Heartworm Disease in Dogs



Heartworm disease is one of the most preventable diseases in veterinary medicine. There are over a dozen products on the market designed for heartworm prevention. Yet this disease continues to be a prevalent problem in dogs.

So what is heartworm disease? It is what it sounds like, a disease of the heart that is caused by parasitic worms. It is a serious and fatal disease that can lead to heart failure. How do dogs acquire this disease? Mosquitoes are the major culprit in spreading this disease. A female mosquito will obtain a blood meal from an infected dog and ingests young heartworms or microfilariae in her gut. The microfilariae will continue to develop inside the mosquito for a period of 10-30 days. After this period they become an infective larvae and any unprotected dog they bite becomes infected. These larvae continue to develop and migrate toward the major blood vessels in the heart over the next 2-3 months. Once they reach the heart, they become adults and reproduce. The offspring produced by the adult heartworms live in the bloodstream until they are picked up by a mosquito through a blood meal where they develop into infective larvae. The adult worms can become as large as 6 to 14 inches long.

So how do you know if your dog is infected? Regarding clinical signs, it can take a few years before dogs show outward signs. The worms affect the dogs by causing an obstruction in the major blood vessels from the heart and sometimes interfere with the valves inside the heart. This causes a decrease in blood supply to the other organs of the body potentially leading to malfunction. Severity of clinical signs depend on the number of adult worms present, the location of the worms, and the degree of damage to the heart. Common signs include a dry, chronic cough, shortness of breath, weakness, and exercise intolerance. In more advanced cases congestive heart failure may be apparent. Signs of this include abnormal heart sounds, distension of the abdomen, weight loss, and anemia.

The majority of cases of heartworm disease can be diagnosed by a blood test. The most common blood test detects antigens or proteins that are produced by the female adult heartworms. Microfilaria also can be found in blood through a microscopic exam. These blood tests are the most reliable for early detection of this disease. The success of treatment improves the earlier it is detected.

Additional diagnostics are often recommended if a dog tests positive. The information provided can help predict how risky treatment may be. An x-ray of the chest is helpful in determining enlargement of the heart and the condition of the arteries providing blood to the lungs. The right side of the heart can be enlarged giving it an appearance of a reverse “D” on x-rays. The blood vessels (pulmonary arteries) are often seen as tortuous and prominent which indicates advanced disease. Lab work is performed to help determine any organ dysfunction prior to treatment.

Treatment for heartworm has changed over the years with the goal to help minimize side effects of treatment. There are two organizations that have been that have been instrumental in developing the most updated treatment protocols based on current research. The Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) are organizations that are focused on establishing guidelines for treatment of internal parasites.

The most effective treatment and the only drug approved by the FDA is a Melarsomine dihydrochloride (also known as Immiticide) injections. This drug is injected deep in the back muscles at scheduled intervals. Previously, if a dog was diagnosed with heartworm disease, injections of the Immiticide would be started right away and heartworm prevention would not be started until the patient was confirmed heartworm negative. Compared to the earlier protocol, the treatment is now staged over several months. The dog does not receive his first Immiticide injection until 2 months after a diagnosis. Heartworm prevention is started right away with the first dose given under observation of a veterinarian for at least 8 hours. The heartworm prevention kills any microfiliaria that is in the blood stream and sometimes the body can react to these dead larvae. The patient is observed for an allergic reaction and treated appropriately if necessary. After 2 months the first Immiticide injection is given then repeated in one month with 2 injections 24 hours apart. Below is a table discussing the current recommended Treatment protocol based on the American Heartworm Society.

