, scarring and blockage of pa, and pieces of heartworm that lodge in pa or clots that form and lodge in pa (known as heartworm emboli). Pulmonary hypertension is then responsible for right sided heart disease, even right sided congestive heart failure. The severity of pulmonary hypertension is dependent on the number of adult heartworms, thus, the larger the number of heartworms, the more severe the disease.
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3. Changes within the lung itself. These changes include the infiltration of lung by eosinophils, a white blood cell that takes part in the immune response to allergy and parasitism. Eosinophils are capable of causing and perpetuating a severe inflammatory response which damages the lung and leads to scarring of the lung. Other white blood cells and inflammatory cells may invade the lung causing damage. The heartworms can actually block an artery (thrombosis) and, in areas where thrombosis has occurred, nodules made up of inflammatory cells known as granulomas may form.
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4. Vena Cava Syndrome. This syndrome strikes fear in the heart of every veterinarian. It can happen when there are excessive numbers of adult heartworms in the pulmonary arteries and the heart to the extent that they block blood returning from the liver and the back of the body. This results in severe pulmonary hypertension, the death of liver cells, and the destruction of red blood cells. Large numbers of adult heartworms can interfere with the closing of the valve on the right side of the heart (tricuspid valve) and cause increased speed of flow through this valve exacerbating these problems.
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5. Serious injury to other organs, especially the kidneys, due to the deposition of damaging immune complexes formed between heartworm proteins and antibodies the dog produces against them.
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If you are one of the lucky ones, your dog can be diagnosed before any symptoms occur. Just in case you were wondering what the symptoms were, here is a list:
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1. None - dogs with only a few adult heartworms are not likely to develop significant symptoms. When symptoms do occur, they are usually delayed at least 1 year post infection and in most cases may not occur for many years.
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2. Cough.
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3. Difficulty breathing.
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4. Intolerance to exercise, tiring easily. Note: 2-4 depend upon the severity of damage to the lung and/or the presence of right sided heart disease.
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5. Weight loss - usually assoc. with severe heart &/or lung disease.
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6. Coughing up blood - usually assoc. with pulmonary embolism.
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7. Bloated abdomen due to fluid collection (ascites), a classic sign of right sided heart failure.
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8. Bloody looking urine from pigments released during red blood cell destruction, pale mucous membranes (anemia), jaundice (icterus), weakness and lethargy, and signs of right sided heart failure (all due to Vena Cava Syndrome).
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The Diagnosis of heartworm disease in dogs:
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1. The best test is one that detects adult heartworm protein (antigen) in the bloodstream. This is the test we run for every dog on an annual basis. This test may be falsely negative if it is run too early in the course of infection (this is why we recommend regular testing), if only a few adults are present, or if only male heartworms are present. This test will detect all dogs in need of treatment.
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2. Prior to the development of the adult heartworm antigen test, blood samples were (and still are) analyzed under the microscope to identify the presence of microfilaria. About 30% or more of all infected dogs will have adult heartworms, but no microfilaria (known as occult infection). The frequency of occult infection makes this test undependable. Occult infections occur because it may be too early for microfilaria to be produced, the dog's immune system may have destroyed the microfilaria, heartworm preventative may have destroyed the microfilaria, or there may be only male or female heartworms present preventing reproduction.
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3. On X-rays of the chest, dogs with significant infection will develop changes including pulmonary artery enlargement and tortuosity (enlarged and winding pulmonary arteries), increase in density of the connective tissue of the lung, prominent pulmonary vessels in general due to hypertension, fluffy densities suggestive of pulmonary artery embolism, generalized infiltrates of the lung due to allergic reaction, enlargement of the right side of the heart, and in severe cases liver enlargement.
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4. Other laboratory findings include increased numbers of eosinophils in the bloodstream, anemia, elevated globulins and total protein, elevated liver enzymes and kidney tests, and the finding of bilirubin or hemoglobin in the urine.
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5. Physical exam findings may be normal. A heart murmur may be heard due to enlargement of the right ventricle or the presence of worms in the tricuspid valve. Symptoms of right sided congestive heart failure may be seen in severe cases including: jugular pulse, ascites, palpable liver enlargement, weight loss, and respiratory distress.
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The treatment of heartworm disease in dogs:
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Before treatment is performed, we strongly recommend some basic work to assess the dog's general condition, the severity of the infection, establish a prognosis, and to be sure there is no hidden condition which could result in unexpected complications to treatment. This includes a physical examination, a second heartworm test to confirm the diagnosis, chest x-rays, and a complete blood count and biochemical profile. Chest x-rays are important to assess the severity of changes in the lung and whether or not right sided heart disease exists. Dogs with mild to moderate disease have a very good prognosis. Dogs with severe disease are more likely to experience complications and represent higher risk patients. Pregnant dogs or dogs with kidney or liver failure or other serious underlying illness will not be treated.
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Until recently, the treatment of heartworm disease was associated with a high rate of serious complications. Now we have a superior treatment available, Immiticide (melarsomine dihydrochloride). This drug was originally developed for use in higher risk patients, but because of it's safety and efficacy, it has become the preferred treatment no matter what the severity of disease. The drug is administered by deep intramuscular injection. Most cases receive a staged treatment where the dog gets one injection, then 30 days later the dog gets two injections 24 hours apart. The dog is typically hospitalized for 2-4 days to allow assessment, treatment, and observation.
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A word of caution. Any dog receiving even a single injection of Immiticide must be rested for at least 30 and preferably 45 days after the injection. That means dogs receiving staged treatment like mentioned above must be rested a minimum of 60 days. This is extremely important. It is absolutely essential that the dog be strictly confined for a minimum of 4 weeks after Immiticide administration to reduce the chance of thromboembolism (explained in this paragraph). The more severe the disease, the more strict the confinement. For most cases, they must be confined to the house or a small pen with no running, no playing, and no jumping. The dog can be leashed walked to do its business, but must immediately return to confinement. The purpose of this period of rest is to allow the lungs to heal. The treatment kills the adult heartworms, and it takes some time for body to clean up the mess that dead heartworms cause. The most severe complication to treatment is pulmonary thromboembolization when a chunk of heartworm breaks away and obstructs the blood flow to a portion of the lung (remember heartworms live in the arteries of the lung). Beginning at about 5 to 10 days post treatment, thromboembolism can occur any time for the next 4-6 weeks. Symptoms of thromboembolism include fever, panting, cough, and even collapse. It is most definitely life threatening. Thromboembolization is more likely if there is severe pulmonary arterial change; evidence of thromboembolism previous to treatment; severe infiltration of the lung; evidence of heart disease; or severe symptoms such as a heavy cough, respiratory distress, weakness, weight loss, or exercise intolerance. Treatment of pulmonary thromboembolism includes rest, oxygen, the careful administration of intravenous fluids, rest, and prednisolone. Symptoms, if they are not lethal, typically resolve within 2 weeks. The two main causes of pulmonary thromboembolism are lack of proper confinement or severe disease.
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Dogs diagnosed with Vena Cava Syndrome are treated by surgical removal of the heartworms via the jugular vein on an emergency basis.
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Once the dog has successfully completed treatment, heartworm prevention is started. Hopefully in your case, you can start your dog on heartworm prevention before infection
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Dr Paul Skellenger is the Veterinarian for Veterinarian Care us and has over 20 years in Veterinarian medicine and experience with heartworms in dogs For additional information you can contact one of our Veterinarians in your area.
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