Source: http://www.heartwormsociety.org/veterinary-resources/canine-guidelines.html#9

Table 1. AHS-Recommended Treatment Protocol for Heartworm Disease

Day Treatment

Day 0 Dog diagnosed and verified as heartworm positive: • Positive antigen (Ag) test verified with microfilaria (MF) test • If no microfilariae are detected, confirm with 2nd Ag test from a different manufacturer Begin exercise restriction.   • The more pronounced the signs, the stricter the exercise restriction If the dog is symptomatic: • Stabilize with appropriate therapy and nursing care • Prednisone prescribed at 0.5 mg/kg BID 1st week, 0.5 mg/kg SID 2nd week, 0.5 mg/kg   EOD 3rd and 4th weeks

Day 1 Administer heartworm preventive. • If microfilariae are detected, pretreat with antihistamine and glucocorticosteroid,   if not already on prednisone, to reduce risk of anaphylaxis   • Observe for at least 8 hours for signs of reaction

Days 1–28 Administer doxycycline 10 mg/kg BID for 4 weeks. • Reduces pathology associated with dead heartworms • Disrupts heartworm transmission

Day 30 Administer heartworm preventive.

Day 60 Administer heartworm preventive. First melarsomine injection 2.5 mg/kg intramuscularly (IM) Prescribe prednisone 0.5 mg/kg BID 1st week, 0.5 mg/kg SID 2nd week, 0.5 mg/kg EOD 3rd and 4th weeks. Decrease activity level even further. • Cage restriction/on leash when using yard

Day 90 Administer heartworm preventive. Second melarsomine injection 2.5 mg/kg IM

Day 91 Third melarsomine injection 2.5 mg/kg IM Prescribe prednisone 0.5 mg/kg BID 1st week, 0.5 mg/kg SID 2nd week, 0.5 mg/kg EOD 3rd and 4th weeks. Continue exercise restriction for 6 to 8 weeks following last melarsomine injections.

Day 120 Test for presence of microfilariae. • If positive treat with a microfilaricide and retest in 4 weeks Establish year-round heartworm prevention.

Day 271 Antigen test 6 months after completion; screen for microfilariae.


Side effects can depend on how advanced the heartworm disease was before treatment. As the worms die off from the injections, they decompose and break apart. The fragments of the worm can lodge in the blood vessels blocking blood flow and lead to blood clots in the lungs. Clots in the lungs leads to coughing, exercise intolerance, and can be deadly. Exercise restriction is required for up to 4 weeks after injections to help minimize these effects. Steroid therapy can help control symptoms if they do occur. A local reaction can occur at the site of injection especially if some of the Immiticide leaks under the skin. This can be painful but can be controlled with anti-inflammatory therapy.

Slow kill therapy is an alternative therapy that some dog owners have opted for when they are concerned about the potential side effects of the Immiticide treatment. Slow kill therapy involves starting a heartworm positive dog on a monthly heartworm preventative and waiting for the adult worms to die a natural death. The life span of a heartworm is 2-3 years. The heartworm preventative serves to prevent any further infection of heartworms but does not kill the adult worms. Both the CAPC and the AHS organization do not recommend the slow kill therapy according to current research findings. One concern with a slow kill therapy is that research has shown a potential for resistance to the heartworm preventative. Heartworm positive dogs can have a high volume of microfilaria circulating in their bloodstream. Constant exposure to the preventative increases the chance that they may develop a gene for resistance. Also it is common for these dogs to form complexes where the protein used to diagnosis heartworm binds with an antibody. This prevents the protein from being detected leading to a falsely negative heartworm test.


So why should you start your dog on heartworm prevention? As you see heartworm disease can have devastating consequences and the only approved treatment has a potential for side effects. The cost of the above treatment protocol can cost anywhere from $500 to over $1000 depending on the size of your dog and severity of signs. A year of heartworm prevention can cost you anywhere from $50 -$100 a year. As you see a case of heartworm disease can cost you as much as 10 x the cost of a preventative!! So please if your dog is not on heartworm prevention please get him/her tested and place on a preventative. It is recommended that dogs start heartworm therapy as early as 8 weeks old and are tested yearly. I will discuss options for heartworm preventatives in future postings.


Dr. Alleyne

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